ABSTRACT
We use the sample of known stars and brown dwarfs within 5 pc of the Sun, supplemented with AFGK stars within 10 pc, to determine which stellar spectral types provide the most habitable real estate—defined as locations where liquid water could be present on Earth-like planets. Stellar temperatures and radii are determined by fitting model spectra to spatially resolved broadband photometric energy distributions for stars in the sample. Using these values, the locations of the habitable zones are calculated using an empirical formula for planetary surface temperature and assuming the condition of liquid water, called here the empirical habitable zone (EHZ). Systems that have dynamically disruptive companions are considered not habitable. We consider companions to be disruptive if the separation ratio of the companion to the habitable zone is less than 5:1. We use the results of these calculations to derive a simple formula for predicting the location of the EHZ for main sequence stars based on V − K color. We consider EHZ widths as more useful measures of the habitable real estate around stars than areas because multiple planets are not expected to orbit stars at identical stellar distances. This EHZ provides a qualitative guide on where to expect the largest population of planets in the habitable zones of main sequence stars. Because of their large numbers and lower frequency of short-period companions, M stars provide more EHZ real estate than other spectral types, possessing 36.5% of the habitable real estate en masse. K stars are second with 21.5%, while A, F, and G stars offer 18.5%, 6.9%, and 16.6%, respectively. Our calculations show that three M dwarfs within 10 pc harbor planets in their EHZs—GJ 581 may have two planets (d with msin i = 6.1 M⊕; g with msin i = 3.1 M⊕), GJ 667 C has one (c with msin i = 4.5 M⊕), and GJ 876 has two (b with msin i = 1.89 MJup and c with msin i = 0.56 MJup). If Earth-like planets are as common around low-mass stars as recent Kepler results suggest, M stars will harbor more Earth-like planets in habitable zones than any other stellar spectral type.
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1. INTRODUCTION
Early astronomers looked to the sky and saw the Moon as a habitable world covered in vast oceans, Venus as a swampy marshland enshrouded in clouds, and Mars with grand canals (Lowell 1895). Not one of these worlds has maintained its promise of abundant life. Instead, the solar system, once thought to be teeming with life, may be barren, although hope remains for environments under the icy crust of Europa (Marion et al. 2003), in the tiger stripes of Enceladus (Parkinson et al. 2007), in water under the Martian surface (Malin & Edgett 2000), or perhaps lurking somewhere as yet unidentified. With the discovery of more than 7001 extrasolar planets since 1989, the real estate market in our solar system is no longer the only place we might look for evidence of life beyond Earth. A likely place to search for planets harboring life is in the habitable zones (HZs) of nearby stars.
The term "habitable zone" was first coined by Huang (1959) as a region around a star where a planet could support life. Since then, there have been many definitions of habitability, most based on the presence of liquid water on the surface of a planet or moon. These habitable worlds could be terrestrial planets, which are known to exist (Borucki et al. 2011), or perhaps moons of gas-giant planets, which are suspected to exist (Weidner & Horne 2010). Examples of nearby stars hosting potentially habitable super-Earths include GJ 581, an M dwarf with potentially two planets in its HZ (Mayor et al. 2009; Vogt et al. 2010a) and GJ 667C, another M dwarf with one planet in its HZ (Anglada-Escudé et al. 2012).
Kasting et al. (1993) did pioneering work in describing HZs around main sequence stars. To approximate the location of the HZ, they introduced a one-dimensional climate model that yields the distances from main sequence stars where liquid water would be present given an initial assumption of a CO2/H2O/N2 atmosphere and an Earth-sized planet. They describe the inner boundary of their model as the point at which the atmosphere becomes saturated with H2O, causing a loss of water via photolysis and hydrogen escape; the outer boundary is marked by the formation of CO2 clouds that cool a planet's surface by increasing its albedo and lowering its convective lapse rate. They give an equation for the distance from a star, D, of the HZ in AU based on the incident flux that a planet receives, L/L☉, the star's luminosity relative to that of the Sun, and Seff, the ratio of outgoing IR flux to the incoming incident flux at the top of the planet's atmosphere:
Using more explicit terms, Equation (1) can be used to show the distance from a star at which a planet would have a given temperature, based on energy balance (Kaltenegger et al. 2002). Equation (2) can then be used to show that the equilibrium temperature, TP, of a planet at a distance, D, from its host star is a function of the stellar effective temperature, Teff, stellar radius, R⋆, and the planet's Bond albedo, A. Thus, if the stellar Teff and R⋆ are known, we can calculate the range of distances where a planet with a given albedo2 would have a surface temperature suitable for liquid water, as described by the planet's temperature, TP:
In this paper, we apply this equation to the nearby population of stars whose stellar properties are well known to estimate the total habitable real estate that they provide. We use the complete sample of all stars currently known to be within 5 pc of the Sun from Henry (2013), and an extended 10 pc sample of AFGK stars, as well as binary properties and photometric and astrometric data. We present our derived Teff and R⋆ for each star in the sample and discuss the methods used to derive each star's empirical habitable zone (EHZ). For multiple star systems, we assess dynamical stability to eliminate stellar systems unsuitable for long-term planetary orbits. Our main goal is to determine, as a function of spectral type, the cumulative available EHZ in the solar neighborhood.
2. MOTIVATIONS FOR THIS STUDY
We do not yet have a detailed understanding of the architecture of all types of planetary systems orbiting various types of stars, although there has been recent progress for planets close to stars via the Kepler mission (e.g., Howard et al. 2012 for planets in orbital periods less than 50 days around FGKM stars). In this paper we assume no bias in the final locations of planets around stars, including formation and migration (i.e., we assume the distribution of planets to be uniform in semimajor axis), to assess the integrated EHZs of various types of stars found in the solar neighborhood. We focus on the presence of the first habitable planet around a given star, although wider EHZs may of course include more than one planet. This is an important assessment to make given the limited telescope time, funding, and energy of astronomers, so here we focus on the question: What set of targets might be most appropriate to observe to improve the odds of detecting at least one habitable planet? This question is posed in wide-ranging arenas, from conversations between students and faculty when developing research projects to discussions of programmatic directions such as those of NASA's Exoplanet Program Analysis Group and the NASA/NSF Exoplanet Task Force.
In this paper we evaluate stars within 10 pc in a consistent fashion to assess what spectral types offer the greatest promise for nearby habitable planets. The nearby sample contains stars that have been characterized carefully and as a population provide the most accurate snapshot of the stellar content of the Galaxy. Thus, this sample provides the foundation for a realistic understanding of the relative merits of examining different types of stars. Our results, coupled with the planet frequency statistics from the Kepler mission (e.g., Borucki & Koch 2011; Howard et al. 2012; Dressing & Charbonneau 2013), can provide statistical measures for the number of habitable planets within larger stellar populations, and particularly among volume-limited samples of nearby stars, as we build a comprehensive sample of the nearest stars (e.g., Henry et al. 2006; Henry 2013). For example, Howard et al. (2012) show that the number of super-Earth-size planets increases with decreasing Teff for orbital periods less than 50 days. Dressing & Charbonneau (2013) show that for cool stars (Teff < 4000 K) the occurrence rate for planets with 0.5–4 R⊕ is 0.9 planets per star for orbital periods less than 50 days. They also calculate that the lower limit on the occurrence rate of Earth-size planets in the HZ of cool stars is 0.04 with a 95% confidence. Using the population we describe here, this implies that there could be two Earth-size planets within the EHZs of M dwarfs within 5 pc of the Sun. Assuming a constant density of M dwarfs to 10 pc (not all of which have yet been identified) the number of Earth-size planets jumps to 16. One of the primary motivations of this study is to determine, in aggregate, how the odds of finding such planets around M dwarfs compares to other spectral types.
The results of our habitable real estate calculations, as outlined in this paper, are particularly valuable for highlighting that the ubiquitous M dwarfs provide many locales meeting the canonical definition for habitability. Current searches for habitable worlds orbiting the nearest stars have yielded many Jovian planets and a few terrestrial worlds, but most of the searches to date have been carried out using the radial velocity technique and have focused on bright FGK stars that provide many photons to spectrographs (and which are in the sweet spot for Kepler). For transit searches, M dwarfs provide higher contrast ratios for a given planet, whereas for radial velocity and astrometric searches they provide larger wobbles for a planet than do more massive FGK stars. Knowing that the M dwarfs, as a group, are stars important to the search for habitable worlds helps direct our focus back to the solar neighborhood, in which three-quarters of all stars are red dwarfs. Because of their proximity, these stars hold great promise for the detailed characterization of exoplanets.
3. PROPERTIES OF OUR NEAREST NEIGHBORS
Primary science goals of the Research Consortium on Nearby Stars (RECONS3) and the Center for High Angular Resolution Astronomy (CHARA4), are to find and determine the fundamental properties of our nearest stellar neighbors. This has led to the most complete assessment of stars within 5 pc of the Sun to date.
3.1. The 5 pc Sample
The modern sample of all known stars and brown dwarfs within 5 pc of the Sun (Henry 2010, 2013), listed in Table 1 (assembled photometry in Table 2) was first published in the Observer's Handbook 2010. The sample, which is updated yearly, was created using the combination of several ground-based and space-based parallax programs, including the General Catalogue of Trigonometric Stellar Parallaxes (van Altena et al. 1995), Hipparcos (Perryman et al. 1997), RECONS (Henry et al. 1997, 2006; Deacon et al. 2005; Jao et al. 2005; Costa et al. 2005), the Hubble Space Telescope (Benedict et al. 1999, 2002), Gatewood (1989, 1994), and Gatewood et al. (1992, 1993). To be included in the sample, a system must have a weighted mean trigonometric parallax measurement of 200 mas or greater with an error of 10 mas or less. To create this sample the parallaxes compiled were combined and weighted based on the individual measurement errors. The spectral type, with reference, for each star or star system is given with the astrometric data, including right ascension, declination, the weighted mean trigonometric parallax, and the number of parallaxes included in the weighted mean, in Table 1. The closing date for the sample is 2012.0.
Table 1. Five Parsec Sample
Name | LHS | R.A. 2000.0 | Decl. | π | err | No. of π | SpType | Ref. |
---|---|---|---|---|---|---|---|---|
Sun | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | G2.0V | ⋅⋅⋅ |
GJ 1 | 1 | 00 05 24.4 | −37 21 27 | .23032 | .00090 | 2 | M1.5V | Hen10 |
GJ 1002 | 2 | 00 06 43.2 | −07 32 17 | .21300 | .00360 | 1 | M5.0V | Hen94 |
GJ 15 A | 3 | 00 18 22.9 | +44 01 23 | .27987 | .00060 | 3 | M1.5V | Hen94 |
GJ 15 B | 4 | 00 18 22.9 | +44 01 23 | .27987 | .00060 | 3 | M3.5V | Hen94 |
GJ 35 | 7 | 00 49 09.9 | +05 23 19 | .23270 | .00181 | 2 | DZ7 | YPC95 |
GJ 54.1 | 138 | 01 12 30.6 | −16 59 57 | .26908 | .00299 | 2 | M4.5V | Hen94 |
GJ 65 A | 9 | 01 39 01.3 | −17 57 01 | .37370 | .00270 | 1 | M5.5V | Hen94 |
GJ 65 B | 10 | 01 39 01.3 | −17 57 01 | .37370 | .00270 | 1 | M6.0V | Hen94 |
GJ 71 | 146 | 01 44 04.1 | −15 56 15 | .27397 | .00017 | 2 | G8.5V | Gra06 |
GJ 83.1 | 11 | 02 00 13.2 | +13 03 08 | .22480 | .00290 | 1 | M4.5V | Hen94 |
SO 0253+1652 | ⋅⋅⋅ | 02 53 00.9 | +16 52 53 | .25941 | .00089 | 3 | M7.0V | Hen04 |
DEN 0255−4700 | ⋅⋅⋅ | 02 55 03.7 | −47 00 52 | .20137 | .00389 | 1 | L7.5V | Cos06 |
GJ 144 | 1557 | 03 32 55.8 | −09 27 30 | .31122 | .00009 | 3 | K2.0V | Gra06 |
GJ 1061 | 1565 | 03 36 00.0 | −44 30 46 | .27201 | .00130 | 2 | M5.0V | Hen06 |
LP 944−020 | ⋅⋅⋅ | 03 39 35.2 | −35 25 41 | .20140 | .00421 | 1 | M9.0V | Sch07 |
GJ 166 A | 23 | 04 15 16.3 | −07 39 10 | .20065 | .00023 | 2 | K0.5V | Gra06 |
GJ 166 B | 24 | 04 15 22.0 | −07 39 35 | .20065 | .00023 | 2 | DA4 | CNS91 |
GJ 166 C | 25 | 04 15 22.0 | −07 39 35 | .20065 | .00023 | 2 | M4.5V | Hen94 |
GJ 191 | 29 | 05 11 40.6 | −45 01 06 | .25567 | .00091 | 2 | M2.0VI | Jao08 |
GJ 234 A | 1849 | 06 29 23.4 | −02 48 50 | .24444 | .00092 | 3 | M4.0V | Mon01 |
GJ 234 B | 1850 | 06 29 23.4 | −02 48 50 | .24444 | .00092 | 3 | M5.5V | Rei04 |
GJ 244 A | 219 | 06 45 08.9 | −16 42 58 | .38002 | .00128 | 2 | A1.0V | Joh53 |
GJ 244 B | ⋅⋅⋅ | 06 45 08.9 | −16 42 58 | .38002 | .00128 | 2 | DA2 | CNS91 |
GJ 273 | 33 | 07 27 24.5 | +05 13 33 | .26623 | .00066 | 3 | M3.5V | Hen94 |
GJ 280 A | 233 | 07 39 18.1 | +05 13 30 | .28517 | .00064 | 4 | F5.0IV–V | Gra01 |
GJ 280 B | ⋅⋅⋅ | 07 39 18.1 | +05 13 30 | .28517 | .00064 | 4 | DA | CNS91 |
GJ 1111 | 248 | 08 29 49.5 | +26 46 37 | .27580 | .00300 | 1 | M6.0V | Rei95 |
GJ 380 | 280 | 10 11 22.1 | +49 27 15 | .20553 | .00049 | 2 | K7.0V | Hen94 |
GJ 388 | 5167 | 10 19 36.4 | +19 52 10 | .20460 | .00280 | 1 | M3.0V | Hen94 |
LHS 288 | 288 | 10 44 21.2 | −61 12 36 | .20970 | .00265 | 2 | M5.5V | Hen04 |
LHS 292 | 292 | 10 48 12.6 | −11 20 14 | .22030 | .00360 | 1 | M6.5V | Rei95 |
DEN 1048−3956 | ⋅⋅⋅ | 10 48 14.6 | −39 56 07 | .24853 | .00118 | 3 | M8.5V | Hen04 |
GJ 406 | 36 | 10 56 29.2 | +07 00 53 | .41910 | .00210 | 1 | M6.0V | Hen94 |
GJ 411 | 37 | 11 03 20.2 | +35 58 12 | .39325 | .00057 | 2 | M2.0V | Hen94 |
GJ 412 A | 38 | 11 05 28.6 | +43 31 36 | .20567 | .00093 | 2 | M1.0V | Hen94 |
GJ 412 B | 39 | 11 05 30.4 | +43 31 18 | .20567 | .00093 | 2 | M5.5V | Hen94 |
GJ 440 | 43 | 11 45 42.9 | −64 50 29 | .21612 | .00109 | 3 | DQ6 | CNS91 |
GJ 447 | 315 | 11 47 44.4 | +00 48 16 | .29814 | .00137 | 2 | M4.0V | Hen94 |
GJ 473 A | 333 | 12 33 17.2 | +09 01 15 | .22790 | .00460 | 1 | M5.0V | Hen10 |
GJ 473 B | ⋅⋅⋅ | 12 33 17.2 | +09 01 15 | .22790 | .00460 | 1 | M7.0V | CNS91 |
GJ 551 | 49 | 14 29 43.0 | −62 40 46 | .76885 | .00029 | 4 | M5.0V | CNS91 |
GJ 559 A | 50 | 14 39 36.5 | −60 50 02 | .74723 | .00117 | 1 | G2.0V | Gra06 |
GJ 559 B | 51 | 14 39 35.1 | −60 50 14 | .74723 | .00117 | 1 | K0.0V | CNS91 |
GJ 628 | 419 | 16 30 18.1 | −12 39 45 | .23438 | .00150 | 2 | M3.0V | Hen94 |
GJ 674 | 449 | 17 28 39.9 | −46 53 43 | .22011 | .00139 | 2 | M2.5V | Mon01 |
GJ 687 | 450 | 17 36 25.9 | +68 20 21 | .22047 | .00083 | 2 | M3.0V | Hen94 |
GJ 699 | 57 | 17 57 48.5 | +04 41 36 | .54551 | .00029 | 2 | M4.0V | Hen94 |
GJ 725 A | 58 | 18 42 46.7 | +59 37 49 | .28383 | .00146 | 3 | M3.0V | Hen94 |
GJ 725 B | 59 | 18 42 46.9 | +59 37 37 | .28383 | .00146 | 3 | M3.5V | Hen94 |
SCR 1845−6357 A | ⋅⋅⋅ | 18 45 02.6 | −63 57 52 | .25950 | .00111 | 2 | M8.5V | Hen04 |
SCR 1845−6357 B | ⋅⋅⋅ | 18 45 02.6 | −63 57 52 | .25950 | .00111 | 2 | T5.5V | Bil06 |
GJ 729 | 3414 | 18 49 49.4 | −23 50 10 | .33722 | .00197 | 2 | M3.5V | Hen10 |
GJ 1245 A | 3494 | 19 53 54.2 | +44 24 55 | .22020 | .00100 | 1 | M5.5V | Hen94 |
GJ 1245 B | 3495 | 19 53 55.2 | +44 24 56 | .22020 | .00100 | 1 | M6.0V | Hen94 |
GJ 1245 C | ⋅⋅⋅ | 19 53 54.2 | +44 24 55 | .22020 | .00100 | 1 | M7.0V | Rei04 |
GJ 820 A | 62 | 21 06 53.9 | +38 44 58 | .28608 | .00048 | 3 | K5.0V | Hen94 |
GJ 820 B | 63 | 21 06 55.3 | +38 44 31 | .28608 | .00048 | 3 | K7.0V | Hen94 |
GJ 825 | 66 | 21 17 15.3 | −38 52 03 | .25344 | .00080 | 2 | M0.0V | Tor06 |
GJ 832 | 3685 | 21 33 34.0 | −49 00 32 | .20203 | .00100 | 2 | M1.5V | Joh10 |
GJ 845 A | 67 | 22 03 21.7 | −56 47 10 | .27607 | .00028 | 2 | K4.0V | Gra06 |
GJ 845 B | ⋅⋅⋅ | 22 04 10.5 | −56 46 58 | .27607 | .00028 | 2 | T1.0V | McC04 |
GJ 845 C | ⋅⋅⋅ | 22 04 10.5 | −56 46 58 | .27607 | .00028 | 2 | T6.0V | McC04 |
GJ 860 A | 3814 | 22 27 59.5 | +57 41 45 | .24806 | .00139 | 2 | M3.0V | Hen94 |
GJ 860 B | 3815 | 22 27 59.5 | +57 41 45 | .24806 | .00139 | 2 | M4.0V | Hen94 |
GJ 866 A | 68 | 22 38 33.4 | −15 18 07 | .28950 | .00440 | 1 | M5.0V | Hen02 |
GJ 866 B | ⋅⋅⋅ | 22 38 33.4 | −15 18 07 | .28950 | .00440 | 1 | M | ⋅⋅⋅ |
GJ 866 C | ⋅⋅⋅ | 22 38 33.4 | −15 18 07 | .28950 | .00440 | 1 | M | ⋅⋅⋅ |
GJ 876 | 530 | 22 53 16.7 | −14 15 49 | .21447 | .00057 | 3 | M4.0V | Mon01 |
GJ 887 | 70 | 23 05 52.0 | −35 51 11 | .30508 | .00070 | 2 | M2.0V | Tor06 |
GJ 905 | 549 | 23 41 55.0 | +44 10 38 | .31637 | .00055 | 3 | M5.5V | Hen94 |
References. Bil06: Biller et al. (2006); CNS91: Catalog of Nearby Stars, Gliese & Jahreiß (1991); Cos06: Costa et al. (2006); Gra01: Gray et al. (2001); Gra06: Gray et al. (2006); Hen94: Henry et al. (1994); Hen02: Henry et al. (2002); Hen04: Henry et al. (2004); Hen06: Henry et al. (2006); Hen10: Henry (2010); Jao08: Jao et al. (2008); Joh53: Johnson & Morgan (1953); Joh10: Johnson et al. (2010); McC04: McCaughrean et al. (2004); Mon01: Montes et al. (2001); Rei95: Reid et al. (1995); Rei04: Reid et al. (2004); Sch07: Schmidt et al. (2007); Tor06: Torres et al. (2006); YPC95: Yale Parallax Catalog, van Altena et al. (1995).
Table 2. Five Parsec Sample: Photometry
Name | U | Uref | B | Bref | V | Vref | R | Rref | I | Iref | J | err | H | err | K | err | Notes |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Sun | −25.97 | Alo95 | −26.10 | Alo95 | −26.75 | Alo95 | −27.27 | Alo95 | −27.56 | Alo95 | −27.928 | Alo95 | −28.211 | Alo95 | −28.274 | Alo95 | ⋅⋅⋅ |
GJ 1 | ⋅⋅⋅ | ⋅⋅⋅ | 10.02 | Bes90 | 8.54 | Bes90 | 7.57 | Bes90 | 6.41 | Bes90 | 5.328 | 0.019 | 4.828 | 0.076 | 4.523 | 0.017 | ⋅⋅⋅ |
GJ 1002 | 17.61 | Leg92 | 15.73 | Wei96 | 13.77 | Bes91 | 12.16 | Bes91 | 10.15 | Bes91 | 8.323 | 0.019 | 7.792 | 0.034 | 7.439 | 0.021 | ⋅⋅⋅ |
GJ 15 A | 10.87 | Leg92 | 9.63 | Leg92 | 8.08 | Leg92 | ⋅⋅⋅ | ⋅⋅⋅ | 5.94 | Leg92 | 5.252 | 0.264 | 4.476 | 0.200 | 4.018 | 0.020 | ⋅⋅⋅ |
GJ 15 B | 14.26 | Leg92 | 12.88 | Wei96 | 11.06 | Wei96 | 9.83 | Wei96 | 8.24 | Wei96 | 6.789 | 0.024 | 6.191 | 0.016 | 5.948 | 0.024 | ⋅⋅⋅ |
GJ 35 | ⋅⋅⋅ | ⋅⋅⋅ | 12.94 | Bes90 | 12.40 | Bes90 | 12.14 | Bes90 | 11.91 | Bes90 | 11.688 | 0.022 | 11.572 | 0.024 | 11.498 | 0.025 | ⋅⋅⋅ |
GJ 54.1 | 15.24 | Leg92 | 13.95 | Bes90 | 12.10 | Bes90 | 10.73 | Bes90 | 8.95 | Bes90 | 7.258 | 0.020 | 6.749 | 0.033 | 6.420 | 0.017 | ⋅⋅⋅ |
GJ 65 A | 14.96 | Leg92 | 13.95 | Bes90 | 12.61* | Hen99 | 10.40 | Bes90 | 8.34 | Bes90 | 6.86* | Hen93 | 6.30* | Hen93 | 5.91* | Hen93 | Joint UBRI |
GJ 65 B | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 13.06* | Hen99 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 7.24* | Hen93 | 6.60* | Hen93 | 6.31* | Hen93 | ⋅⋅⋅ |
GJ 71 | 4.42 | Joh66 | 4.21 | Bes90 | 3.49 | Bes90 | 3.06 | Bes90 | 2.67 | Bes90 | 2.06 | Joh66 | 1.800 | 0.234 | 1.68 | Joh66 | ⋅⋅⋅ |
GJ 83.1 | 15.47 | Leg92 | 14.14 | Bes90 | 12.31 | Bes90 | 10.95 | Bes90 | 9.21 | Bes90 | 7.514 | 0.017 | 6.970 | 0.027 | 6.648 | 0.017 | ⋅⋅⋅ |
SO 0253+1652 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 15.13 | Hen06 | 13.03 | Hen06 | 10.65 | Hen06 | 8.394 | 0.027 | 7.883 | 0.040 | 7.585 | 0.046 | ⋅⋅⋅ |
DEN 0255−4700 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 22.92 | Cos06 | 19.90 | Cos06 | 17.45 | Cos06 | 13.246 | 0.027 | 12.204 | 0.024 | 11.558 | 0.024 | ⋅⋅⋅ |
GJ 144 | ⋅⋅⋅ | ⋅⋅⋅ | 4.61 | Bes90 | 3.73 | Bes90 | 3.22 | Bes90 | 2.79 | Bes90 | 2.20 | Gla75 | 1.75 | Gla75 | 1.65 | Gla75 | ⋅⋅⋅ |
GJ 1061 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 13.09 | Hen06 | 11.45 | Hen06 | 9.46 | Hen06 | 7.523 | 0.020 | 7.015 | 0.044 | 6.610 | 0.021 | ⋅⋅⋅ |
LP 944−020 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 13.29 | Dea07 | 10.725 | 0.021 | 10.017 | 0.021 | 9.548 | 0.023 | ⋅⋅⋅ |
GJ 191 | ⋅⋅⋅ | ⋅⋅⋅ | 10.41 | Bes90 | 8.85 | Bes90 | 7.90 | Bes90 | 6.90 | Bes90 | 5.821 | 0.025 | 5.316 | 0.027 | 5.049 | 0.021 | ⋅⋅⋅ |
GJ 234 A | 14.03 | Leg92 | 12.81 | Bes90 | 11.18* | Hen99 | 9.78 | Bes90 | 8.08 | Bes90 | 6.57* | Hen93 | 5.97* | Hen93 | 5.73* | Hen93 | Joint UBRI |
GJ 234 B | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 14.26* | Hen99 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 8.36* | Hen93 | 7.60* | Hen93 | 7.23* | Hen93 | ⋅⋅⋅ |
GJ 244 AB | −1.4 1 | Joh66 | −1.43 | Bes90 | −1.43 | Bes90 | −1.42 | Bes90 | −1.41 | Bes90 | −1.391 | 0.109 | −1.391 | 0.184 | −1.390 | 0.214 | Joint |
GJ 273 | 12.59 | Wei93 | 11.42 | Bes90 | 9.85 | Bes90 | 8.70 | Bes90 | 7.16 | Bes90 | 5.714 | 0.032 | 5.219 | 0.063 | 4.857 | 0.022 | ⋅⋅⋅ |
GJ 280 AB | 0.82 | Joh66 | 0.79 | Bes90 | 0.37 | Bes90 | 0.12 | Bes90 | −0.12 | Bes90 | −0.40 | Gla75 | −0.60 | Gla75 | −0.65 | Gla75 | Joint |
GJ 1111 | ⋅⋅⋅ | ⋅⋅⋅ | 16.95 | Bes90 | 14.90 | Bes90 | 12.90 | Bes90 | 10.64 | Bes90 | 8.235 | 0.021 | 7.617 | 0.018 | 7.260 | 0.024 | ⋅⋅⋅ |
GJ 380 | 9.25 | Leg92 | 7.97 | Leg92 | 6.59 | Leg92 | 5.74 | Leg92 | 4.97 | Leg92 | 3.98 | Gla75 | 3.32 | Gla75 | 3.19 | Gla75 | ⋅⋅⋅ |
GJ 388 | 11.91 | Leg92 | 10.85 | Leg92 | 9.32 | Leg92 | 8.23 | Leg92 | 6.81 | Leg92 | 5.449 | 0.027 | 4.843 | 0.020 | 4.593 | 0.017 | ⋅⋅⋅ |
LHS 288 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 13.92 | Bes91 | 12.33 | Bes91 | 10.31 | Bes91 | 8.492 | 0.021 | 8.054 | 0.044 | 7.728 | 0.027 | ⋅⋅⋅ |
LHS 292 | ⋅⋅⋅ | ⋅⋅⋅ | 17.70 | Leg92 | 15.73 | Bes91 | 13.67 | Bes91 | 11.33 | Bes91 | 8.857 | 0.021 | 8.263 | 0.036 | 7.926 | 0.033 | ⋅⋅⋅ |
DEN 1048−3956 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 17.39 | Jao05 | 15.05 | Jao05 | 12.55 | Jao05 | 9.538 | 0.022 | 8.905 | 0.044 | 8.447 | 0.023 | ⋅⋅⋅ |
GJ 406 | 17.03 | Leg92 | 15.52 | Bes90 | 13.53 | Bes90 | 11.67 | Bes90 | 9.50 | Bes90 | 7.085 | 0.024 | 6.482 | 0.042 | 6.084 | 0.017 | ⋅⋅⋅ |
GJ 411 | 10.12 | Leg92 | 8.98 | Leg92 | 7.47 | Leg92 | 6.46 | Leg92 | 5.32 | Leg92 | 4.13 | Gla75 | 3.56 | Gla75 | 3.20 | Gla75 | ⋅⋅⋅ |
GJ 412 A | 11.48 | Leg92 | 10.34 | Wei96 | 8.77 | Wei96 | 7.79 | Wei96 | 6.70 | Wei96 | 5.538 | 0.019 | 5.002 | 0.021 | 4.769 | 0.020 | ⋅⋅⋅ |
GJ 412 B | ⋅⋅⋅ | ⋅⋅⋅ | 16.53 | Bes90 | 14.44 | Bes90 | 12.77 | Bes90 | 10.68 | Bes90 | 8.742 | 0.025 | 8.177 | 0.024 | 7.839 | 0.026 | ⋅⋅⋅ |
GJ 440 | 11.04 | Lan92 | 11.68 | Lan92 | 11.50 | Sub09 | 11.34 | Sub09 | 11.20 | Sub09 | 11.188 | 0.024 | 11.130 | 0.025 | 11.104 | 0.026 | ⋅⋅⋅ |
GJ 447 | 14.22 | Leg92 | 12.92 | Bes90 | 11.16 | Bes90 | 9.85 | Bes90 | 8.17 | Bes90 | 6.505 | 0.023 | 5.945 | 0.024 | 5.654 | 0.024 | ⋅⋅⋅ |
GJ 473 A | ⋅⋅⋅ | ⋅⋅⋅ | 15.06* | Tor99 | 13.25* | Tor99 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 7.69* | Tor99 | 7.06* | Tor99 | 6.59* | Tor99 | ⋅⋅⋅ |
GJ 473 B | ⋅⋅⋅ | ⋅⋅⋅ | 15.11* | Tor99 | 13.24* | Tor99 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 7.82* | Tor99 | 7.26* | Tor99 | 7.03* | Tor99 | ⋅⋅⋅ |
GJ 551 | 14.36 | Leg92 | 12.88 | Bes90 | 11.05 | Bes90 | 9.43 | Bes90 | 7.43 | Bes90 | 5.357 | 0.023 | 4.835 | 0.057 | 4.384 | 0.033 | ⋅⋅⋅ |
GJ 559 A | ⋅⋅⋅ | ⋅⋅⋅ | 0.64 | Bes90 | 0.01 | Bes90 | −0.35 | Bes90 | −0.68 | Bes90 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 559 B | ⋅⋅⋅ | ⋅⋅⋅ | 2.18 | Bes90 | 1.34 | Bes90 | 0.87 | Bes90 | 0.46 | Bes90 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 628 | 12.82 | Wei93 | 11.68 | Bes90 | 10.10 | Bes90 | 8.94 | Bes90 | 7.42 | Bes90 | 5.950 | 0.024 | 5.373 | 0.040 | 5.075 | 0.024 | ⋅⋅⋅ |
GJ 674 | ⋅⋅⋅ | ⋅⋅⋅ | 10.90 | Bes90 | 9.37 | Bes90 | 8.30 | Bes90 | 6.97 | Bes90 | 5.711 | 0.019 | 5.154 | 0.033 | 4.855 | 0.018 | ⋅⋅⋅ |
GJ 687 | 11.76 | Leg92 | 10.64 | Wei96 | 9.17 | Wei96 | 8.08 | Wei96 | 6.68 | Wei96 | 5.335 | 0.021 | 4.766 | 0.033 | 4.548 | 0.021 | ⋅⋅⋅ |
GJ 699 | 12.54 | Leg92 | 11.31 | Bes90 | 9.57 | Bes90 | 8.35 | Bes90 | 6.79 | Bes90 | 5.244 | 0.020 | 4.834 | 0.034 | 4.524 | 0.020 | ⋅⋅⋅ |
GJ 725 A | 11.55 | Leg92 | 10.42 | Wei96 | 8.90 | Wei96 | 7.83 | Wei96 | 6.48 | Wei96 | 5.189 | 0.017 | 4.741 | 0.036 | 4.432 | 0.020 | ⋅⋅⋅ |
GJ 725 B | 12.41 | Leg92 | 11.28 | Wei96 | 9.69 | Wei96 | 8.57 | Wei96 | 7.13 | Wei96 | 5.721 | 0.020 | 5.197 | 0.024 | 5.000 | 0.022 | ⋅⋅⋅ |
SCR 1845−6357 AB | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 17.39 | Hen06 | 14.99 | Hen06 | 12.46 | Hen06 | 9.544 | 0.023 | 8.967 | 0.027 | 8.508 | 0.020 | Joint |
GJ 729 | ⋅⋅⋅ | ⋅⋅⋅ | 12.18 | Bes90 | 10.44 | Bes90 | 9.21 | Bes90 | 7.65 | Bes90 | 6.222 | 0.018 | 5.655 | 0.034 | 5.370 | 0.016 | ⋅⋅⋅ |
GJ 1245 A | ⋅⋅⋅ | ⋅⋅⋅ | 15.31 | Leg92 | 13.46* | Hen99 | 11.81 | Wei96 | 9.78 | Wei96 | 8.09* | Hen93 | 7.53* | Hen93 | 7.21* | Hen93 | Joint BRI |
GJ 1245 C | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 16.75* | Hen99 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 9.35* | Hen93 | 8.61* | Hen93 | 8.24* | Hen93 | ⋅⋅⋅ |
GJ 1245 B | ⋅⋅⋅ | ⋅⋅⋅ | 15.98 | Leg92 | 14.01 | Wei96 | 12.36 | Wei96 | 10.27 | Wei96 | 8.275 | 0.025 | 7.728 | 0.031 | 7.387 | 0.018 | ⋅⋅⋅ |
GJ 820 A | 8.63 | Joh66 | 7.40 | Joh66 | 6.03 | Joh66 | 4.86 | Joh66 | 4.03 | Joh66 | 3.114 | 0.268 | 2.540 | 0.198 | 2.248 | 0.318 | ⋅⋅⋅ |
GJ 820 B | 8.62 | Leg92 | 7.40 | Leg92 | 6.03 | Leg92 | ⋅⋅⋅ | ⋅⋅⋅ | 4.41 | Leg92 | 3.546 | 0.278 | 2.895 | 0.218 | 2.544 | 0.328 | ⋅⋅⋅ |
GJ 825 | 9.29 | Le192 | 8.09 | Bes90 | 6.67 | Bes90 | 5.77 | Bes90 | 4.91 | Bes90 | 3.915 | Mou76 | 3.270 | Mou76 | 3.075 | Mou76 | ⋅⋅⋅ |
GJ 832 | 11.36 | Leg92 | 10.18 | Bes90 | 8.66 | Bes90 | 7.66 | Bes90 | 6.47 | Bes90 | 5.349 | 0.032 | 4.766 | 0.256 | 4.501 | 0.018 | ⋅⋅⋅ |
GJ 845 A | 6.74 | Joh66 | 5.73 | Bes90 | 4.68 | Bes90 | 4.06 | Bes90 | 3.53 | Bes90 | 2.894 | 0.292 | 2.349 | 0.214 | 2.237 | 0.240 | ⋅⋅⋅ |
GJ 845 BC | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 11.010 | 0.020 | 11.306 | 0.024 | 11.208 | 0.024 | Joint |
GJ 860 A | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 9.86* | Hen93 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 5.91* | Hen93 | 5.33* | Hen93 | 5.02* | Hen93 | ⋅⋅⋅ |
GJ 860 B | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 11.41* | Hen93 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 7.10* | Hen93 | 6.47* | Hen93 | 6.39* | Hen93 | ⋅⋅⋅ |
GJ 866 AC | 15.83 | Leg92 | 14.33 | Bes90 | 12.94* | Hen99 | 10.70 | Bes90 | 8.64 | Bes90 | 7.06* | Lei90 | 6.46* | Lei90 | 6.05* | Lei90 | Joint UBRI |
GJ 866 B | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 13.34* | Hen99 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 7.62* | Lei90 | 7.02* | Lei90 | 6.61* | Lei90 | ⋅⋅⋅ |
GJ 876 | 12.90 | Wei93 | 11.76 | Bes90 | 10.18 | Bes90 | 9.00 | Bes90 | 7.43 | Bes90 | 5.934 | 0.019 | 5.349 | 0.049 | 5.010 | 0.021 | ⋅⋅⋅ |
GJ 887 | ⋅⋅⋅ | ⋅⋅⋅ | 8.84 | Bes90 | 7.34 | Bes90 | 6.37 | Bes90 | 5.32 | Bes90 | 4.338 | 0.258 | 3.608 | 0.230 | 3.465 | 0.200 | ⋅⋅⋅ |
GJ 905 | 15.65 | Leg92 | 14.20 | Wei96 | 12.29 | Wei96 | 10.77 | Wei96 | 8.82 | Wei96 | 6.884 | 0.025 | 6.247 | 0.027 | 5.929 | 0.020 | ⋅⋅⋅ |
GJ 166 A | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 4.43 | Bes90 | 3.96 | Bes90 | 3.54 | Bes90 | 3.013 | 0.238 | 2.594 | 0.198 | 2.498 | 0.236 | ⋅⋅⋅ |
GJ 166 B | 8.88 | Kid91 | 9.56 | Kid91 | 9.53 | Kid91 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 9.849 | 0.029 | 9.986 | 0.039 | 9.861 | 0.071 | ⋅⋅⋅ |
GJ 166 C | ⋅⋅⋅ | ⋅⋅⋅ | 12.84 | Rei04 | 11.17 | Rei04 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 6.747 | 0.020 | 6.278 | 0.040 | 5.962 | 0.026 | ⋅⋅⋅ |
Notes. JHK and err from 2MASS unless otherwise noted. "Joint" indicates unresolved photometry. *Deconvolved using flux ratios from references given and available optical and infrared photometry. References. Alo95: Alonso et al. (1995); Bes90: Bessel (1990); Bes91: Bessell (1991); Cos06: Costa et al. (2006); Dea07: Deacon & Hambly (2007); Gla75: Glass (1975); Hen93: Henry & McCarthy (1993); Hen99: Henry et al. (1999); Hen06: Henry et al. (2006); Jao05: Jao et al. (2005); Joh66: Johnson et al. (1966); Kid91: Kidder et al. (1991); Lan92: Landolt (1992); Leg92: Leggett (1992); Lei90: Leinert et al. (1990); Mou76: Mould & Hyland (1976); Sub09: Subasavage et al. (2009); Wei93: Weis (1993); Wei96: Weis (1996); Tor99: Torres et al. (1999).
The 5 pc sample contains 67 stars, including the Sun, and 4 presumed brown dwarfs (spectral types L or T) in 50 systems. Of the 50 systems, 34 are single, 11 are binaries, and 5 are triples, giving a multiplicity fraction of 32%. This fraction is consistent with previous volume-limited surveys (35%; Reid & Gizis 1997). The majority (85%) of stars in the sample are main sequence stars; the exceptions include Procyon (GJ 280A), which has a slightly evolved spectral type of F5IV-V, five white dwarfs, and the four L/T dwarfs. The spectral type breakdown includes 1 A, 1 F, 3 Gs (including the Sun), 7 Ks, 50 Ms, 1 L, 3 Ts, and 5 white dwarfs. We do not include white dwarfs and brown dwarfs in subsequent calculations of habitable real estate as they are objects that are cooling continually, resulting in unsustainable HZs on long (∼Gyr) timescales.
3.2. An Estimated 10 pc Sample
Due to the sparse population of all but the M stars in the 5 pc sample, it is difficult to draw meaningful statistics to estimate which stellar spectral types possess the most habitable real estate. Therefore, we extend our sample to 10 pc for A, F, G, and K stars. Using the Hipparcos catalog, which is complete to V = 9 (Perryman et al. 1997), we selected all objects with a parallax greater than 100 mas for inclusion into the sample. We expect that the extended 10 pc sample is complete for spectral types A through K, given Mv = 9.0 for a M0.0V star (Henry et al. 2006). However, rather than using spectral type as a selection criterion, we used a color cutoff of V − K ⩽ 3.5 as the dividing line between K and M dwarfs (Kenyon & Hartmann 1995). As with the 5 pc sample, weighted mean parallaxes from the General Catalogue of Trigonometric Stellar Parallaxes, the Hipparcos catalog and other sources are listed with the astrometry data for the extended 10 pc sample in Table 3, as well as spectral types and references. Available photometric data are listed in Table 4. We note that there are no O-type or B-type stars within 10 pc. Because the M star population is not complete out to 10 pc (Henry et al. 2006), we approximate the total M star population by scaling by a factor of eight from the 5 pc sample. For clarity, we refer to the additional AFGK stars from 5–10 pc as the "extended 10 pc sample" and our estimates of all AFGKM stars within 10 pc as the "estimated 10 pc sample." In total, the stellar population of the estimated 10 pc sample consists of 66 AFGK stars in 57 systems. Broken down by spectral type, the sample contains 4 A stars, 6 F stars, 21 G stars, 35 K stars, and an estimated 400 M stars within 10 pc.
Table 3. Extended 10 pc Sample
Name | LHS | R.A. 2000.0 | Decl. | π | err | No. of π | SpType | Ref. |
---|---|---|---|---|---|---|---|---|
GJ 17 | 5 | 00 20 04.2 | −64 52 29 | .11647 | .00016 | 2 | F9.5V | Gra06 |
GJ 19 | 6 | 00 25 45.1 | −77 15 15 | .13407 | .00011 | 2 | G0.0V | Gra06 |
GJ 33 | 121 | 00 48 23.0 | +05 16 50 | .13426 | .00049 | 2 | K2.5V | Gra06 |
GJ 34 A | 123 | 00 49 06.3 | +57 48 55 | .16823 | .00046 | 2 | G3.0V | Mon01 |
GJ 53 A | 8 | 01 08 16.4 | +54 55 13 | .13267 | .00074 | 2 | G5.0V | Joh53 |
GJ 66 A | ⋅⋅⋅ | 01 39 47.6 | −56 11 47 | .12999 | .00208 | 2 | K5.0V | Mon01 |
GJ 66 B | ⋅⋅⋅ | 01 39 47.6 | −56 11 36 | .12999 | .00208 | 2 | K5.0V | Mon01 |
GJ 68 | 1287 | 01 42 29.8 | +20 16 07 | .13275 | .00049 | 2 | K1.0V | Gra06 |
GJ 105 A | 15 | 02 36 04.9 | +06 53 13 | .13906 | .00044 | 2 | K3.0V | Gra06 |
GJ 139 | 19 | 03 19 55.7 | −43 04 11 | .16547 | .00019 | 2 | G8.0V | Pas94 |
GJ 150 | 1581 | 03 43 14.9 | −09 45 48 | .11063 | .00022 | 2 | K1.0III-IV | Gra06 |
GJ 178 | ⋅⋅⋅ | 04 49 50.4 | +06 57 41 | .12393 | .00017 | 2 | F6.0V | Mon01 |
GJ 183 | 200 | 05 00 49.0 | −05 45 13 | .11477 | .00048 | 2 | K3.0V | CNS91 |
GJ 216 A | ⋅⋅⋅ | 05 44 27.8 | −22 26 54 | .11204 | .00018 | 2 | F6.0V | Mon01 |
GJ 216 B | ⋅⋅⋅ | 05 44 26.5 | −22 25 19 | .11204 | .00018 | 2 | K2.0V | CNS91 |
GJ 222 A | ⋅⋅⋅ | 05 54 23.0 | +20 16 34 | .11522 | .00025 | 3 | G0.0V | CNS91 |
GJ 250 A | 1875 | 06 52 18.1 | −05 10 25 | .11465 | .00043 | 2 | K3.0V | CNS91 |
GJ 423 A | 2390 | 11 18 10.9 | +31 31 45 | .11951 | .00079 | 2 | G0.0V | Bat89 |
GJ 423 B | 2391 | 11 18 11.0 | +31 31 46 | .11951 | .00079 | 2 | G5.0V | Bat89 |
GJ 432 A | 308 | 11 34 29.5 | −32 49 53 | .10461 | .00037 | 2 | K0.0V | CNS91 |
GJ 434 | ⋅⋅⋅ | 11 41 03.0 | +34 12 06 | .10416 | .00026 | 2 | G8.0V | CNS91 |
GJ 442 A | 311 | 11 46 31.1 | −40 30 01 | .10844 | .00022 | 2 | G2.0V | Gra06 |
GJ 451 | 44 | 11 52 58.8 | +37 43 07 | .11013 | .00040 | 2 | G8.0V | Joh53 |
GJ 475 | 2579 | 12 33 44.5 | +41 21 27 | .11848 | .00020 | 2 | G0.0V | CNS91 |
GJ 502 | 348 | 13 11 52.4 | +27 52 41 | .10952 | .00017 | 2 | G0.0V | CNS91 |
GJ 506 | 349 | 13 18 24.3 | −18 18 40 | .11690 | .00022 | 2 | G7.0V | Gra06 |
GJ 566 A | ⋅⋅⋅ | 14 51 23.4 | +19 06 02 | .14757 | .00072 | 2 | G8.0V | Ruc95 |
GJ 566 B | ⋅⋅⋅ | 14 51 23.4 | +19 06 02 | .14757 | .00072 | 2 | K4.0V | Mon99 |
GJ 570 A | 387 | 14 57 28.0 | −21 24 56 | .17062 | .00067 | 3 | K4.0V | Gra06 |
GJ 631 | 3224 | 16 36 21.4 | −02 19 29 | .10249 | .00040 | 2 | K2.0V | Mon01 |
GJ 638 | ⋅⋅⋅ | 16 45 06.4 | +33 30 33 | .10195 | .00070 | 2 | K7.0V | CNS91 |
GJ 663 A | 437 | 17 15 20.9 | −26 36 09 | .16812 | .00040 | 4 | K1.0V | CNS91 |
GJ 663 B | 438 | 17 15 21.0 | −26 36 10 | .16812 | .00040 | 4 | K1.0V | CNS91 |
GJ 664(C)* | 439 | 17 16 13.4 | −26 32 46 | .16812 | .00040 | 4 | K5.0V | CNS91 |
GJ 667 A | 442 | 17 18 57.2 | −34 59 23 | .13822 | .00070 | 2 | K3.0V | CNS91 |
GJ 667 B | 443 | 17 19 01.9 | −34 59 33 | .13822 | .00070 | 2 | K5.0V | CNS91 |
GJ 666 A | 444 | 17 19 03.8 | −46 38 10 | .11371 | .00069 | 2 | G8.0V | CNS91 |
GJ 673 | 447 | 17 25 45.2 | +02 06 41 | .12987 | .00071 | 2 | K7.0V | CNS91 |
GJ 695 A | 3326 | 17 46 27.5 | +27 43 14 | .12032 | .00016 | 2 | G5.0IV | CNS91 |
GJ 702 A | 458 | 18 05 27.4 | +02 29 59 | .19596 | .00087 | 2 | K0.0V | CNS91 |
GJ 702 B | 459 | 18 05 27.4 | +02 29 56 | .19596 | .00087 | 2 | K5.0V | CNS91 |
GJ 713 A | 3379 | 18 21 03.4 | +72 43 58 | .12343 | .00044 | 3 | F7.0V | CNS91 |
GJ 713 B | ⋅⋅⋅ | 18 21 03.4 | +72 43 58 | .12343 | .00044 | 3 | G8.0V | Far10 |
GJ 721 | ⋅⋅⋅ | 18 36 56.3 | +38 47 01 | .12985 | .00032 | 3 | A0.0V | CNS91 |
GJ 764 | 447 | 19 32 21.6 | +69 39 40 | .17379 | .00018 | 2 | K0.0V | CNS91 |
GJ 768 | 3490 | 19 50 47.0 | +08 52 06 | .19540 | .00046 | 3 | A7.0V | CNS91 |
GJ 780 | 485 | 20 08 43.6 | −66 10 55 | .16371 | .00017 | 2 | G8.0IV | Gra06 |
GJ 783 A | 486 | 20 11 11.9 | −36 06 04 | .16626 | .00027 | 2 | K2.5V | Gra06 |
GJ 785 | 488 | 20 15 17.4 | −27 01 59 | .11222 | .00030 | 2 | K2.0V | Gra06 |
GJ 827 | 3674 | 21 26 26.6 | −65 21 58 | .10797 | .00019 | 2 | F9.0V | Gra06 |
GJ 881(A) | ⋅⋅⋅ | 22 57 39.0 | −29 37 20 | .13042 | .00037 | 4 | A4.0V | Gra06 |
GJ 879(B)* | ⋅⋅⋅ | 22 56 24.1 | −31 33 56 | .13042 | .00037 | 4 | K5.0V | CNS91 |
GJ 884 | 3885 | 23 00 16.1 | −22 31 28 | .12175 | .00069 | 2 | K7.0V | Gra06 |
GJ 892 | 71 | 23 13 17.0 | +57 10 06 | .15284 | .00028 | 2 | K3.0V | CNS91 |
Notes. *Designates component to above system with different GJ number. References. Bat89: Batten et al. (1989); CNS91: Catalog of Nearby Stars, Gliese & Jahreiß (1991); Far10: Farrington et al. (2010); Gra06: Gray et al. (2006); Mon99: Montes et al. (1999); Mon01: Montes et al. (2001); Pas94: Pasquini et al. (1994); Ruc95: Ruck & Smith (1995); Joh53: Johnson & Morgan (1953).
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Table 4. Extended 10 pc Sample: Photometry
Name | U | Uref | B | Bref | V | Vref | R | Rref | I | Iref | J | err | H | err | K | err | Notes |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
GJ 17 | 4.82 | Joh66 | 4.80 | Bes90 | 4.22 | Bes90 | 3.89 | Bes90 | 3.57 | Bes90 | 3.17 | Gla75 | 2.87 | Gla75 | 2.78 | Gla75 | ⋅⋅⋅ |
GJ 19 | 3.53 | Joh66 | 3.42 | Bes90 | 2.80 | Bes90 | 2.45 | Bes90 | 2.12 | Bes90 | 1.72 | Gla75 | 1.40 | Gla75 | 1.32 | Gla75 | ⋅⋅⋅ |
GJ 33 | ⋅⋅⋅ | ⋅⋅⋅ | 6.60 | Bes90 | 5.72 | Bes90 | 5.21 | Bes90 | 4.76 | Bes90 | 4.24 | Joh68 | 3.72 | Joh68 | 3.48 | Joh68 | ⋅⋅⋅ |
GJ 34 A | 4.04 | Joh66 | 4.02 | Joh66 | 3.44 | Joh66 | 3.1 | USNOB | 2.8 | USNOB | 2.109 | 0.570 | 2.086 | 0.504 | 1.988 | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 53 AB | 5.96 | Joh66 | 5.87 | Joh66 | 5.18 | Joh66 | 4.7 | USNOB | 4.4 | USNOB | 3.86 | Joh68 | 3.39 | Joh68 | 3.36 | Joh68 | Joint |
GJ 66 A | ⋅⋅⋅ | ⋅⋅⋅ | 6.69 | Hog00 | 5.80 | Hog00 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 4.043 | 0.378 | ⋅⋅⋅ | ⋅⋅⋅ | 3.510 | 0.282 | ⋅⋅⋅ |
GJ 66 B | ⋅⋅⋅ | ⋅⋅⋅ | 6.84 | Mer86 | 5.96 | Mer86 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 3.573 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 3.558 | 0.270 | ⋅⋅⋅ |
GJ 68 | 6.57 | Joh66 | 6.08 | Joh66 | 5.24 | Joh66 | 4.7 | USNOB | 4.3 | USNOB | 3.855 | 0.24 | 3.391 | 0.226 | 3.285 | 0.266 | ⋅⋅⋅ |
GJ 105 AC | ⋅⋅⋅ | ⋅⋅⋅ | 6.78 | Bes90 | 5.81 | Bes90 | 5.24 | Bes90 | 4.74 | Bes90 | 4.07 | Gla75 | 3.52 | Gla75 | 3.45 | Gla75 | Joint |
GJ 139 | 5.19 | Joh66 | 4.97 | Bes90 | 4.26 | Bes90 | 3.85 | Bes90 | 3.47 | Bes90 | 2.95 | Gla75 | 2.59 | Gla75 | 2.52 | Gla75 | ⋅⋅⋅ |
GJ 150 | ⋅⋅⋅ | ⋅⋅⋅ | 4.45 | Bes90 | 3.53 | Bes90 | 3.03 | Bes90 | 2.59 | Bes90 | 1.99 | Gla75 | 1.53 | Gla75 | 1.45 | Gla75 | ⋅⋅⋅ |
GJ 178 | 3.64 | Joh66 | 3.65 | Bes90 | 3.19 | Bes90 | 2.92 | Bes90 | 2.67 | Bes90 | 2.35 | Gla75 | 2.15 | Gla75 | 2.07 | Gla75 | ⋅⋅⋅ |
GJ 183 | ⋅⋅⋅ | ⋅⋅⋅ | 7.29 | Bes90 | 6.23 | Bes90 | 5.44 | Bes90 | 4.69 | Bes90 | 4.389 | 0.244 | 3.797 | 0.214 | 3.706 | 0.228 | ⋅⋅⋅ |
GJ 216 A | 4.07 | Joh66 | 4.06 | Bes90 | 3.59 | Bes90 | 3.30 | Bes90 | 3.02 | Bes90 | 2.804 | 0.276 | 2.606 | 0.236 | 2.508 | 0.228 | ⋅⋅⋅ |
GJ 216 B | ⋅⋅⋅ | ⋅⋅⋅ | 7.12 | Bes90 | 6.18 | Bes90 | 5.63 | Bes90 | 5.17 | Bes90 | 4.845 | 0.198 | 4.158 | 0.202 | 4.131 | 0.264 | ⋅⋅⋅ |
GJ 222 AB | 5.04 | Joh66 | 5.00 | Joh66 | 4.41 | Joh66 | 4.0 | USNOB | 3.8 | USNOB | 3.34 | Joh68 | 3.04 | Joh68 | 2.97 | Joh68 | Joint |
GJ 250 A | ⋅⋅⋅ | ⋅⋅⋅ | 7.64 | Bes90 | 6.59 | Bes90 | 5.98 | Bes90 | 5.45 | Bes90 | 5.013 | 0.252 | 4.294 | 0.258 | 4.107 | 0.036 | ⋅⋅⋅ |
GJ 423 AC | ⋅⋅⋅ | ⋅⋅⋅ | 4.78 | Lep05 | 4.27 | Lep05 | 3.9 | USNOB | 3.7 | USNOB | 2.462 | 0.294 | 2.231 | 0.204 | 2.142 | 0.230 | Joint |
GJ 423 BD | ⋅⋅⋅ | ⋅⋅⋅ | 5.36 | Lep05 | 4.74 | Lep05 | 4.4 | USNOB | 4.0 | USNOB | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Joint |
GJ 432 A | ⋅⋅⋅ | ⋅⋅⋅ | 6.78 | Bes90 | 5.97 | Bes90 | 5.52 | Bes90 | 5.10 | Bes90 | 4.784 | 0.228 | 4.138 | 0.214 | 4.022 | 0.036 | ⋅⋅⋅ |
GJ 434 | 6.28 | Joh66 | 6.05 | Bes90 | 5.33 | Bes90 | 4.9 | USNOB | 4.5 | USNOB | 3.99 | Joh68 | 3.61 | Joh68 | 3.60 | Joh68 | ⋅⋅⋅ |
GJ 442 A | ⋅⋅⋅ | ⋅⋅⋅ | 5.56 | Bes90 | 4.90 | Bes90 | 4.5 | USNOB | 4.2 | USNOB | 3.931 | 0.276 | 3.490 | 0.238 | 3.489 | 0.278 | ⋅⋅⋅ |
GJ 451 | 7.37 | Joh66 | 7.20 | Joh66 | 6.45 | Joh66 | 6.0 | USNOB | 5.6 | USNOB | 4.89 | Gla75 | 4.43 | Gla75 | 4.37 | Gla75 | ⋅⋅⋅ |
GJ 475 | 4.91 | Joh66 | 4.86 | Joh66 | 4.27 | Joh66 | 3.9 | USNOB | 3.6 | USNOB | 3.23 | Joh66 | 2.905 | 0.198 | 2.84 | Joh66 | ⋅⋅⋅ |
GJ 502 | 4.92 | Joh66 | 4.84 | Joh66 | 4.26 | Joh66 | 3.9 | USNOB | 3.6 | USNOB | 3.24 | Gla75 | 2.90 | Gla75 | 2.87 | Gla75 | ⋅⋅⋅ |
GJ 506 | 5.69 | Joh66 | 5.43 | Bes90 | 4.72 | Bes90 | 4.33 | Bes90 | 3.97 | Bes90 | 3.334 | 0.200 | 2.974 | 0.176 | 2.956 | 0.236 | ⋅⋅⋅ |
GJ 566 A | 5.68 | Lut71 | 5.44 | Lut71 | 4.72 | Lut71 | 4.52 | Bre64 | 4.24 | Bre64 | 2.660 | 0.448 | 2.253 | 0.698 | 1.971 | 0.600 | Joint JHK |
GJ 566 B | 9.29 | Lut71 | 8.14 | Lut71 | 6.97 | Lut71 | 6.30 | Bre64 | 5.86 | Bre64 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 570 A | 7.88 | Joh66 | 6.82 | Joh66 | 5.71 | Joh66 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 3.663 | 0.258 | 3.085 | 0.196 | 3.048 | 0.224 | ⋅⋅⋅ |
GJ 631 | ⋅⋅⋅ | ⋅⋅⋅ | 6.57 | Bes90 | 5.76 | Bes90 | 5.31 | Bes90 | 4.9 | Bes90 | 4.33 | Joh66 | 4.053 | 0.208 | 3.87 | Joh66 | ⋅⋅⋅ |
GJ 638 | ⋅⋅⋅ | ⋅⋅⋅ | 9.48 | Joh53 | 8.11 | Joh65 | 7.3 | USNOB | 6.6 | USNOB | 5.48 | 0.023 | 4.878 | 0.018 | 4.712 | 0.021 | ⋅⋅⋅ |
GJ 663 AB | ⋅⋅⋅ | ⋅⋅⋅ | 5.93 | Tor06 | 5.08 | Tor06 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Joint |
GJ 664(C)* | ⋅⋅⋅ | ⋅⋅⋅ | 7.48 | Bes90 | 6.32 | Bes90 | 5.62 | Bes90 | 5.04 | Bes90 | 4.155 | 0.25 | ⋅⋅⋅ | ⋅⋅⋅ | 3.466 | 0.256 | ⋅⋅⋅ |
GJ 667 AB | 7.77 | Joh66 | 6.95 | Joh66 | 5.91 | Joh66 | 4.97 | Joh66 | 4.38 | Joh66 | 3.903 | 0.262 | 3.230 | 0.206 | 3.123 | 0.278 | Joint |
GJ 666 A | ⋅⋅⋅ | ⋅⋅⋅ | 6.35 | Bes90 | 5.47 | Bes90 | 5.00 | Bes90 | 4.54 | Bes90 | 4.077 | 0.996 | 3.146 | 0.664 | 3.421 | 0.282 | ⋅⋅⋅ |
GJ 673 | ⋅⋅⋅ | ⋅⋅⋅ | 8.89 | Bes90 | 7.53 | Bes90 | 6.69 | Bes90 | 5.94 | Bes90 | 4.934 | 0.024 | 4.341 | 0.044 | 4.14 | Gla75 | ⋅⋅⋅ |
GJ 695 AD | 4.56 | Joh66 | 4.17 | Joh66 | 3.42 | Joh66 | 2.9 | USNOB | 2.6 | USNOB | 2.13 | Joh66 | 1.559 | 0.184 | 1.77 | Joh66 | Joint |
GJ 702 A | 5.31 | Egg65 | 4.98 | Egg65 | 4.20 | Egg65 | 3.87 | Bre64 | 3.61 | Bre64 | 2.343 | 0.296 | 1.876 | 0.244 | 1.791 | 0.304 | Joint JHK |
GJ 702 B | ⋅⋅⋅ | ⋅⋅⋅ | 7.15 | Egg65 | 6.00 | Egg65 | 5.26 | Bre64 | 4.82 | Bre64 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 713 AB | 4.01 | Joh66 | 4.07 | Joh66 | 3.58 | Joh66 | 3.3 | USNOB | 3.0 | USNOB | 2.588 | 0.260 | 2.372 | 0.188 | 2.216 | 0.252 | Joint |
GJ 721 | 0.02 | Joh66 | 0.02 | Bes90 | 0.03 | Bes90 | 0.04 | Bes90 | 0.04 | Bes90 | 0.02 | Joh66 | −0.029 | 0.146 | 0.02 | Joh66 | ⋅⋅⋅ |
GJ 764 | 5.86 | Oja84 | 5.46 | Oja93 | 4.68 | Oja93 | 4.2 | USNOB | 3.8 | USNOB | 3.32 | Joh66 | 3.039 | 0.214 | 2.78 | Joh66 | ⋅⋅⋅ |
GJ 768 | 1.07 | Joh66 | 0.99 | Bes90 | 0.77 | Bes90 | 0.64 | Bes90 | 0.50 | Bes90 | 0.39 | Joh66 | 0.102 | 0.220 | 0.26 | Joh66 | ⋅⋅⋅ |
GJ 780 | 4.76 | Joh66 | 4.31 | Bes90 | 3.55 | Bes90 | 3.14 | Bes90 | 2.79 | Bes90 | 2.35 | Gla75 | 2.03 | Gla75 | 1.93 | Gla75 | ⋅⋅⋅ |
GJ 783 A | ⋅⋅⋅ | ⋅⋅⋅ | 6.19 | Joh66 | 5.32 | Joh66 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 3.518 | 0.300 | 2.999 | 0.422 | 3.008 | 0.602 | ⋅⋅⋅ |
GJ 785 | ⋅⋅⋅ | ⋅⋅⋅ | 6.61 | Bes90 | 5.73 | Bes90 | 5.23 | Bes90 | 4.81 | Bes90 | 4.112 | 0.294 | 3.582 | 0.266 | 3.501 | 0.232 | ⋅⋅⋅ |
GJ 827 | 4.58 | Joh66 | 4.71 | Bes90 | 4.22 | Bes90 | 3.92 | Bes90 | 3.61 | Bes90 | 3.27 | Gla75 | 3.00 | Gla75 | 2.90 | Gla75 | ⋅⋅⋅ |
GJ 881(A) | 1.30 | Joh66 | 1.24 | Bes90 | 1.15 | Bes90 | 1.10 | Bes90 | 1.07 | Bes90 | 1.02 | Gla75 | 1.03 | Gla75 | 0.99 | Gla75 | ⋅⋅⋅ |
GJ 879(B)* | ⋅⋅⋅ | ⋅⋅⋅ | 7.56 | Bes90 | 6.46 | Bes90 | 5.8 | Bes90 | 5.78 | Bes90 | 4.533 | 0.037 | 3.804 | 0.210 | 3.805 | 0.240 | ⋅⋅⋅ |
GJ 884 | ⋅⋅⋅ | ⋅⋅⋅ | 9.25 | Bes90 | 7.86 | Bes90 | 7.00 | Bes90 | 6.23 | Bes90 | 5.346 | 0.021 | 4.696 | 0.076 | 4.478 | 0.016 | ⋅⋅⋅ |
GJ 892 | 7.46 | Oja84 | 6.56 | Oja93 | 5.57 | Oja93 | 4.9 | USNOB | 4.5 | USNOB | 3.80 | Gla75 | 3.27 | Gla75 | 3.18 | Gla75 | ⋅⋅⋅ |
Notes. JHK and err from 2MASS unless otherwise noted. "Joint" indicates unresolved photometry. *Designates component to above system with different GJ number. References. Bes90: Bessel (1990); Bre64: Breckinridge & Kron (1964); Egg65: Eggen (1965); Gla75: Glass (1975); Hog00: Høg et al. (2000); Joh53: Johnson & Morgan (1953); Joh66: Johnson et al. (1966); Joh68: Johnson et al. (1968); Lep05: Lépine & Shara (2005); Lut71: Lutz (1971); Mer86: Mermilliod (1986); Oja84: Oja (1984); Oja93: Oja (1993); Tor06: Torres et al. (2006); USNOB: Monet et al. (2003).
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3.3. Photometry and Energy Distributions
The UBVRI photometry used in this paper, listed in Tables 2 and 4, was extracted from sources in the literature with preference given to large surveys and measurements consistent with photometry obtained as part of the CTIO Parallax (CTIOPI) program, which uses the Johnson–Kron–Cousins system. R and I magnitudes from the USNO-B1.0 catalog (Monet et al. 2003) are incorporated in the extended 10 pc sample and both are rounded to the nearest 0.1 mag. R magnitudes are averaged from the first and second epochs.
The majority of JHK photometry is taken from the Two Micron All Sky Survey (2MASS) database, identified as those values with errors listed explicitly in Tables 2 and 4 (Cutri et al. 2003). In cases of close binaries with magnitude difference measurements in the literature (e.g., Henry & McCarthy 1993; Henry et al. 1999), the optical and 2MASS photometry is used with the published magnitude differences to split the component fluxes into individual magnitudes. Note that stars brighter than ∼5 mag are saturated in 2MASS images and typically have relatively large photometric errors (⩾0.2 mag). Where possible, we use JHK measurements in the literature for these stars (Johnson et al. 1966, 1968; Glass 1975; Mould & Hyland 1976). These magnitudes, listed in their unconverted form in Tables 2 and 4 and with specific references listed in the error columns, were then converted to 2MASS magnitudes using color transformations from Carpenter (2001). Of the 16 multiple systems in the 5 pc sample, 8 have spatially unresolved photometry in some or all passbands and are marked as "Joint" in the Notes section of Table 2. Similarly, 10 of the 19 multiple systems in the extended 10 pc sample have spatially unresolved photometry in some or all passbands and are listed as "Joint" in the Notes section of Table 4.
4. METHODOLOGY TO DERIVE THE EHZ
The HZ around a star is primarily a function of the total energy output of a star that reaches the surface of a planet. This can be determined if the stellar temperature and radius are known, which we calculate for our sample of stars as described in Section 4.1. However, a range of other factors, such as atmospheric pressure, composition, and cloud cover play roles in determining the surface temperature of a planet. To account for this, in Section 4.2 we follow previous studies and approximate these effects by using empirical temperature constraints provided by planets in the solar system.
4.1. SED Fitting Used to Derive Stellar Temperature and Radius
Although spectral types are often used to estimate stellar effective temperatures, the various methods for determining spectral types are inhomogeneous and often depend upon the spectral range used. As an alternative, we fit synthetic stellar spectra to broadband energy distributions to determine effective temperatures and radii. In particular, we use photometric measurements spanning UBVRIJHK (0.3 to 2.4 μm) in conjunction with model spectra generated using the PHOENIX code (Hauschildt et al. 1999). This prescription is only used for single stars and for stars in multiple systems in the 5 pc and extended 10 pc samples with at least four spatially resolved photometric measurements in different filter bandpasses. Estimates for unresolved multiples are discussed in Section 4.3.
These available models span the temperature ranges of Teff of 10,000 K–7000 K in 200 K increments and 6900 K–2000 K in 100 K increments. The model spectra have a resolution of 2 Å and range from 10 Å to 500 μm. The models were convolved with UBVRIJHK filter responses to create synthetic photometry. For the UBVRI synthetic photometry, we took zero points from Bessell et al. (1998) and filter responses from CTIO.5JHK filter responses and zero points were from 2MASS.6 The zero points adopted are listed in Table 5. We adopt log g values of 4.5 or 5.0 for all stars.
Table 5. UBVRIJHK Zero Points
Filter | Zero Point | λeff | Ref. |
---|---|---|---|
(Å) | |||
U | 4.18e-9 | 3660 | Bessell et al. (1998) |
B | 6.32e-9 | 4380 | Bessell et al. (1998) |
V | 3.63e-9 | 5450 | Bessell et al. (1998) |
R | 2.18e-9 | 6410 | Bessell et al. (1998) |
I | 1.13e-9 | 7980 | Bessell et al. (1998) |
J | 3.14e-10 | 12350 | Cohen et al. (2003) |
H | 1.11e-10 | 16620 | Cohen et al. (2003) |
K | 4.29e-11 | 21590 | Cohen et al. (2003) |
Note. Zero points have units of erg cm2 s−1 Å−1.
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Solar metallicity is adopted for all stars except GJ 191 (Fe/H = −0.98; Woolf & Wallerstein 2005) and GJ 451 (Fe/H = −1.16; Valenti & Fischer 2005), for which we adopt metallicities of −1.0. The assumption of solar metallicity for the remainder of our 10 pc sample is based on the 36 FGK stars that overlap with Valenti & Fischer (2005). These stars have an average metallicity of Fe/H = −0.048 with a standard deviation of 0.168 dex.
The flux values in the model spectra are given as a surface flux that must be scaled by a radius to a known distance for fitting with observed integrated flux values. A stellar radius grid from 0.001 R☉ to 3.00 R☉ with a step size of 0.001 R☉ is calculated for each model spectrum. Each spectrum and radius combination is then convolved with filter response and the zero point data referenced above to derive consequent fluxes observed at Earth. These integrated fluxes are then fit via a χ2 minimization routine, written in IDL and described in Equation (3), to compare the model flux with the observed flux across each filter bandpass. Here, O is the observed integrated flux from the photometric measurements, E is the estimated integrated flux from the model grids, ν is the number of photometric points (degrees of freedom), and σ is the average error in the photometric measurement. Examples of fits for AFGKM stars in the 5 pc sample are shown in Figure 1:
The output is an effective temperature from the model and a radius that best fits the measured photometric data. We assume no interstellar extinction for our sample, and choose four photometric points as the minimum number needed to make a fit. When deriving radii, all stars in the sample are assumed to be spherical and radiate isotropically, which is not the case for rapidly rotating stars that may be oblate and experience gravity darkening (e.g., Altair, see Monnier et al. 2007; Vega, see Aufdenberg et al. 2006). This is most common among stars earlier than mid-F spectral type, as these stars are fully radiative and consequently do not possess an efficient rotational braking mechanism (e.g., Wilson 1966). However, all but one of the early-type stars (i.e., earlier than spectral type F5) within 10 pc have projected rotational velocities less than 100 km s−1, which correspond to projected oblateness values ≲2% (Absil et al. 2008). These apparently slow rotating stars include Sirius A (GJ 244, A1V, 16 km s−1; Royer et al. 2002), Procyon A (GJ 280, F5V-IV, 4.9 km s−1; Fekel 1997), Vega (GJ 721, A0V, 24 km s−1; Royer et al. 2007), and Fomalhaut (GJ 881, A4V, 93 km s−1; Royer et al. 2007). We note that despite having a small vsin i value, Vega is believed to be rapidly rotating with a nearly pole on orientation (Aufdenberg et al. 2006). An additional rapidly rotating star is Altair (GJ 768, A7V, 217 km s−1; Royer et al. 2007) which has an oblateness of 18% determined from interferometric measurements by the CHARA Array (Monnier et al. 2007). As expected, our derived radii and effective temperatures for Altair and Vega are intermediate to the polar and equatorial values listed in Monnier et al. (2007) and Aufdenberg et al. (2006), respectively. Because the EHZ boundaries are a function of the square root of the total luminosity (see Section 4.2), our adopted methodology should yield reliable estimates of the habitable real estate these stars provide, even if the stellar temperatures vary with stellar latitude.
As a consequence of the limited temperature resolution of the PHOENIX grids used (ΔT = 200 K from 10,000 K to 7000 K and ΔT = 100 K from 6900 K to 2000 K), the fitting routine can determine a slightly larger radius coupled with a cooler temperature, or vice versa. Systematic uncertainties in the PHOENIX models are such that a ΔT of less than 100 K is unreliable (P. H. Hauschildt 2009, private communication). As the Stefan–Boltzmann law shows R⋆2Teff4 ∼ L⋆, the luminosity determined from the spectral energy distribution (SED) remains the same as long as the radii and effective temperatures move in opposite directions, and therefore the EHZ, being a function of the square root of the luminosity, does not change. Of key importance, the output radii and temperatures determined here allow us to compare our results directly to those found via interferometric techniques.
Of the stars investigated, 28 have spatially resolved angular diameters from long baseline interferometric instruments such as CHARA, the Palomar Testbed Interferometer, and VLTI (Table 6). Fifteen of these measurements are from the recent effort of Boyajian et al. (2012). Because these measurements determine sizes to within a few percent, we use them to test the accuracy of the radii determined in our fitting process. The published radii are on average 7.4% larger than our derived radii, which shows a systematic effect. Our derived model Teff's are on average 2.4% hotter than derived from interferometric measurements. A comparison of our model versus published values is shown in Figure 2. The temperature uncertainties correspond to spectral type uncertainties of 1–2 spectral subclasses, similar to the error associated with most spectral classification methods. As the average values indicate, the overprediction of temperature leads to the expected under-prediction of radius, and the combination yields a more accurate luminosity, and thus consistent EHZ. Given this agreement, we adopt our model values for Teff and R/R☉ in final calculations of the habitable real estate.
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Standard image High-resolution imageTable 6. Comparison of Derived Temperatures and Radii for nearby Stars Using Our SED Fits and Interferometric Results
Star | SpType | V − K | TeffModel | TeffModelFixR | TeffPublished | R/R☉Model | R/R☉Pub | Ref. |
---|---|---|---|---|---|---|---|---|
GJ 244 A | A1.0V | −0.040 | 10000 | 9600 | 9900 ± 200 | 1.645 | 1.711 ± 0.013 | Kervella et al. (2003a) |
GJ 280 A | F5.0IV–V | 1.028 | 6700 | 6500 | 6524 | 1.898 | 2.048 ± 0.025 | Kervella et al. (2003b) |
GJ 559 A | G2.0V | 1.490 | 6000 | 5700 | 5810 ± 50 | 1.103 | 1.224 ± 0.003 | Kervella et al. (2003b) |
GJ 71 | G8.5V | 1.696 | 5700 | 5400 | 5264 ± 100 | 0.708 | 0.816 ± 0.013 | Di Folco et al. (2004) |
GJ 631 | K2.0V | 1.890 | 5500 | 5300 | 5337 ± 41 | 0.710 | 0.759 ± 0.012 | Boyajian et al. (2012) |
GJ 166 A | K0.5V | 1.932 | 5200 | 5100 | 5143 ± 14 | 0.735 | 0.806 ± 0.004 | Boyajian et al. (2012) |
GJ 559 B | K0.0V | 1.940 | 5400 | 5100 | 5260 ± 50 | 0.775 | 0.863 ± 0.005 | Kervella et al. (2003b) |
GJ 144 | K2.0V | 1.954 | 5200 | 5000 | 5135 ± 100 | 0.689 | 0.743 ± 0.005 | Di Folco et al. (2004) |
GJ 33 | K2.5V | 2.240 | 5200 | 5000 | 4950 ± 14 | 0.643 | 0.695 ± 0.004 | Boyajian et al. (2012) |
GJ 105 A | K3.0V | 2.360 | 5000 | 4700 | 4662 ± 17 | 0.677 | 0.795 ± 0.006 | Boyajian et al. (2012) |
GJ 892 | K3.0V | 2.390 | 5000 | 4800 | 4699 ± 16 | 0.698 | 0.778 ± 0.005 | Boyajian et al. (2012) |
GJ 702 A | K0.0V | 2.409 | 5500 | 5700 | 5407 ± 52 | 0.754 | 0.831 ± 0.004 | Boyajian et al. (2012) |
GJ 845 A | K5.0V | 2.443 | 4900 | 4600 | 5468 ± 59 | 0.608 | 0.732 ± 0.007 | Demory et al. (2009) |
GJ 570 A | K4.0V | 2.662 | 4700 | 4700 | 4507 ± 58 | 0.734 | 0.739 ± 0.019 | Boyajian et al. (2012) |
GJ 820 B | K7.0V | 3.486 | 4300 | 4200 | 4040 ± 80 | 0.530 | 0.595 ± 0.008 | Kervella et al. (2008) |
GJ 380 | K7.0V | 3.628 | 4200 | 4200 | 4081 ± 15 | 0.599 | 0.642 ± 0.005 | Boyajian et al. (2012) |
GJ 820 A | K5.0V | 3.782 | 4000 | 4100 | 4400 ± 100 | 0.709 | 0.665 ± 0.005 | Kervella et al. (2008) |
GJ 702 B* | K5.0V | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 4393 ± 149 | ⋅⋅⋅ | 0.670 ± 0.009 | Boyajian et al. (2012) |
GJ 191 | M2.0VI | 3.801 | 3800 | 3600 | 3570 ± 156 | 0.249 | 0.291 ± 0.025 | Ségransan et al. (2003) |
GJ 887 | M2.0V | 3.875 | 3800 | 3700 | 3797 ± 45 | 0.414 | 0.459 ± 0.011 | Demory et al. (2009) |
GJ 411 | M2.0V | 3.969 | 3700 | 3500 | 3465 ± 17 | 0.338 | 0.392 ± 0.004 | Boyajian et al. (2012) |
GJ 412 A | M1.0V | 4.001 | 3700 | 3600 | 3497 ± 39 | 0.353 | 0.398 ± 0.009 | Boyajian et al. (2012) |
GJ 15 A | M1.5V | 4.062 | 3800 | 3700 | 3730 ± 49 | 0.323 | 0.379 ± 0.006 | Berger et al. (2006) |
GJ 725 A | M3.0V | 4.468 | 3400 | 3400 | 3407 ± 15 | 0.344 | 0.356 ± 0.004 | Boyajian et al. (2012) |
GJ 687 | M3.0V | 4.622 | 3400 | 3200 | 3413 ± 28 | 0.406 | 0.418 ± 0.007 | Boyajian et al. (2012) |
GJ 725 B | M3.5V | 4.690 | 3300 | 3200 | 3104 ± 28 | 0.275 | 0.323 ± 0.006 | Boyajian et al. (2012) |
GJ 699 | M4.0V | 5.046 | 3100 | 3100 | 3224 ± 10 | 0.198 | 0.187 ± 0.001 | Boyajian et al. (2012) |
GJ 551 | M5.0V | 6.666 | 2700 | 2900 | 3098 ± 56 | 0.167 | 0.141 ± 0.007 | Demory et al. (2009) |
Notes. TeffModel refers to temperatures from our model, while, TeffModelFixR refers to temperatures derived while holding the radius to values obtained through long baseline interferometry. *GJ 702B does not have enough resolved photometry for a model solution.
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A few stars are worthy of note. Procyon is slightly evolved (F5.0IV-V) and has a highly constrained log g of 4.05 ± 0.04 (Fuhrmann et al. 1997) measured via high-resolution spectroscopy in concert with masses determined using astrometry from the Procyon–white-dwarf orbit. We adjusted the log g to 4.0 for this star and recomputed the Teff and R. This, nevertheless, yielded values within the model grid resolution identical to those of log g = 4.5. The model spectra used in this work have a low temperature limit of 2000 K, which is the value derived for the three intrinsically faintest stars in this sample: SCR 1845−6357A (M8.5V), DEN 1048−3956 (M8.5V), and LP 944−020 (M9.0V).
4.2. Habitable Zones of Single Stars
Kasting et al. (1993) use a one-dimensional climate model to calculate HZs around single main sequence stars. A one-dimensional climate model characterizes a planet's global temperature by dividing the planet into latitudinal bands, and treats the planet as uniform with respect to longitude. These one-dimensional radiative-convective models are a good approximation of global temperature, but more complicated three-dimensional global climate models are needed to account for the complex physical interactions associated with oceans, clouds, and land surface processes. These inputs add parameters such as land/ocean surface coverage and clouds, which can vary from planet to planet, complicating the overall goal to characterize the EHZ of a star. Here we use the one-dimensional model to generalize the EHZ, and adopt a modified version of the one-dimensional model based on the "Venus and early Mars criterion" from Selsis et al. (2007). In that work, they argue that empirical evidence shows that Venus has not had water on its surface for at least 1 billion years, and Mars had water on its surface around 4 billion years ago. The solar fluxes at those times were 8% and 28% lower, respectively (Baraffe et al. 1998). Venus (0.72 AU today) and Mars (1.52 AU today) would need to be at distances of ∼0.75 AU and ∼1.77 AU, respectively, to receive these levels of solar flux today.
Selsis et al. (2007) provide a method for estimating the inner and outer edges of HZs for stars with Teff = 3700 K–7200 K. The stars in the sample discussed here range in temperatures from 2000 K to 10,000 K. We have therefore chosen to derive new relations for the HZ boundaries that span the entire stellar temperature regime of our sample, and thereby provide a consistent methodology for all stars in the sample.
In defining our EHZ inner and outer boundaries, we assume a planet with an atmosphere, radius, mass, and Bond albedo (0.3) that matches Earth's (Kasting 1996). This leads to the EHZ Equations (4) and (5), used to determine the empirical surface temperature of an Earth-like planet that satisfies the "Venus criterion" (we adopt 0.80 AU on the suggestion of J. Kasting 2010, private communication) and "Mars criterion" (1.77 AU). The resulting inner and outer radii of the EHZ correspond to equilibrium temperatures of 285 K to 195 K, respectively.
Note that these only depend on R⋆ and , or essentially the square root of the star's luminosity. Values for both samples are listed in Tables 7 and 8. Only stars used in the cumulative EHZ calculations are listed, except for the cases of Sirius (GJ 244A) and Procyon (GJ 280A), which provide useful benchmarks.
Table 7. Empirical Habitable Zones (EHZs) for the 5 pc Sample
Model | Model FixR | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Name | SpType | R | Teff | EHZ Inner R | EHZ Outer R | EHZ Lin | R | Teff | EHZ Inner R | EHZ Outer R | EHZ Lin |
(R/R☉) | (K) | (AU) | (AU) | (AU) | (R/R☉) | (K) | (AU) | (AU) | (AU) | ||
Sun | G2.0V | 0.917 | 6000 | 0.789 | 1.686 | 0.897 | 1.00 | 5800 | 0.804 | 1.718 | 0.914 |
DEN 1048−3956 | M8.5V | 0.134 | 2000 | 0.013 | 0.027 | 0.014 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 1 | M1.5V | 0.349 | 3700 | 0.114 | 0.244 | 0.130 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 15 A | M1.5V | 0.323 | 3800 | 0.112 | 0.239 | 0.127 | 0.387 | 3700 | 0.127 | 0.271 | 0.144 |
GJ 15 B | M3.5V | 0.197 | 3200 | 0.048 | 0.103 | 0.055 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 54.1 | M4.5V | 0.181 | 2900 | 0.036 | 0.078 | 0.042 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 65 A | M5.5V | 0.248 | 2600 | 0.040 | 0.086 | 0.046 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 65 B | M6.0V | 0.225 | 2700 | 0.039 | 0.084 | 0.045 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 71 | G8.0V | 0.700 | 5700 | 0.545 | 1.164 | 0.619 | 0.816 | 5400 | 0.570 | 1.218 | 0.648 |
GJ 83.1 | M4.5V | 0.195 | 2900 | 0.039 | 0.084 | 0.045 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 144 | K2.0V | 0.630 | 5400 | 0.440 | 0.940 | 0.500 | 0.735 | 5000 | 0.440 | 0.941 | 0.500 |
GJ 166 A | K0.5V | 0.735 | 5200 | 0.476 | 1.017 | 0.541 | 0.806 | 5000 | 0.483 | 1.031 | 0.549 |
GJ 166 C | M4.5V | 0.249 | 3200 | 0.061 | 0.130 | 0.069 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 191 | M1.5VI | 0.240 | 3800 | 0.083 | 0.177 | 0.094 | 0.291 | 3500 | 0.085 | 0.182 | 0.097 |
GJ 234 A | M4.0V | 0.287 | 3000 | 0.062 | 0.132 | 0.070 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 234 B | M5.5V | 0.155 | 2700 | 0.027 | 0.058 | 0.031 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 244 A* | A1.0V | 1.645 | 10000 | 3.942 | 8.420 | 4.478 | 1.711 | 9600 | 3.779 | 8.071 | 4.293 |
GJ 273 | M3.5V | 0.316 | 3200 | 0.077 | 0.165 | 0.088 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 280 A* | F5.0IV–V | 1.906 | 6700 | 2.050 | 4.380 | 2.329 | 2.048 | 6500 | 2.073 | 4.429 | 2.356 |
GJ 380 | K7.0V | 0.648 | 4100 | 0.261 | 0.558 | 0.297 | 0.642 | 4100 | 0.259 | 0.552 | 0.294 |
GJ 388 | M3.0V | 0.452 | 3300 | 0.118 | 0.251 | 0.133 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 406 | M6.0V | 0.162 | 2500 | 0.024 | 0.052 | 0.028 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 411 | M2.0V | 0.338 | 3700 | 0.111 | 0.237 | 0.126 | 0.392 | 3500 | 0.115 | 0.246 | 0.130 |
GJ 412 A | M1.0V | 0.353 | 3700 | 0.116 | 0.247 | 0.131 | 0.398 | 3600 | 0.124 | 0.264 | 0.140 |
GJ 412 B | M5.5V | 0.152 | 2600 | 0.025 | 0.053 | 0.028 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 447 | M4.0V | 0.213 | 3000 | 0.046 | 0.098 | 0.052 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 473 A | M5.0V | 0.212 | 2700 | 0.037 | 0.079 | 0.042 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 473 B | M7.0V | 0.177 | 2900 | 0.036 | 0.076 | 0.040 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 551 | M5.5V | 0.167 | 2700 | 0.029 | 0.062 | 0.033 | 0.141 | 2900 | 0.028 | 0.061 | 0.032 |
GJ 559 A | G2.0V | 0.884 | 6700 | 0.951 | 2.031 | 1.080 | 1.224 | 5700 | 0.953 | 2.036 | 1.083 |
GJ 559 B | K0.0V | 0.774 | 5400 | 0.541 | 1.155 | 0.614 | 0.863 | 5200 | 0.559 | 1.194 | 0.635 |
GJ 628 | M3.0V | 0.321 | 3200 | 0.079 | 0.168 | 0.089 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 674 | M2.5V | 0.355 | 3400 | 0.098 | 0.210 | 0.112 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 687 | M3.0V | 0.406 | 3400 | 0.112 | 0.240 | 0.128 | 0.418 | 3400 | 0.116 | 0.247 | 0.132 |
GJ 699 | M4.0V | 0.198 | 3100 | 0.046 | 0.097 | 0.051 | 0.187 | 3100 | 0.043 | 0.092 | 0.048 |
GJ 725 A | M3.0V | 0.344 | 3400 | 0.095 | 0.204 | 0.109 | 0.356 | 3400 | 0.099 | 0.211 | 0.112 |
GJ 725 B | M3.5V | 0.275 | 3300 | 0.072 | 0.153 | 0.081 | 0.323 | 3200 | 0.079 | 0.169 | 0.090 |
GJ 729 | M3.5V | 0.214 | 3100 | 0.049 | 0.105 | 0.056 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 820 A | K5.0V | 0.709 | 4000 | 0.272 | 0.581 | 0.309 | 0.665 | 4100 | 0.268 | 0.572 | 0.304 |
GJ 820 B | K7.0V | 0.530 | 4300 | 0.235 | 0.502 | 0.267 | 0.595 | 4200 | 0.252 | 0.537 | 0.286 |
GJ 825 | M0.0V | 0.516 | 4100 | 0.208 | 0.444 | 0.236 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 832 | M1.5V | 0.423 | 3600 | 0.131 | 0.280 | 0.149 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 845 A | K5.0V | 0.608 | 4900 | 0.350 | 0.747 | 0.397 | 0.732 | 4600 | 0.371 | 0.793 | 0.422 |
GJ 860 A | M3.0V | 0.328 | 3300 | 0.085 | 0.183 | 0.098 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 860 B | M4.0V | 0.194 | 3200 | 0.048 | 0.102 | 0.054 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 866 B | MV | 0.215 | 2700 | 0.037 | 0.080 | 0.043 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 876 A | M4.0V | 0.390 | 3100 | 0.090 | 0.191 | 0.101 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 887 | M1.5V | 0.414 | 3800 | 0.143 | 0.306 | 0.163 | 0.491 | 3700 | 0.161 | 0.344 | 0.183 |
GJ 905 | M5.5V | 0.216 | 2700 | 0.038 | 0.080 | 0.042 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 1002 | M5.0V | 0.142 | 2900 | 0.029 | 0.061 | 0.032 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 1061 | M5.0V | 0.178 | 2700 | 0.031 | 0.066 | 0.035 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 1111 | M6.0V | 0.162 | 2400 | 0.022 | 0.048 | 0.026 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 1245 A | M5.5V | 0.194 | 2700 | 0.034 | 0.072 | 0.038 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 1245 B | M6.0V | 0.150 | 2700 | 0.026 | 0.056 | 0.030 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 1245 C | M7.0V | 0.171 | 2100 | 0.018 | 0.039 | 0.021 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
LHS 288 | M5.5V | 0.150 | 2700 | 0.026 | 0.056 | 0.030 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
LHS 292 | M6.5V | 0.146 | 2400 | 0.020 | 0.043 | 0.023 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
LP 944−020 | M9.0V | 0.091 | 2000 | 0.009 | 0.019 | 0.010 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
SCR 1845−6357 A | M8.5V | 0.129 | 2000 | 0.012 | 0.026 | 0.014 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
SO 0253+1652 | M7.0V | 0.156 | 2400 | 0.022 | 0.046 | 0.024 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
Notes. This table shows model radius, temperature, inner and outer HZ radius, and the HZ width in AU in Columns 3–7. Columns 8–12 show radius, temperature, inner and outer HZ radius, and the HZ width in AU based on the method of holding the stellar radius to the values obtained through long baseline interferometry (see Table 6). *Sirius (GJ 244A) and Procyon (GJ 280A) are included as benchmarks, but are not used in final EHZ calculations because of companion white dwarfs.
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Table 8. Empirical Habitable Zones (EHZs) for the Extended 10 pc Sample
Model | Model FixR | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Name | SpType | R | Teff | EHZ Inner R | EHZ Outer R | EHZ Lin | R | Teff | EHZ Inner R | EHZ Outer R | EHZ Lin |
(R/R☉) | (K) | (AU) | (AU) | (AU) | (R/R☉) | (K) | (AU) | (AU) | (AU) | ||
GJ 17 | F9.5V | 0.989 | 6100 | 0.882 | 1.884 | 1.002 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 19 | G0.0V | 1.709 | 6000 | 1.474 | 3.149 | 1.675 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 33 | K2.5V | 0.643 | 5200 | 0.417 | 0.890 | 0.473 | 0.695 | 5000 | 0.416 | 0.889 | 0.473 |
GJ 34 A | G3.0V | 0.989 | 6100 | 0.882 | 1.884 | 1.002 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 66 A | K5.0V | 0.740 | 5100 | 0.461 | 0.985 | 0.524 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 66 B | K5.0V | 0.794 | 4900 | 0.457 | 0.976 | 0.519 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 68 | K1.0V | 0.739 | 5400 | 0.516 | 1.103 | 0.587 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 105 A | K3.0V | 0.677 | 5000 | 0.406 | 0.866 | 0.460 | 0.795 | 4700 | 0.421 | 0.899 | 0.478 |
GJ 139 | G8.0V | 0.812 | 5700 | 0.632 | 1.350 | 0.718 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 178 | F6.0V | 1.244 | 6600 | 1.299 | 2.774 | 1.475 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 183 | K3.0V | 0.791 | 4800 | 0.437 | 0.933 | 0.496 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 216 A | F6.0V | 1.137 | 6600 | 1.187 | 2.534 | 1.347 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 216 B | K2.0V | 0.593 | 5300 | 0.399 | 0.853 | 0.454 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 250 A | K3.0V | 0.597 | 4900 | 0.344 | 0.734 | 0.390 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 432 A | K0.0V | 0.627 | 5500 | 0.455 | 0.971 | 0.516 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 434 | G8.0V | 0.793 | 5700 | 0.617 | 1.319 | 0.702 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 442 A | G2.0V | 0.792 | 6000 | 0.683 | 1.459 | 0.776 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 451 | G8.0V | 0.542 | 5400 | 0.378 | 0.807 | 0.429 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 475 | G0.0V | 0.952 | 6100 | 0.849 | 1.813 | 0.964 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 502 | G0.0V | 1.029 | 6100 | 0.918 | 1.960 | 1.042 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 506 | G7.0V | 0.927 | 5700 | 0.722 | 1.542 | 0.820 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 566 A | G8.0V | 0.534 | 6200 | 0.491 | 1.049 | 0.558 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 566 B | K4.0V | 0.376 | 5000 | 0.225 | 0.481 | 0.256 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 570 A | K4.0V | 0.734 | 4700 | 0.389 | 0.830 | 0.441 | 0.739 | 4700 | 0.391 | 0.836 | 0.444 |
GJ 631 | K2.0V | 0.710 | 5500 | 0.515 | 1.099 | 0.584 | 0.759 | 5300 | 0.511 | 1.091 | 0.580 |
GJ 638 | K7.0V | 0.620 | 4100 | 0.250 | 0.534 | 0.284 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 663 A | K1.0V | ⋅⋅⋅ | ⋅⋅⋅ | 0.523 | 1.118 | 0.595 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 663 B | K1.0V | ⋅⋅⋅ | ⋅⋅⋅ | 0.523 | 1.118 | 0.595 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 664 (C)* | K5.0V | 0.581 | 4600 | 0.295 | 0.629 | 0.334 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 666 A | G8.0V | 0.841 | 5200 | 0.545 | 1.164 | 0.619 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 667 A | K3.0V | ⋅⋅⋅ | ⋅⋅⋅ | 0.399 | 0.853 | 0.454 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 667 B | K5.0V | ⋅⋅⋅ | ⋅⋅⋅ | 0.285 | 0.607 | 0.322 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 673 | K7.0V | 0.644 | 4100 | 0.259 | 0.554 | 0.295 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 702 A | K0.0V | 0.754 | 5500 | 0.547 | 1.168 | 0.621 | 0.670 | 5700 | 0.522 | 1.114 | 0.593 |
GJ 702 B | K5.0V | 0.411 | 5100 | 0.256 | 0.735 | 0.479 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 721 | A0.0V | 2.543 | 9800 | 5.852 | 12.50 | 6.648 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 764 | K0.0V | 0.694 | 5500 | 0.503 | 1.075 | 0.572 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 768 | A7.0V | 1.701 | 7800 | 2.480 | 5.297 | 2.817 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 783 A | K2.5V | 0.751 | 5000 | 0.450 | 0.961 | 0.511 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 785 | K2.0V | 0.802 | 5100 | 0.450 | 1.068 | 0.618 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 827 | F9.0V | 0.970 | 6400 | 0.952 | 2.034 | 1.082 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 879 (B)* | K5.0V | 0.592 | 4700 | 0.313 | 0.669 | 0.356 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 881 (A) | A4.0V | 1.635 | 9200 | 3.316 | 7.084 | 3.768 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 884 | K7.0V | 0.553 | 4200 | 0.234 | 0.499 | 0.265 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ |
GJ 892 | K3.0V | 0.698 | 5000 | 0.418 | 0.893 | 0.475 | 0.778 | 4800 | 0.430 | 0.918 | 0.488 |
Notes. This table shows model radius, temperature, inner and outer HZ radius, and the HZ width in AU in Columns 3–7 for the extended 10 pc sample. HZs for GJ 663A, GJ 663B, GJ 667A, and GJ 667B are estimated using the V–K relationship (see Section 5.2). *Designates wide component to above system with different GJ number.
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4.3. Habitable Zones of Multiple-star Systems
Although stars in multiple systems are often avoided in planet searches, planets have been found in binary and multiple systems (e.g., Patience et al. 2002; Raghavan et al. 2006; Eggenberger et al. 2007). The potentially dynamically disruptive effects of any close stellar companion must be considered when assessing the possibility of formation, long-term dynamical stability, and ultimately the habitability of planets in multiple star systems. The α Centauri triple system, at a distance from the Sun of 1.34 pc for the G2V–K0V pair (GJ 559 AB) and 1.30 pc for the wide M5V companion (GJ 551), includes the nearest set of Sun-like stars, and provides a test case to study the habitability of multiple stars. The G–K pair has an orbit with a semimajor axis of 1757 and eccentricity of 0.518 (Pourbaix et al. 2002). This gives periastron and apastron distances of 11.33 AU and 26.67 AU, respectively. Barbieri et al. (2002) show for α Centauri A that planets can form in stable orbits on a timescale of 5 Myr. Using numerical simulations, they show that not only could planets form, but in some models they formed directly in the HZ. Quintana et al. (2007) similarly find that binary separations greater than 10 AU did not inhibit the formation of terrestrial planets at 2 AU. Consistent with this, Wiegert & Holman (1997) show that the orbit of a planet can be stable as long as the ratio of the semimajor axis of the binary to that of the planet is more than 5:1.
The 36 multiple systems in the 5 pc and extended 10 pc samples consist of 27 binaries, 6 triples, and 3 quadruple systems (GJ 423, GJ 570, and GJ 695), for a total of 84 stars/brown dwarfs. However, nine are not main sequence stars, and are excluded because of their evolutionary states (e.g., sub-giants, white dwarfs, and brown dwarfs). Additionally, 16 are M star companions in the extended 10 pc AFGK sample. Because the habitable real estate of M stars out to 10 pc is estimated by scaling the 5 pc results, we do not consider these M stars in our EHZ calculations. This leaves 59 stars in multiple systems with potential HZs. For clarity, we emphasize that the EHZ of each star in a system is calculated separately.
Of the 59 stars, 43 (73%) have at least 4 spatially resolved photometric measurements in different filter bandpasses. For these stars, the same prescription used in Section 4.2 is used to calculate the EHZs. For the 16 stars that are not photometrically spatially resolved from their nearest companion(s), we estimate the EHZ locations based on spectral type information for the components. Using the assembled spectral types listed in Tables 1 and 3, we estimate the components' V − K color using the spectral type versus V − K colors relations of Kenyon & Hartmann (1995), and estimate the location of the EHZs using the relations described in Section 5.2.
To assess the dynamical stability of any planets in these EHZs, we compare the locations of the outer EHZ boundaries to the binary separations listed in Table 9. We consider a planet to be dynamically stable if the periastron distance of a star's nearest companion is greater than five times the outer EHZ boundary. In cases where the periastron distance cannot be calculated (because its full orbit solution is not known), we use the projected separation. Fortunately, these exceptions are all large separation multiples, and thus minor errors in the adopted separation are likely irrelevant for dynamical stability considerations. The ratios of periastron distances to outer EHZ boundaries are illustrated in Figure 3. For each star and its nearest companion, the ratios range from 0.001 to 164,107. 49 of the 59 stars have ratios to their nearest companions of greater than 5, and thus EHZs in which planets should be in dynamically stable orbits. The remaining 10 stars (GJ 53A, GJ 222A, GJ 244A, GJ 280A, GJ 423A, GJ 423B, GJ 713A, GJ 713B, GJ 866A, and GJ 866C) are considered to have EHZs in which planets would not be in dynamically stable orbits, so are excluded in our estimate of total EHZ real estate. Seven of these have ratios less than 1.0, hinting at the possibility of circumbinary planets. However, the majority of the EHZ outer radii are only a few times the binary separations, making it unlikely that these systems would have dynamically stable circumbinary planets in the EHZ. Nonetheless, it is worth noting that a few massive planets have been reported in circumbinary orbits (Lee et al. 2009; Qian et al. 2010).
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Standard image High-resolution imageTable 9. Orbital Properties of Multiple Systems
Star | Sep | a | e | i | Ω | ω | P | T | Ref. |
---|---|---|---|---|---|---|---|---|---|
(°) | (°) | (°) | (yr) | ||||||
GJ 15 AB | 40'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Eggen (1996) |
GJ 34 AB | ⋅⋅⋅ | 1199 | 0.497 | 34.76 | 98.43 | 88.59 | 480 | 1889.6 | Strand (1969) |
GJ 53 AB | ⋅⋅⋅ | 101 | 0.561 | 106.8 | 47.3 | 152.7 | 21.75 | 1975.74 | Drummond et al. (1995) |
GJ 65 AB | ⋅⋅⋅ | 206 | 0.615 | 127.3 | 150.5 | 285.4 | 26.52 | 1971.88 | Worley & Behall (1973) |
GJ 66 AB | ⋅⋅⋅ | 782 | 0.534 | 142.8 | 13.1 | 18.37 | 483.66 | 1813.5 | van Albada (1957) |
GJ 105 AC-B | 165'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Golimowski et al. (2000) |
GJ 105 AC | 33 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Golimowski et al. (2000) |
GJ 166 A-BC | 83'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Baize & Petit (1989) |
GJ 166 BC | 694 | ⋅⋅⋅ | 0.410 | 108.9 | 150.9 | 327.8 | 252.1 | 1849.6 | Heintz (1974) |
GJ 216 AB | 95'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Eggen (1956) |
GJ 222 AB | ⋅⋅⋅ | 0688 | 0.451 | 95.94 | 126.36 | 111.57 | 14.11 | 1999.9 | Han & Gatewood (2002) |
GJ 234 AB | ⋅⋅⋅ | 104 | 0.371 | 51.8 | 30.7 | 223 | 16.12 | 1999.38 | Ségransan et al. (2000) |
GJ 244 AB | ⋅⋅⋅ | 756 | 0.592 | 136.5 | 55.57 | 147.27 | 50.09 | 1894.13 | Gatewood & Gatewood (1978) |
GJ 250 AB | 58'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Eggen (1956) |
GJ 280 AB | ⋅⋅⋅ | 4496 | 0.365 | 31.9 | 284.8 | 88.8 | 40.38 | 1967.86 | Irwin et al. (1992) |
GJ 412 AB | 28'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Gould (2003) |
GJ 423 AC-BD | ⋅⋅⋅ | 2533 | 0.421 | 112.1 | 101.3 | 127.3 | 59.84 | 1995.05 | Heintz (1996) |
GJ 423 AC | ⋅⋅⋅ | 0056 | 0.53 | 94.9 | 263.5 | 143.0 | 1.832 | 1986.50 | Mason et al. (1995) |
GJ 423 BD | *274,000 km | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 0.015 | ⋅⋅⋅ | Mason et al. (1995) |
GJ 432 AB | 17'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Henry et al. (2002) |
GJ 442 AB | 254 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Poveda et al. (1994) |
GJ 473 AB | ⋅⋅⋅ | 0926 | 0.295 | 103.00 | 143.48 | 347.2 | 15.64 | 1992.30 | Torres et al. (1999) |
GJ 559 AB | ⋅⋅⋅ | 1757 | 0.518 | 79.2 | 204.85 | 231.65 | 79.91 | 1875.66 | Pourbaix et al. (2002) |
GJ 551-GJ 559 AB | 218 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Poveda et al. (1994) |
GJ 566 AB | ⋅⋅⋅ | 494 | 0.51 | 139 | 347 | 203 | 151.6 | 1909.3 | Söderhjelm (1999) |
GJ 570 A-BC | ⋅⋅⋅ | 3234 | 0.20 | 72.53 | 317.31 | 252.1 | 2130 | 1689 | Hale (1994) |
GJ 570 BC | ⋅⋅⋅ | 0151 | 0.756 | 107.6 | 195.9 | 127.56 | 0.846 | 1996.51 | Forveille et al. (1999) |
GJ 570 ABC-D | 2583 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Burgasser et al. (2000) |
GJ 663 AB | ⋅⋅⋅ | 130 | 0.916 | 99.79 | −85.8 | −90.2 | 470.9 | 1677.9 | Irwin et al. (1996) |
GJ 663 AB-GJ 664 | 742 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Luyten (1957) |
GJ 666 AB | ⋅⋅⋅ | 1042 | 0.779 | 35.64 | 131.78 | 333.44 | 693.24 | 1907.2 | Wieth-Knudsen (1957) |
GJ 667 AB | ⋅⋅⋅ | 181 | 0.58 | 128 | 313 | 247 | 42.15 | 1975.9 | Söderhjelm (1999) |
GJ 695 AD | 143 | ⋅⋅⋅ | 0.32 | 68.0 | 81.8 | 92 | 65 | 1951.0 | Turner et al. (2001); Heintz (1994) |
GJ 695 AD-BC | 34'' | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Raghavan et al. (2010) |
GJ 695 BC | ⋅⋅⋅ | 136 | 0.178 | 66.2 | 60.7 | 174.0 | 43.2 | 1965.4 | Couteau (1959) |
GJ 702 AB | ⋅⋅⋅ | 455 | 0.499 | 121.16 | 302.12 | 14.0 | 88.38 | 1895.94 | Pourbaix (2000) |
GJ 713 AB | ⋅⋅⋅ | 0124 | 0.428 | 74.42 | 230.30 | 119.3 | 0.768 | 1984.83 | Farrington et al. (2010) |
GJ 725 AB | ⋅⋅⋅ | 1388 | 0.53 | 66.0 | 136.9 | 234.6 | 408 | 1775.0 | Heintz (1987) |
GJ 783 AB | 71 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Poveda et al. (1994) |
GJ 820 AB | ⋅⋅⋅ | 2443 | 0.414 | 52.7 | 173.4 | 153.17 | 691.61 | 1689.14 | Baize (1950) |
GJ 845 A-BC | 4023 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Scholz et al. (2003) |
GJ 845 BC | 0732 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | McCaughrean et al. (2004) |
GJ 860 AB | ⋅⋅⋅ | 2383 | 0.410 | 167.2 | 154.5 | 211.0 | 44.67 | 1970.22 | Heintz (1986) |
GJ 866 AC | ⋅⋅⋅ | ∼001 | 0.000 | ∼117 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | 1991.71 | Delfosse et al. (1999) |
GJ 866 AC-B | ⋅⋅⋅ | 0346 | 0.446 | 112 | 161.5 | 337.6 | 2.25 | 1997.53 | Delfosse et al. (1999) |
GJ 881-GJ 879 | 197 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Poveda et al. (1994) |
GJ 1245 AC-B | 7969 | ⋅⋅⋅ | 0.320 | 135.0 | 80.0 | 38.0 | 15.22 | 1983.1 | Tokovinin (1997) |
GJ 1245 AC | 0800 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Tokovinin (1997) |
SCR 1845-6357 AB | 1170 | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | ⋅⋅⋅ | Biller et al. (2006) |
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5. DISCUSSION
The described HZ calculations are used to assess the total habitable real estate in the solar neighborhood and to determine the amount of habitable real estate as a function of spectral type. Evolved stars, brown dwarfs, and multiple stars with separations detrimental to the orbital stability of a planet in the EHZ, as described above, are excluded in these assessments. In the 5 pc sample, stars removed from the analysis due to close companions include GJ 244A, GJ 244B, GJ 280A, GJ 280B, GJ 866A, and GJ 866C. The distribution by spectral type, after the removal of these stars in the 5 pc sample is as follows: 0 A, 0 F, 3 G, 7 K, and 48 M stars. Similarly, stars removed from the extended 10 pc sample analysis due to close companions include GJ 53A, GJ 53B, GJ 222A, GJ 222B, GJ 423A, GJ 423 B, GJ 423C, GJ 423D, GJ 713A, and GJ 713B. Stars with evolved spectral types such as GJ 150, GJ 695A, and GJ 780 are also removed from subsequent calculations. By spectral type, the total stellar samples considered in the final EHZ assessment are 3 A, 4 F, 14 G, 34 K, and an estimated 384 M stars.
5.1. The EHZ "Width"
Using our initial assumptions of a terrestrial "Earth-like" planet as the basis for our EHZ, we estimate the habitable real estate using linear AU separations from the central star, essentially the width of the EHZ. In Table 10 we present the EHZ width totals for each spectral type in the 5 pc and total 10 pc samples.
Table 10. Total EHZ by Spectral Type
SpType | 5 pc Sample | EHZ | Total 10 pc Sample | EHZ |
---|---|---|---|---|
No. of Stars | (AU) | No. of Stars | (AU) | |
A | 0(1) | ⋅⋅⋅ | 3 (4) | 13.2 |
F | 0(1) | ⋅⋅⋅ | 4 (6) | 4.9 |
G | 3 | 2.6 | 14 (21) | 11.9 |
K | 7 | 2.9 | 34 (35) | 15.4 |
M | 48(50) | 3.3 | 384*(400)* | 26.1* |
Notes. The numbers in parentheses provide the total number of stars known in each subsample, prior to excluding unresolved binaries and resolved stars. *The totals for the M type population to 10 pc are a factor of eight greater than the population within 5 pc, estimated via scaling by the volume (R3).
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The cumulative EHZ width for stars in the 5 pc subsample is 8.8 AU, including 2.6 AU for the three G stars (including the Sun) and 2.9 AU for the seven K stars. The 48 M dwarfs in the 5 pc sample en masse provide 3.3 linear AU available for habitable planets, or 38% of the available EHZ. The dominant contribution of M dwarfs to the EHZ width is demonstrated clearly using the estimated 10 pc sample. The total EHZ width for the estimated 10 pc sample is 71.5 AU, including 13.2 AU for the 3 A, 4.9 AU for the 4 F stars, 11.9 AU for the 14 G stars, and 15.4 AU for the 34 K stars. The estimated 384 M stars en masse provide 26.1 AU of linear EHZ. This accounts for 36.5% of the total EHZ. Thus, by spectral type, M stars en masse provide the largest EHZ real estate.
5.2. Predicting the Size of the Habitable Zone from V − K Colors
Given the rapid pace of exoplanet discovery, it would be helpful to have a tool to easily and accurately predict the location of the EHZ to determine whether or not a planet resides within it. Predicting the EHZ of a star based on spectral classification can be problematic due to the inhomogeneity in classification and spectral types being determined over different wavelength ranges. As shown in Henry et al. (2006), the V − K color is a useful temperature diagnostic for the A through M stars that dominate the solar neighborhood. This relation can also be very helpful in determining a rough estimate of the EHZ based on observable photometry, and may be easily scaled to larger populations.
We use the results from the 5 pc and extended 10 pc samples to derive a relation between V − K color and the size and location of the EHZ. As in the computation of total habitable real estate, we removed binary stars with unresolved photometry as well as stars known to be evolved. We do, however, use stars such as Sirius (GJ 244) and Procyon (GJ 280) for which EHZs have been determined, even though their companions corrupt their EHZs. Their white dwarf companions do not significantly contribute to their luminosities or V − K, and their inclusion improves the statistics of our fit. We fit a second order polynomial, described by Equation (6), to the V − K colors and computed EHZ widths shown in Figure 4:
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Standard image High-resolution imageUsing the same method, a relationship for V − K color and the inner and outer radii of the EHZs are described by Equations (7) and (8), respectively. These relations are only valid for main sequence stars:
As a check on these relations, we use these to calculate the total EHZ width by spectral type of the estimated 10 pc sample and compare these values to the direct calculations described in Sections 4.2 and 4.3. We find the total EHZ widths from the empirical relations differ on average from the calculated total widths by −12.7%, −6.4%, 1.6%, 4.7%, and 0.3% for A, F, G, K, and M stars respectively; negative values correspond to underpredictions of the EHZ width totals. The higher percentage differences for A type and F type totals are due to the relatively small populations within 10 pc and the effects of large 2MASS photometric errors due to brightness. The results imply that this relation is useful for quickly estimating the amount of habitable real estate for a population with known V − K values. We also test how well the predicted inner and outer boundaries from our relations agree for any given star. On average, these predictions yield values consistent to 3% with a dispersion of 22% for both inner and outer boundaries for AFGKM stars in our samples. These dispersions can be interpreted as the uncertainty in the locations of these boundaries from these relations.
5.3. Planets in the EHZs of Nearby Stars
Of the 14 confirmed planetary systems within 10 pc of the Sun for which orbits have been determined, 5 contain multiple planets (GJ 139, GJ 506, GJ 581, GJ 667C, and GJ 876). All 14 systems are listed in Table 11 with published values for semimajor axis and eccentricity, as well as calculated inner and outer radii of the EHZ from this work. Three of the systems, GJ 581, GJ 667C, and GJ 876 have planets in the EHZ.
Table 11. Exoplanets within 10 pc
Star | Planet | Semimajor Axis | e | Ref. | HZinner | HZouter |
---|---|---|---|---|---|---|
(AU) | (AU) | (AU) | ||||
GJ 139 | b | 0.1207 ± 0.0020 | 0.0 | Pepe et al. (2011) | 0.632 | 1.350 |
GJ 139 | c | 0.2036 ± 0.0034 | 0.0 | Pepe et al. (2011) | 0.632 | 1.350 |
GJ 139 | d | 0.3499 ± 0.0059 | 0.0 | Pepe et al. (2011) | 0.632 | 1.350 |
GJ 144 | b | 3.39 ± 0.36 | 0.702 ± 0.039 | Benedict et al. (2006) | 0.439 | 0.938 |
GJ 176 | b | 0.066 | 0 | Forveille et al. (2009) | 0.136 | 0.290 |
GJ 442 A | b | 0.46 ± 0.04 | 0.34 ± 0.14 | Tinney et al. (2011) | 0.689 | 1.472 |
GJ 506 | b | 0.05 ± 5.0e-6 | 0.12 ± 0.11 | Vogt et al. (2010b) | 0.689 | 1.472 |
GJ 506 | c | 0.2175 ± 0.0001 | 0.14 ± 0.06 | Vogt et al. (2010b) | 0.719 | 1.537 |
GJ 506 | d | 0.476 ± 0.001 | 0.35 ± 0.09 | Vogt et al. (2010b) | 0.719 | 1.537 |
GJ 559 B | b* | 0.04185 ± 0.0003 | 0 | Dumusque et al. (2012) | 0.541 | 1.155 |
GJ 581 | b | 0.04 | 0 | Mayor et al. (2009) | 0.083 | 0.179 |
GJ 581 | c | 0.07 | 0.17 ± 0.07 | Mayor et al. (2009) | 0.083 | 0.179 |
GJ 581 | d | 0.22 | 0.38 ± 0.09 | Mayor et al. (2009) | 0.083 | 0.179 |
GJ 581 | e | 0.03 | 0 | Mayor et al. (2009) | 0.083 | 0.179 |
GJ 581 | f* | 0.758 ± 0.015 | ⋅⋅⋅ | Vogt et al. (2010a) | 0.083 | 0.179 |
GJ 581 | g* | 0.1460 ± 1.4e-4 | ⋅⋅⋅ | Vogt et al. (2010a) | 0.083 | 0.179 |
GJ 667 C | b | 0.049 | 0.172 ± 0.043 | Anglada-Escudé et al. (2012) | 0.096 | 0.205 |
GJ 667 C | c | 0.123 ± 0.020 | <0.27 | Anglada-Escudé et al. (2012) | 0.096 | 0.205 |
GJ 674 | b | 0.039 | 0.20 ± 0.02 | Bonfils et al. (2007) | 0.098 | 0.210 |
GJ 785 | b | 0.310 ± 0.005 | 0.30 ± 0.09 | Howard et al. (2011) | 0.500 | 1.068 |
GJ 832 | b | 3.4 ± 0.4 | 0.12 ± 0.11 | Bailey et al. (2009) | 0.131 | 0.280 |
GJ 849 | b | 2.35 | 0.06 ± 0.09 | Butler et al. (2006a) | 0.136 | 0.290 |
GJ 876 | b | 0.2083 ± 2.0e-5 | 0.0292 ± 1.5e-3 | Rivera et al. (2010) | 0.090 | 0.191 |
GJ 876 | c | 0.1296 ± 2.6e-5 | 0.2549 ± 8.0e-4 | Rivera et al. (2010) | 0.090 | 0.191 |
GJ 876 | d | 0.0208 ± 1.5e-7 | 0.207 ± 0.055 | Rivera et al. (2010) | 0.090 | 0.191 |
GJ 876 | e | 0.3443 ± 0.0013 | 0.055 ± 0.012 | Rivera et al. (2010) | 0.090 | 0.191 |
GJ 881 | b | 115 | 0.11 | Kalas et al. (2008) | 3.316 | 7.084 |
Notes. Rows in bold font indicate the planet is in the EHZ for at least part of its orbit. *Planet detection controversial.
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GJ 581, an M2.5V star at a distance of 6.25 pc, has six proposed planets, but the existence of GJ 581g and GJ 581f are currently debated (see: Vogt et al. 2010a; Andrae et al. 2010; Gregory 2011; Anglada-Escudé 2010; Tuomi 2011). If real, GJ 581g, with a semimajor axis of 0.146 AU, orbits in the EHZ in a presumed circular orbit. Using CHARA, von Braun et al. (2011) recently measured the size of the star and derived HZ boundaries of Rin = 0.11 AU and Rout = 0.21 AU. Our EHZ is somewhat closer in to the star, Rin = 0.083 AU and Rout = 0.179 AU, but still places GJ 581g in the EHZ. The differences are due to our calculated luminosity (L = 0.11 L☉) being 8% lower than von Braun et al. (2011), as they adopted an extinction of AV = 0.174 for GJ 581, which they note as unexpected for a star at this distance. GJ 581d, with semimajor axis of 0.22 AU and eccentricity of 0.38, also moves in and out of the EHZ of GJ 581.
GJ 667C, an M1.5V star at a distance of 7.23 pc, hosts two planets, and possibly four (Anglada-Escudé et al. 2012). Although GJ 667Cb does not lie within the EHZ, GJ 667Cc (msin i of 4.5 M⊕) lies within the EHZ for the majority of its orbit. With a semimajor axis of 0.123 AU and eccentricity <0.27, it may lie completely within the EHZ once the eccentricity is more highly constrained.
GJ 876, an M3.5V star at a distance of 4.66 pc, hosts four planets (Rivera et al. 2010). Our calculations show an EHZ spanning 0.090–0.191 AU. Rivera et al. (2010) report orbital fits for two planets near the EHZ with semimajor axes of 0.13 AU (GJ 876c) and 0.21 AU (GJ 876b), and eccentricities of 0.25 and 0.03, respectively. As shown in Figure 5, this allows for GJ 876c to be in the EHZ of its host star for the full duration of its orbit, while GJ 876b lies just outside the EHZ. Although these planets are not considered terrestrial (msin i of 0.56 and 1.89 MJ), the possibility exists that they could have terrestrial-like moons that could be habitable.
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Standard image High-resolution image5.4. Complications to Habitability
The previous sections provide estimates of EHZs based on the requirement of liquid water on a planetary surface. Of course, a planet's location in the EHZ of a host star does not guarantee its habitability. A host of other factors, such as planet size, atmosphere, magnetic fields, and even plate tectonics, play vital roles in determining the habitability of a planet in the EHZ. Without a sufficiently thick atmosphere, biologically harmful radiation can penetrate to the surface of a planet. On Earth, atmospheric CO2 levels are kept in check by the carbon-silicate cycle. Known to regulate climate temperatures through a negative feedback, this cycle allows for as much as 60 bars (Kasting 1996) of CO2 to be locked away in rock and sediments. This slow carbon-silicate cycle requires that water be present, because without water, the atmospheric CO2 cannot be sequestered as carbonate. A magnetosphere on Earth also plays an important role by deflecting harmful charged particles. Tectonic activity may be one of the key factors in keeping a planet habitable (Doyle et al. 1998). Without water, the lithosphere of a planet may become a stagnant lid, halting tectonic activity and the sequestration of CO2.
There is a temporal constraint on habitability as well, as the HZ may change considerably over the lifetime of a star (Kasting et al. 1993; Kasting 1996; Tarter et al. 2007; Selsis et al. 2007). Putting these complications aside, the requirement of liquid water on the surface of a planet is a good first order approximation to habitability.
6. SUMMARY
We assess the sample of stars currently known to be within 5 pc of the Sun for the purpose of determining the habitable real estate and its dependence on spectral type. Because of the sparse population of high mass stars within 5 pc of the Sun, we expand this sample for AFGK stars to 10 pc; there are no O or B stars within 10 pc of the Sun. After eliminating evolved stars, substellar objects, and close multiples in which planets in HZs would be dynamically unstable, we use the final sample to estimate the EHZs for stars in both the 5 pc and extended 10 pc samples. We do not consider circumbinary habitable zones in this work, but EHZs are calculated in the same fashion as single stars for each of the 49 components in multiple systems that satisfy our dynamical stability constraint.
Using PHOENIX models convolved with filter response curves, we fit observed UBVRIJHK photometry for each object, assuming spherical, non-rapidly rotating stars with solar metallicity and log g values of 4.0–5.0, with the two exceptions being the metal poor stars GJ 191 and GJ 451. This fitting process allows us to determine a radius and Teff for each star that is then used to determine its surrounding EHZ, calculated using a modified "Venus and early Mars criterion" from Selsis et al. (2007).
We use estimates of linear AU to map the EHZ of each star and sum by spectral type en masse: 48 M dwarf stars used in the 5 pc sample provide more habitable real estate (3.3 AU) than the 3 G dwarfs stars (2.6 AU) and 7 K stars (2.9 AU) found within 5 pc of the Sun. Even after extending the sample of AFGK stars to 10 pc, the anticipated sample of M dwarfs within 10 pc (not all have been identified) possess more EHZ real estate than any other spectral type, spanning ∼26 AU compared to 13.2 AU, 11.9 AU, and 15.4 AU for each of the A, G, and K types (the F stars provide only 4.9 linear AU of EHZ). The result is a natural consequence of the large relative numbers of M dwarfs, and the frequency of close companions that declines with mass.
As a population, M dwarfs provide more options and more habitable real estate than their more massive counterparts. Furthermore, recent results from Kepler show that for stars with Teff > 4000 K within 5 pc of the Sun, there is likely to be at least two Earth-size planets in the HZ. That number increases to 16 within 10 pc (Dressing & Charbonneau 2013).
Using the 5 pc and extended 10 pc samples, we derive relations between V − K color and EHZ width and inner and outer limits. Comparisons of color predicted locations suggest they are comparable to uncertainties associated with habitability assumptions (e.g., Section 4.2). Thus, these color relations are practical tools for estimating the EHZs of stars using commonly available photometric measurements. The relations for the inner and outer radii of the EHZ are helpful for quickly estimating whether or not a known planet or disk is within the EHZ. The relation for EHZ size is useful in predicting the habitable real estate available in a stellar population. In particular, we consider the results for the 14 extrasolar planetary systems known within 10 pc of the Sun. The three systems with planets in the calculated EHZs—GJ 581, GJ 667C, and GJ 876—are all M dwarfs. In total, as many as four planets circling these stars spend at least part of their time in the EHZs, providing an ideal set of targets for future efforts to detect biosignatures.
The authors thank J. Kasting for extremely helpful advice and guidance. They also thank the RECONS team for the generous amount of data and support provided. Data used in this paper were acquired via support from the National Science Foundation (grants AST 05-07711 and AST 09-08402) and through the continuous cooperation of the SMARTS Consortium. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France; data from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and IPAC, funded by NASA and NSF; and the Sixth Catalog of Orbits of Visual Binary Stars, operated by the US Naval Observatory. This project was funded in part by NSF/AAG grant No. 0908018.
Footnotes
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(2013 May) http://www.exoplanets.org
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A = 0.3 for the hypothetical planets used in this paper.
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