A NEW LOOK AT THE LOCAL WHITE DWARF POPULATION

In Table 1, 26 entries presently lack published parallaxes. However, many of these 26 stars are part of several ongoing parallax programs. Indeed, 15 of these stars (flagged in Table 1) have preliminary parallaxes that indicate that they lie within 20 pc, to a high degree of confidence. Thus, in the not too distant future virtually all of the local sample will have trigonometric parallaxes. Although the parallaxes for most stars in Table 1 are still derived from the Yale catalog (Van Altena et al. 1994) and the Hipparcos catalog (Perryman 1997), there have been some notable additions. Smart et al. (2003) determined parallaxes for WD 0322−019 and WD 0423+120 and Ducourant et al. (2007) provided a parallax for WD 2211−392. In addition, several new parallaxes are available from the RECONS program: WD 0121−429 and WD 2008−600 (Subasavage et al. 2007) and WD 0552−041 and WD 0738−172 (Costa et al. 2005). New preliminary parallaxes are contained in Subasavage et al. (2008, in preparation).

Table 2 lists the effective temperatures (T_eff), surface gravities (log g) and masses, along with the corresponding uncertainties, for each star in our sample. These parameters are taken from a variety of sources in the published literature, which are referenced in Table 2. Because of the wide range of temperatures and spectral types, a mixture of spectroscopic and photometric data have been used in estimating the astrophysical properties listed in Table 2. For some of the warmer DA and DAZ stars as well as some DQ and DZ stars spectroscopically derived temperatures and gravities are used. Particularly for cooler white dwarfs and non-DA stars, the effective temperatures and gravities are deduced primarily from photometric data and/or taken from the literature. The spectral type designations given in Table 1 are consistent with our adopted T_eff values.

Bergeron et al. (2001; BLR), Liebert et al. (2005; LBH), and Bergeron et al. (2007; BGB) noted that spectroscopic gravities obtained for DA stars with temperatures below 12,000–13,000 K seem to systematically overestimate the actual surface gravities and the corresponding masses of these stars. The origin of this effect is thought to be due to the onset of convection, resulting in the mixing of spectroscopically undetectable He into the H-rich photospheres. One approach to mitigating this effect is to make use of trigonometric parallaxes to determine gravities and masses using the mass–radius relation. BLR employed this method on a large sample of white dwarfs. Approximately one half of the stars in our local sample are in BLR and we have preferentially used their results in Table 2 (Ref. 1). Likewise, we have relied on the results of Dufour et al. (2005, Ref. 11) and (2007, Ref. 10) for most of our DQ and DZ stars, which also employ trigonometric parallaxes.

Where no spectroscopic data exist, we have relied on photometric data. Principal among these cases are stars observed by Subasavage et al. (2007, Ref. 13). It is has been noted that for cool DAs where spectroscopy is suspect, estimates of temperature and gravity based on photometry alone do not seem to show any systematic increase in gravity and mass (Engelbrecht & Koester 2007). Finally, Table 2 also lists the dominant photospheric composition, either H-rich or He-rich. These determinations are for the most part taken from the references given in the table.

Table 3 lists the trigonometric distance measurements and our photometric distances for the stars in our sample. Where possible we also have estimated photometric distances for each H-rich star (DA and DC and DAZ stars). In contrast to the empirical color–magnitude relation based distance estimates used by HOS, we have employed much more precise distance estimates based on spectroscopic temperatures and surface gravities and multi-band synthetic photometry. The synthetic photometry technique is fully described in HBG and has been critically verified against trigonometric parallaxes. Briefly, we use the precise photometric calibrations of DA stars on the Vega Hubble Space Telescope (HST) photometric scale, and calculate distance moduli in various available photometric bands; UBVRI, 2MASS JHK_s, and SDSS ugriz. It is a prerequisite that these multi-band distance moduli mutually agree for a distance to be adopted. Many of the photometric distances in Table 3 are taken directly from HBG. We also provide a final adopted distance and uncertainty using either the trigonometric or photometric distance, or the weighted mean of the two.

WD 0121−429. Subasavage et al. (2007) have noted Zeeman splitting in the Hα and Hβ lines of this star. A preliminary trigonometric parallax distance of 17.7 ± 0.7 pc and an implied mass of 0.43 ± 0.3 M_☉ are given, raising the possibility that this star could be a helium core white dwarf and a member of a double degenerate system.

WD 0135−052. This is a well-known unresolved double-degenerate spectroscopic system composed of two DA stars of similar temperature (Saffer et al. 1988). It thus appears over luminous relative to its temperature and gravity.

WD 0310−668. Kawka et al. (2007) reported a possible detection of a 6 kG magnetic field in this star. We follow Kawka et al. in not including this star among the white dwarfs with known magnetic fields.

WD 0413−077. 40 Eri B is a well-known DA white dwarf with well-determined but inconsistent Yale and Hipparcos trigonometric parallaxes. Valyvin et al. (2003) report the existence of a small magnetic field of a few kG. We follow Kawka et al. (2007) and include this star among the known magnetic white dwarfs.

WD 0423+120. The parallax and photometric distances strongly disagree (17.36 versus 11.88 pc respectively) making the star appear overly luminous. This strongly suggests that the star may be a double degenerate. If we assume it is composed of two equally luminous white dwarfs, the photometric distance becomes 16.8 pc, in good agreement with the parallax distance.

WD 0644+375. The parallax distance and the photometric distances (15.41 ± 0.70 and 18.42 ± 0.28, respectively) are not in satisfactory agreement. The Hipparcos and the Yale parallaxes both agree and therefore would seem to be secure. This leaves the photometric distance estimate in doubt. The star has three highly consistent spectroscopic determinations of its temperature and gravity, yet the star appears under luminous by about 43%.

WD 0743−336. This is a Sirius-like system composed of a G0V primary, HR 3018 (Kunkel et al. 1984).

WD 0806−661. This is an extreme DQ star for which no reliable spectroscopic analysis presently exists. The assumed mass is the mean of the DQ star masses found by Dufour et al. (2005). Although the photometric distance to this star is 21.1 ± 3.5 pc, the preliminary photometric distance is within 20 pc.

WD 0839−327. Bragaglia et al. (1990) noted possible radial variations in the DA star and suggested that it was a double degenerate system. Kawka et al. (2007) determined a photometric distance of 7 pc for this star, significantly less than the trigonometric parallax estimate of 8.87 ± 0.77 pc and estimated the temperature and absolute magnitude of a putative degenerate companion. We find a high level of consistency between our photometric distance (8.48 ± 0.37 pc) and the trigonometric distance. If this star is a double-degenerate system, then the companion must be very cool and faint.

WD 1009−184. A newly recognized Sirius-like system. The primary LHS 2231 is a K7V star (Hawley et al. 1996). Our distance comes from the Hipparcos parallax of the primary. Recent RECONS measurements of WD 1009−184 show it to be a wide common proper motion of companion LHS 2231.

WD 1033+714. There are relatively little observational data on this star. It is listed as a DC9 and its distance is determined photometrically in Liebert et al. (1988). We have used the available BVRI and 2MASS JHK_s data to re-estimate the effective temperature of 5000 ± 500 K and to determine the distance, based on the assumption that the gravity is log g = 8.0 ± 0.3.

WD 1124+595. GD 309 (SDSS_J112652.43+591917.0) is a DA with a photometric distance of less than 20 pc. It is not presently listed as a white dwarf in The White Dwarf Catalog. ⁵ Eisenstein et al. (2006) find GD 309 to be a 10,500 K DA white dwarf with a g-band magnitude of 15.2. The photometric distance calculated by HBG is 17.9 pc.

WD 1132−325. The distance for the white dwarf in this Sirius-like system comes from the Hipparcos parallax of the K0V companion (VB 4, LHS 308, HIP 56472). The spectral type and parameters of the white dwarf are uncertain; Henry et al. (2002) suggest it is a DC.

WD 1544−377. This object is a white dwarf in a Sirius-like system containing the G6V common proper motion companion HR5865 found by Luyten (1969) and studied by Wegner (1973).

WD 1620−391. There is an inconsistency between the Hipparcos and the Yale parallaxes for this DA white dwarf. Hipparcos gives π = 78.04 ± 2.07 mas while Yale gives π = 65.5 ± 7.1 mas. Significantly, this discrepancy is also reflected in the parallaxes of the G5 V common proper motion companion (HR 6094) of WD 1620−391, where the corresponding values are 77.69 ± 0.76 mas and 63.5 ± 10.1 mas. A weighted mean of the more precise Hipparcos parallaxes for both stars gives a distance estimate for the white dwarf of 12.87 ± 0.05 pc. This distance is fully consistent with the photometric distance of 13.03 ± 0.03 pc.). Adopting the mean Yale parallax of the system yields a distance of 15.44 pc which would imply that the star is overluminous by a factor of 1.5. WD 1620−391 is a well-studied DA white dwarf used as a spectrophotometric standard star, and there is no spectroscopic or other evidence that it is in fact overluminous. Indeed, it is possible to match its entire spectral energy distribution with a single model atmosphere from the extreme ultraviolet to the near infrared (Sing et al. 2002). We conclude that the Yale parallaxes for WD 1620−391 and HR 6094 are underestimates.

WD 1814+134. This is a high proper motion white dwarf identified as LSR J1817+1328 in Lépine et al. (2003), who estimated a distance of 18 ± 9 pc. Lépine (2007, private communication) has obtained a preliminary parallax that is consistent with the photometric distance of 15.6 ± 2.5 pc found by Subasavage et al. (2007).

WD 2007−303. Jordan et al. (2007) note a possible 2σ detection of a 2.4 kG magnetic field in this star. However, until this can be confirmed, we list it as non-magnetic.

WD 2008−600. Subasavage et al. (2007) estimate a photometric temperature (5078 ± 221 K) for this DC star, as well as a preliminary trigonometric parallax of 17.1 ± 0.4 pc.

WD 2048+263. BRL suspected this to be a double-degenerate system. Their conclusion was based on the low gravity and mass, as well as the suspected dilution of the Balmer Hα profile of the visible DA white dwarf by a possible DC companion. We find the photometric distance to match the trigonometric distance. Thus, the star does not appear over-luminous, so the contribution from any putative companion appears to be minimal. Therefore we list the star as single.

WD 2138−332. This is a calcium-rich DZ white dwarf. Subasavage et al. (2007) estimate a photometric temperature (7188 ± 291 K) for this DZ star, as well as a photometric distance of 17.3 ± 2.7 pc.

WD 2226−754. This is a member of a double-degenerate system which includes WD 2226−755. Our independent photometric distances for the two stars are 12.8 ± 2.2 and 14.0 ± 2.2 pc, respectively. We have not formally included WD 2226−754 in our list of stars within 13 pc because preliminary trigonometric parallaxes indicate that both stars lie somewhat beyond 13 pc.

As in HOS, we display the distribution in celestial coordinates of the local sample on an equal-area Hammer–Aitoff projection (see Figure 1). HOS found a ∼5:4 preponderance of stars in the northern hemisphere versus those in the southern hemisphere for 109 stars. With 126 stars we find a slightly smaller ratio, with 68 stars north of the equator versus 58 south. We have also calculated the location of the centroid of the local sample: α = 29.9° and δ = +37.3° with a distance of 1.98 pc. The expected offset distance for 126 stars, based on a fully isotropic distribution, is 1.38 pc. Thus, there is a ∼2σ displacement in a generally northward direction with respect to the Sun.

Zoom In Zoom Out Reset image size

HOS argued that the sample of known white dwarfs within a radius of 13 pc was complete. Since that time six stars from that sample (see the Appendix) either moved outside the 13 pc boundary, or turned out not to be white dwarfs. Three new stars have been added to the 13 pc sample. One of these, WD 0000−345, was in the HOS sample but simply migrated over the 13 pc boundary. However, the other two, WD 0821−688 and WD 1202−232, are new discoveries from the RECONS program (Subasavage et al. 2007). Their photometry indicates these objects are closer than 13 pc. These recent discoveries indicate that the 13 pc sample may not be fully complete and that additional stars may be found.

Here, our space density is not explicitly based on the integer count of stars within 13 pc, but rather the normalization of the +3 log–log slope in Figure 2 with respect to the cumulative plot of the likely number of stars (based on the sum of the distance-probability distributions of the stars). Plotting probabilities rather than star numbers also mitigates the net effect of any migrations across the 13 pc boundary due to future improved distance estimates. Likewise, the uncertainty in the space density is defined to be consistent with the band of N^1/2 statistics of the sample shown in Figure 2. Any addition (or loss) of a few stars from the 13 pc sample will thus cause a proportional change in the mean stellar density, but will not significantly alter the uncertainty of the sample. Only by extending completeness out to larger distances and including more stars will the estimate of the space density be improved substantially. This situation may very well be realized in the next few years (see Section 5).

Zoom In Zoom Out Reset image size

HOS estimated that the 2002 sample was 65% complete out to 20 pc. Based on our slightly larger sample and correspondingly smaller space density (see Section 4.2), we now estimate that the current 20 pc sample is ∼80% complete and that there are ∼33 ± 13 white dwarfs remaining to be discovered between 13 and 20 pc. This is shown graphically in Figure 2 were we show the cumulative log ΣN versus log distance plot, which is compared to the expected complete distribution with a slope of +3 and normalized at 13 pc. Some of these potential new local sample members are no doubt among the stars listed in Table 4. In plotting the cumulative distribution in Figure 2, we include the stars from Table 4 (and their distance uncertainties). We conclude from this comparison that there are likely 130.7 stars within 20 pc among those listed in both Tables 3 and 4. If we omit the stars in Table 4, and only consider those in Table 3, then the likely number of stars drops to 120.

Using the adopted distances in Table 3 (and counting unresolved double degenerates twice) we have computed a cumulative log ΣN versus log distance plot (Figure 2) and compared the resulting distance distribution with a straight line of slope 3.0 expected for a distribution of stars with a uniform space density of 4.8 ± 0.5 × 10⁻³ stars pc⁻³. This is to be compared with the space density of 5.0 ± 0.7 × 10⁻³ pc⁻³ determined from the original HOS sample. Interestingly, our revised space density is in good agreement with the value of 4.6 ± 0.5 × 10⁻³ stars pc⁻³ obtained by Harris et al. (2006) by integrating their detailed SDSS white dwarf luminosity function. We find a total of 44 white dwarfs (including double degenerates) within 13 pc (compared with 46 in HOS). The uncertainty in the space density is due to the N^1/2 Poisson statistics.

The mass density of white dwarfs corresponding to the space density in Section 4.2 is 3.2 ± 0.5 × 10⁻³ M_☉ pc⁻³. This estimate is based on the mean mass of the local sample. For the 126 stars in Table 2 the mean mass is 0.665 M_☉, which is essentially the same as the value of 0.65 M_☉ found by HOS. The corresponding mean surface gravity is found to be 8.132.

Both the mean mass and gravity are significantly larger than values typically found for large samples of hot DA stars. For example, LBH find a mean mass of 0.603 M_☉ and a mean surface gravity of 7.883 from a sample of 298 DA white dwarfs with temperatures above 13,000 K. As mentioned in Section 2.2, our local sample consists largely of cool white dwarfs where spectroscopic gravities and masses where spectroscopic determinations regularly produce systematically larger estimates. In order to investigate if there is such a temperature effect evident in our data we have examined several subsamples of the local population. First, following LBH, we split the sample into two groups that have temperature estimates above and below 13,000 K. Interestingly, we find that for the 13 hot DA stars (all with spectroscopic gravities and masses) the mean mass is 0.660 M_☉—essentially the same as the mean entire sample. However, 13 stars is a small number and perhaps the mean mass will diminish if this sample size is eventually increased by extending the boundary, from 20 to 25 or 30 pc. Nevertheless, the present sample does not show the expected temperature-related mass dependence found by LBH and Bergeron et al. (2007). The 35 stars for which we directly use the BLR masses also yield a mean mass of 0.676 M_☉. Recall that stars in BLR have masses that are determined with respect to trigonometric parallaxes. BLR find a mean mass of 0.65 M_☉ for their entire sample of such stars. However, BLR also find a difference between the mean-mass of the H-rich and He-rich stars, with 0.61 M_☉ and 0.72 M_☉, respectively. BLR attribute the lower mass of the H-rich stars to the inclusion of low mass (He-core) white dwarfs which are the product of common envelope evolution. In the local sample we find relatively equal mean masses between the H-rich and He-rich stars, with 0.68 M_☉ and 0.64 M_☉, respectively. The only subsample of stars in Table 2 with a somewhat smaller mean mass is the DQ and DZ white dwarf, respectively taken from Dufour et al. (2005) and Dufour et al. (2007). For these 12 stars, which also employ trigonometric parallax determined masses, we find a mean mass of 0.60 M_☉. Another sample of stars with lower masses in Table 2 is that observed photometrically by Subasavage et al. (2007). The mean mass for this sample of 11 stars is 0.59 M_☉; however, this result is a simple consequence of assuming log g = 8.0 in the analysis of these stars. In summary, the relatively larger mean mass found here appears to be a genuine feature of the local sample and not an artifact of a systematic bias due to spectroscopy or any other feature of the analysis. The question raised by this higher average mass in the local sample is what parameter other than temperature makes the critical difference between the local sample and other analyses of larger samples which are not volume-limited? One possibility is that for a given temperature higher-mass stars have smaller radii and lower luminosities and are less likely to be included in magnitude-limited samples.

The frequency of binary systems among the local sample is of considerable interest. Thirty-one binary systems, including double degenerates, are present in the sample, giving a binary fraction of 25%. There are seven double-degenerate systems; three resolved and four unresolved, counting WD 0121−429 (Subasavage et al. 2007) as a likely unresolved system. Additionally, as mentioned in Section 2.4, WD 0423+120 is likely to be a double-degenerate system. This is approximately 6% of the entries in Table 1. Interestingly, there are now eight Sirius-like systems consisting of a white dwarf and main-sequence companions with spectral types A0V to K7V. All are resolved common proper motion pairs with six out of the eight at distances of ∼13 pc or closer. Although a small sample, this indicates that Sirius-like systems, containing K and earlier main-sequence stars, may prove at least as common as double-degenerate systems.

In Table 5 we describe the contents of the local sample in terms of various categories of spectral type and binary status. The bulk of the stars (54%) are hydrogen-rich DA or DAZ stars. There is only one He-rich star with an explicit DB spectral classification (WD 1977−077, DBQA5). There are, however, 14 stars with a DQ designation, which are He-rich with spectral features due to atomic and/or molecular carbon. Thus, in Table 2 there are 81 H-rich versus 40 He-rich white dwarfs. The DC stars, which show continuous or near continuous featureless spectra, make up 19% of the sample.

Magnetic degenerates that show either Zeeman splitting (DH) or polarization (DP) make up 14% of our sample. This is in good agreement with the results of Liebert et al. (2003) who suggested upwards of 10%, if additional low field strength stars are to be found. Kawka et al. (2003, 2007) looked carefully at the question of the magnetic fraction and searched for low field strength stars. Kawka et al. (2007) identified 15 magnetic white dwarfs within 20 pc. All of the Kawka et al. stars are included in our sample, but we have identified an additional star: WD 0121−429; Subasavage et al. (2007). Jordan et al. (2007) have also searched for low-level kG magnetic fields in a number of white dwarfs, including five DA stars in our local sample. These authors find few, if any, new examples and estimate that the fraction of low field kG stars to be 11–15%.

Eleven stars have been removed from the original HOS list. In Table A1 we list these stars with their original distance estimates and spectroscopic designations. We also list the references and reasons for the removal of these stars.