THE SOLAR NEIGHBORHOOD. XXVI. AP Col: THE CLOSEST (8.4 pc) PRE-MAIN-SEQUENCE STAR

To measure a spectral type, AP Col was observed on the CTIO 1.5 m on UT 2004 March 14 using the 32/I grating setup (6000–9600 Å, R = 1500). The resulting spectrum was reduced using standard IRAF procedures and then classified as M4.5Ve using the ALLSTAR code (Henry et al. 2002), which compares it to spectral standards on the Kirkpatrick et al. (1991) system.

To measure a preliminary radial velocity, Hα, and Li i λ6708 line strengths, AP Col was observed several times during 2011 January–March with the Wide Field Spectrograph (WiFeS) on the Australian National University 2.3 m at Siding Spring Observatory. WiFeS (Dopita et al. 2007, 2010) is a new dual-beam image slicing integral field spectrograph that provides a nominal 25'' × 38'' field of view with 05 pixels to two gratings and camera assemblies simultaneously using a beam splitter, one "beam" optimized for red spectra, the other for blue. AP Col was observed in single-beam mode with the R7000 grating, yielding a resolving power of R ≃ 7000 and wavelength coverage of 5500–7000 Å. Following S. J. Murphy et al. (2011, in preparation), we used WiFeS in single-star mode with twice the spatial binning (1'' spatial pixels) and optimally extracted and combined the five image slices (effectively a 5'' diameter aperture around the object) that contain the majority of the stellar flux.

WiFeS has a measured radial velocity precision capability of ∼2 km s⁻¹ per epoch at this resolution and signal-to-noise ratio (Table 1; S. J. Murphy et al. 2011, in preparation). We therefore observed AP Col seven times on different nights to improve the mean velocity and check for changes in Hα emission line strength. These observations motivated further high-resolution observations with the Lick Hamilton echelle and Keck HIRES echelle, discussed in the next sections.

In addition to the R7000 observations, AP Col was observed on 2011 January 25–26 with the B3000 and R3000 gratings in dual-beam mode, yielding a resolving power of R ≃ 3000 and wavelength coverage of 3400–9560 Å. The spectra from each beam were independently flux-calibrated and corrected for telluric features, and combined into a single spectrum. Details about all the WiFeS observations are presented in Table 1.

Contemporaneous with the low-resolution WiFeS spectra, additional measurements of radial velocity, Hα, and Li i λ6708 EW were obtained at the Lick Observatory Shane 3 m telescope with the Hamilton echelle spectrograph (Vogt 1987), which is located at the telescope's Coudé focus. The spectra were bias-subtracted, flat-fielded, extracted, and wavelength-calibrated with ThAr arclamp spectra. Further details on data reduction for the Hamilton echelle with IRAF tasks are outlined in detail in Lick Technical Report No. 74.⁸

Details about the observations are presented in Table 1. We note that AP Col, at declination −34, is at the southern limit of what can reasonably be observed from Lick Observatory; the average air mass during observations was 3.3.

More precise measurements of radial velocity and vsin i were obtained on the Keck-I 10 m telescope on Mauna Kea using the HIRES echelle spectrograph (Vogt et al. 1994). All HIRES data were reduced with standard IRAF echelle reduction tasks: data are bias-subtracted, then flat-fielded with "wide-decker" flats (flats taken with twice the decker height of the science data). Data are extracted and then wavelength-calibrated with ThAr arclamp spectra. Observational parameters are given in Table 1. The iodine cell was in the light path during the observations of AP Col; as a result, strong iodine absorption features are present from ∼5000 to 6000 Å.

Archival HIRES observations of the M4.5V star GJ 83.1 were retrieved from the Keck Observatory Archive⁹ for use in radial velocity cross-correlation. GJ 83.1 has a radial velocity known to be stable to <100 m s⁻¹ (Nidever et al. 2002). The archival observations from UT 2009 August 10 (PI: Haghighipour) were performed with an identical instrument setup.

Stars that have not yet reached the main sequence (i.e., equilibrium between gravitational collapse and thermonuclear fusion) will appear overluminous compared to other stars of the same color because their photospheric surfaces are larger than main-sequence stars. Such young stars are therefore elevated relative to the main sequence on an H-R diagram.

To properly compare AP Col to other known young stars without relying on theoretical models requires accurate data. Therefore, we compiled a catalog of known young stars in associations from Zuckerman & Song (2004) and Torres et al. (2008). Position, proper motion, and photometry for each star were extracted from the All Sky Compiled Catalog (Kharchenko 2001), while trigonometric parallaxes¹⁴ were obtained from multiple sources, principally Hipparcos (van Leeuwen 2007), the General Catalog of Trigonometric Parallaxes (van Altena et al. 1995), Weinberger et al. (2011), and unpublished data from the CTIOPI observing program. To produce empirical isochrones for nearby associations, multiples, suspected multiples, and obvious outliers (generally noted as such in the Hipparcos catalog; Zuckerman & Song 2004 or Torres et al. 2008) were removed from the subset of young stars with parallaxes. The sample was then approximated by fifth-order polynomials to form rough isochrones, which are listed in Table 4.

Table 4. Polynomial Coefficients for Empirical Association Isochrones

Association	No. of Stars in Fit	Min. V−K	Max. V−K	c0	c1	c2	c3	c4	c5
β Pic	32	0.0	6.3	1.6265	2.1789	−0.6631	0.2978	−0.0550	0.00370
TW Hya	16	0.0	8.3	1.4555	0.8992	1.2706	−0.5648	0.0915	−0.00485
AB Dor	41	1.2	5.1	−5.7448	13.6063	−6.5304	1.6629	−0.1949	0.00845
Tuc–Hor	31	−0.6	3.9	1.3926	1.8747	−0.0707	0.4269	−0.2199	0.02960
Cha	10	−0.3	6.6	0.4050	1.2320	2.2331	−1.2254	0.2256	−0.01376

Notes. Coefficients are for the equation M_V = c₀ + c₁(V − K) + c₂(V − K)² + c₃(V − K)³ + c₄(V − K)⁴ + c₅(V − K)⁵.

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Figure 3 shows AP Col and the other members of the RECONS 10 pc sample (stars in systems with parallaxes greater than 100 mas and errors less than 10 mas; Henry et al. 2006). Known young stars with trigonometric parallaxes are also shown on the graph, along with error bars based on their parallactic and photometric errors (AP Col's errors are smaller than the plotted symbol), and polynomial fits to three of the associations. As the fifth-order polynomials are highly sensitive to the colors of their most extreme members, we have not plotted them past the reddest young star in each association. The Cha isochrone relies on too few points to be reliable, and there are no quality Tucana–Horologium (Tuc–Hor) members cooler than M1 (the single point near AU Mic appears to be a binary). The reddest object in β Pic isochrone is TWA 22AB, originally misclassified as a TW Hya member, with a parallax reported by Teixeira et al. (2009). Only the TW Hya association extends redder than the plot in Figure 3; its reddest member is TWA 27 (V − K_S = 8.25, M_V = 16.60), with parallaxes in Gizis et al. (2007), Biller & Close (2007), and Ducourant et al. (2008).

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As can be seen in Figure 3, AP Col is clearly older than members of the TW Hya cluster (∼8 Myr), and consistent with but likely older than β Pic (∼12 Myr). There are no comparably red AB Dor or Tuc–Hor members with known parallaxes; despite this, the position of AB Dor at the high-metallicity upper envelope of the main sequence suggests that AP Col is younger than the ∼70 Myr AB Dor Association.

The Li i λ6708 EW of AP Col is 0.28 ± 0.02 Å, the weighted mean of all our high-resolution spectral measurements. Lithium is easily destroyed but not readily produced by stellar thermonuclear fusion, and is thus only detected in the photospheres of objects that have not yet consumed their primordial supply of the element. This is usually interpreted as a consequence of youth, or in the case of brown dwarfs, because their cores never reach the temperatures necessary to fuse it. Lithium depletes fastest in mid-M stars, where it is thought that the persistence of full convection throughout the star's evolution to the main sequence means that all the lithium is cycled through the core and quickly destroyed once temperatures rise high enough (Jeffries & Naylor 2001). Larger, hotter stars develop radiative cores that take longer to deplete their lithium and can trap it in their photospheres for a billion years; cooler fully convective stars such as AP Col have longer nuclear burning timescales. Thus, as a coeval stellar population ages, the temperature range of lithium-depleted stars widens around the mid-M stars, with a particularly sharp drop on the cooler side. The detection of lithium in a K- or M-type star is therefore a strong indicator of youth.

By way of comparison, Figure 4 presents WiFeS spectra in the vicinity of the Li i λ6708 Å feature for AP Col, Cha 9 (an ∼6 Myr old member of the Cha Association), and GJ 402 (a field radial velocity standard), all of the same approximate spectral type. The strength of the lithium absorption in AP Col is intermediate between the young Cha member and the old field dwarf, and suggests an intermediate age.

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The dependence of lithium burning efficiency on stellar age and temperature leads to the concept of a lithium depletion boundary (LDB), where only a small change in stellar mass and temperature can lead to the appearance or disappearance of the Li i λ6708 feature (Song et al. 2002). For instance, at the age of the β Pic association (∼12 Myr), the cool edge of the LDB lies at the spectral type M4.5 (Song et al. 2002; Torres et al. 2003), while at the age of the Pleiades (∼100–130 Myr) it has shifted to M6.5 (Stauffer et al. 1998; Barrado y Navascués et al. 2004). While Yee & Jensen (2010), Song et al. (2002) and others have noted that lithium depletion ages are systematically larger than those derived by isochrone fitting or kinematic expansion ages, relative age ranks are not affected.

We plot in Figure 5 the lithium measurements for AP Col, several of the young stellar associations in the solar vicinity from da Silva et al. (2009), and the young cluster IC 2391 from Barrado y Navascués et al. (2004). The β Pic LDB at M4.5 (dashed line) is clearly visible as the discontinuity in EW around 3300 K, while the IC 2391 lithium depletion boundary at ∼M5 (3200 K) is shown by a dotted line. AP Col lies between the two boundaries at T_eff ≃ 3250 K (based on the V−I color and the conversion of Kenyon & Hartmann 1995), implying an age between β Pic (∼12 Myr) and IC 2391 (50 ± 5 Myr lithium depletion age; Barrado y Navascués et al. 2004), though consistent with either.

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Certain spectral features are sensitive to the surface gravity of a star, and may therefore be used as age proxies for stars still contracting toward the main sequence (Hayashi 1966), at least for relative dating (e.g., Lawson et al. 2009). The Na i λ8183/8195 doublet is particularly useful for comparative gravity studies (e.g., Lawson et al. 2009; Murphy et al. 2010). For spectral types cooler than ∼M3, there is a marked decrease in the strength of the Na i doublet between dwarfs (strong), pre-main-sequence stars (intermediate), and giants (weak/absent). To measure the strength of Na i absorption, we adopt the spectral index of Lyo et al. (2004), formed by the ratio of the average flux in two 24 Å wide bands: one on the doublet and the other on the immediately adjacent pseudo-continuum. Figure 6 shows the Na i λ8200 index for AP Col compared to the mean trends of other young associations in Lawson et al. (2009). To match the resolution used in that study, we have smoothed and resampled the WiFeS R3000 data to R ∼ 900 and the same wavelength scale used by Lawson et al. (2009). Although close to the dwarf locus, AP Col nevertheless lies at intermediate gravities between β Pic and field dwarfs; we can again constrain the age to greater than that of β Pic and less than the Pleiades, whose M-type members have gravity features indistinguishable from field stars (Slesnick et al. 2006a).

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Alkali metal lines such as Na i can also be affected by stellar activity, where emission fills in the absorption line cores, leading to lower EWs (Reid & Hawley 1999). Our WiFeS observations span a factor of three in Hα EW, but no correlation between that activity indicator and our Na i doublet EW measurements could be found. Slesnick et al. (2006b) note that the Na i λ8183/8195 doublet can be affected by telluric absorption over the region 8161–8282 Å, leading to artificially low Na i index values for stars observed at large air masses. Our WiFeS R3000 spectra were observed at sec(z) ≃ 1; nevertheless, we have checked the telluric correction of the spectra and find no excess that could affect the index measurements.

Young stars are expected to rotate rapidly, with decreasing rotation as they age. Reiners et al. (2009) suggest vsin i > 20 km s⁻¹ is a rapidly rotating M star. From the Keck-I HIRES spectra, we measure a vsin i ∼ 11 ± 1 km s⁻¹, indicating that AP Col is not necessarily a rapidly rotating star. Because it is not in a binary system, this spin is not due to tidal synchronization with a companion; it is a remnant of the star's formation.

While gyrochronology relations exist (e.g., Mamajek 2009) for solar-type stars, none have been developed for M dwarfs, nor stars with saturated X-ray emission such as AP Col. Gyrochronology cannot (yet) be used to estimate an age for AP Col.

Young stars are known to have large amounts of X-ray emission, Hα emission, flares, and photometric variability, when compared to field stars. All of these indicate chromospheric activity, fast rotation rates, and powerful magnetic fields. This activity also manifests itself photometrically, and young stars have long been recognized for long-term large-amplitude variations and flares since they were first discovered as "Orion-type" variables.

Unfortunately, M dwarfs undergo a long adolescence, and as recognized by Zuckerman & Song (2004) and Hawley et al. (1996), X-rays remain saturated at log (L_x/L_bol) ∼ −3 in M dwarfs past the age of the Hyades (∼600 Myr). Hα persists at varying levels for equally long times, and the variability and flaring continues long afterward into the intermediate-age UV Ceti flare stars. Thus, these activity indicators are necessary but insufficient indicators of youth, fairly irrelevant in the face of the measured lithium absorption discussed above, and are discussed mainly for completeness.

AP Col is cross-identified as the ROSAT All Sky Survey object 1RXS J060452.1-343331, and has log (L_x/L_bol) = −2.95 ± 0.16 and log (L_x) = 28.49 (17% error). These values match those of Riaz et al. (2006) and agree with the range of X-ray variability published by Scholz et al. (2005), log (L_x/L_bol) = −3 to −4.

As seen in Table 1, during our spectroscopic observations, the Hα EW of AP Col varied from −6 Å in apparent quiescence, to −35 Å during the strong flare on 2011 January 25, with an average EW of −9.1 ± 5.2 Å, in agreement with the −12.1 Å single-epoch measurement of Riaz et al. (2006).

AP Col is a known UV Ceti flare star, and Ball & Bromage (1995) observed five flares over nine hours of U-band observations, the largest of which was 2.5 mag above quiescence. A 12 hr X-ray flare was also measured by ROSAT and presented by Scholz et al. (2005); during this event, AP Col increased in X-ray luminosity by roughly an order of magnitude and slowly dropped back to normal levels. We have also measured an energetic white-light flare in Lick Hamilton echelle data taken on 2011 January 25 (C. Melis et al. 2011, in preparation).

Finally, we have measured the relative photometric variability of AP Col using the V-filter data from the CTIOPI astrometric frames. The standard deviation of the variability is 1.7% (Figure 7). As shown in Figure 8, this is typical when compared to field M dwarfs observed during CTIOPI (Jao et al. 2011). The 158 CTIOPI astrometry frames constitute a total observing time of 14715 s over 27 nights (4 hr, with a median time of 450 s in five observations per night) spanning 6.48 yr. We additionally checked the ASAS3 database (Pojmanski 1997) for photometry on AP Col, and find no evidence of large flares in their data set either (see Figure 7). We report our own variability in Table 2, as our 0.91 m telescope aperture lends itself to better photometry than the 8 cm ASAS telescopes.

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Young stars often have protostellar disks that show up as near- and mid-IR excesses. These tend to vanish within ∼10 Myr (Haisch et al. 2001, 2005). IRAS and WISE photometry from the preliminary data release show no obvious signs of infrared excess around AP Col. This is not unexpected if AP Col is older than ∼10 Myr, as suggested by our other age indicators.