Extremely low mass (ELM) white dwarfs (WDs) with masses < 0.25 M sub(sm circle in cirle) 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 seven ELM WDs with follow-up observations, six are short-period binaries and four 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 an 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.
Nearby High-speed Stars in Gaia DR2 Bromley, Benjamin C.; Kenyon, Scott J.; Brown, Warren R. ...
Astrophysical journal/The Astrophysical journal,
11/2018, Letnik:
868, Številka:
1
Journal Article
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We investigate the nature of nearby (10-15 kpc) high-speed stars in the Gaia DR2 archive identified on the basis of parallax, proper motion and radial velocity. Together with a consideration of their ...kinematic, orbital, and photometric properties, we develop a novel strategy for evaluating whether high-speed stars are statistical outliers of the bound population or unbound stars capable of escaping the Galaxy. Out of roughly 1.5 million stars with radial velocities, proper motions, and 5 parallaxes, we identify just over 100 high-speed stars. Of these, only two have a nearly 100% chance of being unbound, with an indication that they are not just bound outliers; both are likely hyper-runaway stars. The rest of the high-speed stars are likely statistical outliers. We use the sample of high-speed stars to demonstrate that radial velocity alone provides a poor discriminant of nearby, unbound stars. However, these stars are efficiently identified from the tangential velocity, using just parallax and proper motion. Within the full Gaia DR2 archive of stars with 5 parallax and proper motion but no radial velocity, we identify a sample of 19 with speeds significantly larger than the local escape speed of the Milky Way based on tangential motion alone.
Abstract
We report the discovery of the brightest detached binary white dwarfs with periods less than an hour, which provide two new gravitational-wave verification binaries for the Laser ...Interferometer Space Antenna (LISA). The first one, SMSS J033816.16−813929.9 (hereafter J0338), is a 30.6 minute orbital period,
g
= 17.2 mag detached double white dwarf binary with a Gaia parallax measurement that places it at a distance of 533 pc. The observed radial velocity and photometric variability provide precise constraints on the system parameters. J0338 contains a 0.230 ± 0.015
M
⊙
white dwarf with a
0.38
−
0.03
+
0.05
M
⊙
companion at an inclination of 69° ± 9°. The second system, SDSS J063449.92+380352.2 (hereafter J0634), is a 26.5 minute orbital period,
g
= 17.0 mag detached double white dwarf binary at a distance of 435 pc. J0634 contains a
0.452
−
0.062
+
0.070
M
⊙
white dwarf with a
0.209
−
0.021
+
0.034
M
⊙
companion at an inclination of 37° ± 7°. The more massive white dwarf in J0634 is hotter than its companion, even though tidal dissipation is predicted to be relatively inefficient at such periods. This suggests that the more massive white dwarf formed last. J0338 and J0634 will be detected by LISA with a signal-to-noise ratio of 5 and 19, respectively, after four years. We identified these two systems based on their overluminosity and
u
-band photometry. Follow-up of
u
-band selected Gaia targets will likely yield additional LISA verification binaries.
Abstract
A recent analysis of the 100 pc white dwarf sample in the SDSS footprint demonstrated for the first time the existence of a well-defined ultracool—or IR-faint—white dwarf sequence in the ...Hertzsprung–Russell diagram. Here we take advantage of this discovery to enlarge the IR-faint white dwarf sample threefold. We expand our selection to the entire Pan-STARRS survey footprint as well as the Montreal White Dwarf Database 100 pc sample and identify 37 candidates with strong flux deficits in the optical. We present follow-up Gemini optical spectroscopy of 30 of these systems and confirm all of them as IR-faint white dwarfs. We identify an additional set of 33 objects as candidates based on their colors and magnitudes. We present a detailed model atmosphere analysis of all 70 newly identified IR-faint white dwarfs together with 35 previously known objects reported in the literature. We discuss the physics of model atmospheres and show that the key physical ingredient missing in our previous generation of model atmospheres was the high-density correction to the He
−
free–free absorption coefficient. With new model atmospheres calculated for the purpose of this analysis, we now obtain significantly higher effective temperatures and larger stellar masses for these IR-faint white dwarfs than the
T
eff
and
M
values reported in previous analyses, thus solving a two-decade-old problem. In particular, we identify in our sample a group of ultramassive white dwarfs in the Debye cooling phase with stellar parameters never measured before.
We assess the photometric variability of nine stars with spectroscopic Teff and log g values from the ELM Survey that locates 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 (RV) variations that 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 cut-off for surface reflection in a white dwarf, and this target is likely a high-amplitude δ Scuti pulsator with an overestimated surface gravity. We estimate the probability to be less than 0.0008 that the lack of measured RV 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 R -too large to be consistent with an ELM white dwarf. The only object for which our time series photometry adds support to ELM white dwarf classification is SDSS J105435.78−212155.9, which has consistent signatures of Doppler beaming and ellipsoidal variations. We conclude 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 present a detailed spectroscopic analysis of 61 low-mass white dwarfs and provide precise atmospheric parameters, masses, and updated binary system parameters based on our new model atmosphere ...grids and the most recent evolutionary model calculations. For the first time, we measure systematic abundances of He, Ca, and Mg for metal-rich, extremely low mass white dwarfs and examine the distribution of these abundances as a function of effective temperature and mass. Based on our preliminary results, we discuss the possibility that shell flashes may be responsible for the presence of the observed He and metals. We compare stellar radii derived from our spectroscopic analysis to model-independent measurements and find good agreement except for white dwarfs with T sub(eff) lap 10,000 K. We also calculate the expected gravitational wave strain for each system and discuss their significance to the eLISA space-borne gravitational wave observatory. Finally, we provide an update on the instability strip of extremely low mass white dwarf pulsators.
Three multi-carboxylic acid functionalised ligands have been designed, synthesised and utilised to prepare the new barium-based MOFs, MFM-510, -511, and -512, which show excellent stability to ...water-vapour. MFM-510 and MFM-511 show moderate proton conductivities (2.1 × 10
and 5.1 × 10
S cm
, respectively) at 99% RH and 298 K, attributed to the lack of free protons or hindered proton diffusion within the framework structures. In contrast, MFM-512, which incorporates a pendant carboxylic acid group directed into the pore of the framework, shows a two orders of magnitude enhancement in proton conductivity (2.9 × 10
S cm
). Quasi-elastic neutron scattering (QENS) suggests that the proton dynamics of MFM-512 are mediated by "free diffusion inside a sphere" confirming that incorporation of free carboxylic acid groups within the pores of MOFs is an efficient albeit synthetically challenging strategy to improve proton conductivity.
We present the discovery of 17 low-mass white dwarfs (WDs) in short-period (P < or =, slant 1 day) binaries. Our sample includes four objects with remarkable log g Asymptotically = to 5 surface ...gravities and orbital solutions that require them to be double degenerate binaries. All of the lowest surface gravity WDs have metal lines in their spectra implying long gravitational settling times or ongoing accretion. Notably, six of the WDs in our sample have binary merger times <10 Gyr. Four have > ~0.9 M sub(middot in circle) companions. If the companions are massive WDs, these four binaries will evolve into stable mass transfer AM CVn systems and possibly explode as underluminous supernovae. If the companions are neutron stars, then these may be millisecond pulsar binaries. These discoveries increase the number of detached, double degenerate binaries in the ELM Survey to 54; 31 of these binaries will merge within a Hubble time.
The merger fraction of ultramassive white dwarfs Kilic, Mukremin; Moss, Adam G; Kosakowski, Alekzander ...
Monthly Notices of the Royal Astronomical Society,
01/2023, Letnik:
518, Številka:
2
Journal Article
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ABSTRACT
We search for merger products among the 25 most massive white dwarfs in the Montreal White Dwarf Database 100 pc sample through follow-up spectroscopy and high-cadence photometry. We find an ...unusually high fraction, 40 per cent, of magnetic white dwarfs among this population. In addition, we identify four outliers in transverse velocity and detect rapid rotation in five objects. Our results show that $56^{+9}_{-10}$ per cent of the $M\approx 1.3\, {\rm M}_{\odot }$ ultramassive white dwarfs form through mergers. This fraction is significantly higher than expected from the default binary population synthesis calculations using the α prescription (with αλ = 2), and provides further support for efficient orbital shrinkage, such as with low values of the common-envelope efficiency.
We predict the distinctive three-dimensional space motions of hypervelocity stars (HVSs) and runaway stars moving in a realistic Galactic potential. For nearby stars with distances less than 10 kpc, ...unbound stars are rare; proper motions alone rarely isolate bound HVSs and runaways from indigenous halo stars. At large distances of 20-100 kpc, unbound HVSs are much more common than runaways; radial velocities easily distinguish both from indigenous halo stars. Comparisons of the predictions with existing observations are encouraging. Although the models fail to match observations of solar-type HVS candidates from SEGUE, they agree well with data for B-type HVS and runaways from other surveys. Complete samples of g <, ~ 20 stars with Gaia should provide clear tests of formation models for HVSs and runaways and will enable accurate probes of the shape of the Galactic potential.
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