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Register a new usersdA in SDSS DR12 are Overwhelmingly Not Extremely Low-Mass (ELM) White Dwarfs
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In a search for new white dwarfs in DR12 of the Sloan Digital Sky Survey, Kepler et al. (2016) found atmospheric parameters for thousands of objects with effective temperatures below 20,000 K and surface gravities between 5.5 < log(g) < 6.5. They classified these objects as cool subdwarfs -- sdA -- and speculated that many may be extremely low-mass (ELM) white dwarfs (helium-core white dwarfs with masses below 0.3 Msun). We present evidence -- using radial velocities, photometric colors, and reduced proper motions -- that the vast majority (>99%) of these objects are unlikely to be ELM white dwarfs. Their true identity remains an interesting question.
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We begin the search for extremely-low mass ($Mleq0.3M_{odot}$, ELM) white dwarfs (WDs) in the southern sky based on photometry from the VST ATLAS and SkyMapper surveys. We use a similar color-selection method as the Hypervelocity star survey. We switched to an astrometric selection once Gaia Data Release 2 became available. We use the previously known sample of ELM white dwarfs to demonstrate that these objects occupy a unique parameter space in parallax and magnitude. We use the SOAR 4.1m telescope to test the Gaia-based selection, and identify more than two dozen low-mass white dwarfs, including 6 new ELM white dwarf binaries with periods as short as 2 h. The better efficiency of the Gaia-based selection enables us to extend the ELM Survey footprint to the southern sky. We confirm one of our candidates, J0500$-$0930, to become the brightest ($G=12.6$ mag) and closest ($d=72$ pc) ELM white dwarf binary currently known. Remarkably, the Transiting Exoplanet Survey Satellite (TESS) full-frame imaging data on this system reveals low-level ($<0.1$%) but significant variability at the orbital period of this system ($P=9.5$ h), likely from the relativistic beaming effect. TESS data on another system, J0642$-$5605, reveals ellipsoidal variations due to a tidally distorted ELM WD. These demonstrate the power of TESS full-frame images in confirming the orbital periods of relatively bright compact object binaries.
Extremely low mass (ELM) white dwarfs (WDs) with masses <0.25 Msun are rare objects that result from compact binary evolution. Here, we present a targeted spectroscopic survey of ELM WD candidates selected by color. The survey is 71% complete and has uncovered 18 new ELM WDs. Of the 7 ELM WDs with follow-up observations, 6 are short-period binaries and 4 have merger times less than 5 Gyr. The most intriguing object, J1741+6526, likely has either a pulsar companion or a massive WD companion making the system a possible supernova Type Ia or .Ia progenitor. The overall ELM Survey has now identified 19 double degenerate binaries with <10 Gyr merger times. The significant absence of short orbital period ELM WDs at cool temperatures suggests that common envelope evolution creates ELM WDs directly in short period systems. At least one-third of the merging systems are halo objects, thus ELM WD binaries continue to form and merge in both the disk and the halo.
Extremely low-mass white dwarfs (ELM WDs) are helium WDs with a mass less than $sim$$0.3rm;M_odot$. Most ELM WDs are found in double degenerates (DDs) in the ELM Survey led by Brown and Kilic. These systems are supposed to be significant gravitational-wave sources in the mHz frequency. In this paper, we firstly analyzed the observational characteristics of ELM WDs and found that there are two distinct groups in the ELM WD mass and orbital period plane, indicating two different formation scenarios of such objects, i.e. a stable Roche lobe overflow channel (RL channel) and common envelope ejection channel (CE channel). We then systematically investigated the formation of ELM WDs in DDs by a combination of detailed binary evolution calculation and binary population synthesis. Our study shows that the majority of ELM WDs with mass less than $0.22rm;M_odot$ are formed from the RL channel. The most common progenitor mass in this way is in the range of $1.15-1.45rm;M_odot$ and the resulting ELM WDs have a peak around $0.18rm;M_odot$ when selection effects are taken into account, consistent with observations. The ELM WDs with a mass larger than $0.22rm;M_odot$ are more likely to be from the CE channel and have a peak of ELM WD mass around $0.25rm;M_odot$ which needs to be confirmed by future observations. By assuming a constant star formation rate of 2$rm;M_odot yr^{-1}$ for a Milky Way-like galaxy, the birth rate and local density are $5times10^{-4}rm;yr^{-1}$ and $1500rm;kpc^{-3}$, respectively, for DDs with an ELM WD mass less than $0.25rm;M_odot$.
We assess the photometric variability of nine stars with spectroscopic Teff and log(g) values from the ELM Survey that locate them near the empirical extremely low-mass (ELM) white dwarf instability strip. We discover three new pulsating stars: SDSS J135512.34+195645.4, SDSS J173521.69+213440.6 and SDSS J213907.42+222708.9. However, these are among the few ELM Survey objects that do not show radial velocity variations to confirm the binary nature expected of helium-core white dwarfs. The dominant 4.31-hr pulsation in SDSS J135512.34+195645.4 far exceeds the theoretical cutoff for surface reflection in a white dwarf, and this target is likely a high-amplitude delta Scuti pulsator with an overestimated surface gravity. We estimate the probability to be less than 0.0008 that the lack of measured radial velocity variations in four of eight other pulsating candidate ELM white dwarfs could be due to low orbital inclination. Two other targets exhibit variability as photometric binaries. Partial coverage of the 19.342-hr orbit of WD J030818.19+514011.5 reveals deep eclipses that imply a primary radius > 0.4 solar radii--too large to be consistent with an ELM white dwarf. The only object for which our time series photometry adds support to the ELM white dwarf classification is SDSS J105435.78-212155.9, with consistent signatures of Doppler beaming and ellipsoidal variations. We interpret that the ELM Survey contains multiple false positives from another stellar population at Teff < 9000 K, possibly related to the sdA stars recently reported from SDSS spectra.
We have conducted a search for pulsar companions to 15 low-mass white dwarfs (LMWDs; M < 0.4 M_Sun) at 820 MHz with the NRAO Green Bank Telescope (GBT). These LMWDs were spectroscopically identified in the Sloan Digital Sky Survey (SDSS), and do not show the photometric excess or spectroscopic signature associated with a companion in their discovery data. However, LMWDs are believed to evolve in binary systems and to have either a more massive WD or a neutron star as a companion. Indeed, evolutionary models of low-mass X-ray binaries, the precursors of millisecond pulsars (MSPs), produce significant numbers of LMWDs (e.g., Benvenuto & De Vito 2005), suggesting that the SDSS LMWDs may have neutron star companions. No convincing pulsar signal is detected in our data. This is consistent with the findings of van Leeuwen et al. (2007), who conducted a GBT search for radio pulsations at 340 MHz from unseen companions to eight SDSS WDs (five are still considered LMWDs; the three others are now classified as ordinary WDs). We discuss the constraints our non-detections place on the probability P_MSP that the companion to a given LMWD is a radio pulsar in the context of the luminosity and acceleration limits of our search; we find that P_MSP < 10 +4 -2 %.
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