arXiv subjects

Warren R. Brown

Publications and source records attributed to Warren R. Brown.

At least 73 records · Page 4Linked to original sources

arXiv2015

Proper Motions and Trajectories for 16 Extreme Runaway and Hypervelocity Stars

We measure proper motions with the Hubble Space Telescope for 16 extreme radial velocity stars, mostly unbound B stars in the Milky Way halo. Twelve of these stars have proper motions statistically consistent with zero, and thus have radial trajectories statistically consistent with a Galactic center "hypervelocity star" origin. The trajectory of HE 0437-5439 is consistent with both Milky Way and Large Magellanic Cloud origins. A Galactic center origin is excluded at 3-sigma confidence for two of the lowest radial velocity stars in our sample, however. These two stars are probable disk runaways and provide evidence for ~500 km/s ejections from the disk. We also measure a significant proper motion for the unbound sdO star US 708. Its 1,000 km/s motion is in some tension with proposed supernova ejection models, but can be explained if US 708 was ejected from the stellar halo. In the future, we expect Gaia will better constrain the origin of these remarkable unbound stars.

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arXiv2014

PSR J1738+0333: The First Millisecond Pulsar + Pulsating White Dwarf Binary

We report the discovery of the first millisecond pulsar with a pulsating white dwarf companion. Following the recent discoveries of pulsations in extremely low-mass (ELM, <0.3 Msol) white dwarfs (WDs), we targeted ELM WD companions to two millisecond pulsars with high-speed Gemini photometry. We find significant optical variability in PSR J1738+0333 with periods between roughly 1790-3060 s, consistent in timescale with theoretical and empirical observations of pulsations in 0.17 Msol He-core ELM WDs. We additionally put stringent limits on a lack of variability in PSR J1909-3744, showing this ELM WD is not variable to <0.1 per cent amplitude. Thanks to the accurate distance and radius estimates from radio timing measurements, PSR J1738+0333 becomes a benchmark for low-mass, pulsating WDs. Future, more extensive time-series photometry of this system offers an unprecedented opportunity to constrain the physical parameters (including the cooling age) and interior structure of this ELM WD, and in turn, the mass and spin-down age of its pulsar companion.

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arXiv2014

Predicted Space Motions for Hypervelocity and Runaway Stars: Proper Motions and Radial Velocities for the GAIA Era

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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arXiv2014

Precise Atmospheric Parameters for the Shortest Period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations

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 those white dwarfs with Teff < 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.

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arXiv2014

The Identification of RR Lyrae and Delta Scuti Stars from Variable GALEX Ultraviolet Sources

We identify the RR Lyrae and delta Scuti (DSCT) stars in three catalogs of GALEX variable sources. The NUV amplitude of RR Lyrae stars is about twice that in V-band, so we find a larger percentage of low amplitude variables than catalogs such as Abbas et al (2014). Interestingly, the (NUV-V)_0 color is sensitive to metallicity and can be used to distinguish between variables of the same period but differing [Fe/H]. This color is also more sensitive to T_eff than optical colors and can be used to identify the red edge of the instability gap. We find 8 DSCT stars, 17 RRc stars, 1 RRd star and 84 RRab stars in the GALEX variable catalogs of Welsh et al (2005) and Wheatley et al (2008). We also classify 6 DSCT stars, 5 RRc stars and 18 RRab stars among the 55 variable GALEX sources identified as "stars" or RR Lyraes in the catalog of Gezari et al (2013). We provide ephemerides and light curves for the 26 variables that were not previously known.

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arXiv2014

Radius constraints from high-speed photometry of 20 low-mass white dwarf binaries

We carry out high-speed photometry on 20 of the shortest-period, detached white dwarf binaries known and discover systems with eclipses, ellipsoidal variations (due to tidal deformations of the visible white dwarf), and Doppler beaming. All of the binaries contain low-mass white dwarfs with orbital periods less than 4 hr. Our observations identify the first eight tidally distorted white dwarfs, four of which are reported for the first time here, which we use to put empirical constraints on the mass-radius relationship for extremely low-mass (<0.30 Msun) white dwarfs. We also detect Doppler beaming in several of these binaries, which confirms the high-amplitude radial-velocity variability. All of these systems are strong sources of gravitational radiation, and long-term monitoring of those that display ellipsoidal variations can be used to detect spin-up of the tidal bulge due to orbital decay.

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arXiv2014

A New Gravitational Wave Verification Source

We report the discovery of a detached 20 min orbital period binary white dwarf. WD0931+444 (SDSS J093506.93+441106.9) was previously classified as a WD + M dwarf system based on its optical spectrum. Our time-resolved optical spectroscopy observations obtained at the 8m Gemini and 6.5m MMT reveal peak-to-peak radial velocity variations of 400 km/s every 20 min for the WD, but no velocity variations for the M dwarf. In addition, high-speed photometry from the McDonald 2.1m telescope shows no evidence of variability nor evidence of a reflection effect. An M dwarf companion is physically too large to fit into a 20 min orbit. Thus, the orbital motion of the WD is almost certainly due to an invisible WD companion. The M dwarf must be either an unrelated background object or the tertiary component of a hiearchical triple system. WD0931+444 contains a pair of WDs, a 0.32 Msol primary and a >0.14 Msol secondary, at a separation of >0.19 Rsol. After J0651+2844, WD0931+444 becomes the second-shortest period detached binary WD currently known. The two WDs will lose angular momentum through gravitational wave radiation and merge in <9 Myr. The log h ~ -22 gravitational wave strain from WD0931+444 is strong enough to make it a verification source for gravitational wave missions in the milli-Hertz frequency range, e.g. the evolved Laser Interferometer Space Antenna (eLISA), bringing the total number of known eLISA verification sources to nine.

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arXiv2014

Dusty WDs in the $WISE$ All Sky Survey $\cap$ SDSS

A recent cross-correlation between the SDSS DR7 White Dwarf Catalog with the Wide-Field Infrared Survey Explorer ($WISE$) all-sky photometry at 3.4, 4.6, 12, and 22 microns performed by Debes et al. (2011) resulted in the discovery of 52 candidate dusty white dwarfs (WDs). The 6'' $WISE$ beam allows for the possibility that many of the excesses exhibited by these WDs may be due to contamination from a nearby source, however. We present MMT$+$SWIRC $J$- and $H$-band imaging observations (0.5-1.5'' PSF) of 16 of these candidate dusty WDs and confirm that four have spectral energy distributions (SEDs) consistent with a dusty disk and are not accompanied by a nearby source contaminant. The remaining 12 WDs have contaminated $WISE$ photometry and SEDs inconsistent with a dusty disk when the contaminating sources are not included in the photometry measurements. We find the frequency of disks around single WDs in the $WISE$ $\cap$ SDSS sample to be 2.6-4.1%. One of the four new dusty WDs has a mass of $1.04 M_{\odot}$ (progenitor mass $5.4 M_{\odot}$) and its discovery offers the first confirmation that massive WDs (and their massive progenitor stars) host planetary systems.

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arXiv2014

MMT Hypervelocity Star Survey III: A Complete Survey of Faint B-type Stars in the Northern Milky Way Halo

We describe our completed spectroscopic survey for unbound hypervelocity stars (HVSs) ejected from the Milky Way. Three new discoveries bring the total number of unbound HVSs to 21. We place new constraints on the nature of HVSs and on their distances using moderate resolution MMT spectroscopy. Half of the HVSs are fast rotators; they are certain 2.5-4 Msun main sequence stars at 50 - 120 kpc distances. Correcting for stellar lifetime, our survey implies that unbound 2.5-4 Msun stars are ejected from the Milky Way at a rate of 1.5e-6 /yr. The observed HVSs are likely ejected continuously over the past 200 Myr and do not share a common flight time. The anisotropic spatial distribution of HVSs on the sky remains puzzling. Southern hemisphere surveys like SkyMapper will soon allow us to map the all-sky distribution of HVSs. Future proper motion measurements with Hubble Space Telescope and Gaia will provide strong constraints on origin. All existing observations are consistent with HVS ejections from encounters with the massive black hole in the Galactic center.

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arXiv2013

SDSS J074511.56+194926.5: Discovery of a Metal-Rich and Tidally Distorted Extremely Low Mass White Dwarf

We present the discovery of an unusual, tidally-distorted extremely low mass white dwarf (WD) with nearly solar metallicity. Radial velocity measurements confirm that this is a compact binary with an orbital period of 2.6975 hrs and a velocity semi-amplitude of K = 108.7 km/s. Analysis of the hydrogen Balmer lines yields an effective temperature of Teff = 8380 K and a surface gravity of log g = 6.21 that in turn indicate a mass of M = 0.16 Msol and a cooling age of 4.2 Gyr. In addition, a detailed analysis of the observed metal lines yields abundances of log Mg/H = -3.90, log Ca/H = -5.80, log Ti/H = -6.10, log Cr/H = -5.60, and log Fe/H = -4.50, similar to the sun. We see no evidence of a debris disk from which these metals would be accreted though the possibility cannot entirely be ruled out. Other potential mechanisms to explain the presence of heavy elements are discussed. Finally, we expect this system to ultimately undergo unstable mass transfer and merge to form a ~0.3-0.6 Msol WD in a few Gyr.

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arXiv2013

Found: The Progenitors of AM CVn and Supernovae .Ia

We present optical and X-ray observations of two tidally distorted, extremely low-mass white dwarfs (WDs) with massive companions. There is no evidence of neutron stars in our Chandra and XMM observations of these objects. SDSS J075141.18$-$014120.9 (J0751) is an eclipsing double WD binary containing a 0.19 Msol WD with a 0.97 Msol companion in a 1.9 h orbit. J0751 becomes the fifth eclipsing double WD system currently known. SDSS J174140.49+652638.7 (J1741) is another binary containing a 0.17 Msol WD with an unseen M > 1.11 Msol WD companion in a 1.5 h orbit. With a mass ratio of ~0.1, J1741 will have stable mass transfer through an accretion disk and turn into an interacting AM Canum Venaticorum (AM CVn) system in the next ~160 Myr. With a mass ratio of 0.2, J0751 is likely to follow a similar evolutionary path. These are the first known AM CVn progenitor binary systems and they provide important constraints on the initial conditions for AM CVn. Theoretical studies suggest that both J0751 and J1741 may create thermonuclear supernovae in ~10^8 yr, either .Ia or Ia. Such explosions can account for ~1% of the Type Ia supernova rate.

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arXiv2013

A new class of pulsating white dwarf of extremely low mass: the fourth and fifth members

We report the discovery of two new pulsating extremely low-mass (ELM) white dwarfs (WDs), SDSS J161431.28+191219.4 (hereafter J1614) and SDSS J222859.93+362359.6 (hereafter J2228). Both WDs have masses <0.25 Msun and thus likely harbor helium cores. Spectral fits indicate these are the two coolest pulsating WDs ever found. J1614 has Teff = 8880 +/- 170 K and log g = 6.66 +/- 0.14, which corresponds to a roughly 0.19 Msun WD. J2228 is considerably cooler, with a Teff = 7870 +/- 120 K and log g = 6.03 +/- 0.08, which corresponds to a roughly 0.16 Msun WD, making it the coolest and lowest-mass pulsating WD known. There are multiple ELM WDs with effective temperatures between the warmest and coolest known ELM pulsators that do not pulsate to observable amplitudes, which questions the purity of the instability strip for low-mass WDs. In contrast to the CO-core ZZ Ceti stars, which are believed to represent a stage in the evolution of all such WDs, ELM WDs may not all evolve as a simple cooling sequence through an instability strip. Both stars exhibit long-period variability (1184-6235 s) consistent with non-radial g-mode pulsations. Although ELM WDs are preferentially found in close binary systems, both J1614 and J2228 do not exhibit significant radial-velocity variability, and are perhaps in low-inclination systems or have low-mass companions. These are the fourth and fifth pulsating ELM WDs known, all of which have hydrogen-dominated atmospheres, establishing these objects as a new class of pulsating WD.

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arXiv2013

The Origin of HVS17, an Unbound Main Sequence B Star at 50 kpc

We analyze Keck ESI spectroscopy of HVS17, a B-type star traveling with a Galactic rest frame radial velocity of +445 km/s in the outer halo of the Milky Way. HVS17 has the projected rotation of a main sequence B star and is chemically peculiar, with solar iron abundance and sub-solar alpha abundance. Comparing measured T_eff and logg with stellar evolution tracks implies that HVS17 is a 3.91 +-0.09 Msun, 153 +-9 Myr old star at a Galactocentric distance of r=48.5 +-4.6 kpc. The time between its formation and ejection significantly exceeds 10 Myr and thus is difficult to reconcile with any Galactic disk runaway scenario involving massive stars. The observations are consistent, on the other hand, with a hypervelocity star ejection from the Galactic center. We show that Gaia proper motion measurements will easily discriminate between a disk and Galactic center origin, thus allowing us to use HVS17 as a test particle to probe the shape of the Milky Way's dark matter halo.

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arXiv2013

The Runaway Binary LP 400-22 is Leaving the Galaxy

We present optical spectroscopy, astrometry, radio, and X-ray observations of the runaway binary LP 400-22. We refine the orbital parameters of the system based on our new radial velocity observations. Our parallax data indicate that LP 400-22 is significantly more distant (3 sigma lower limit of 840 pc) than initially predicted. LP 400-22 has a tangential velocity in excess of 830 km/s; it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a recycled millisecond pulsar companion, indicating that LP 400-22 is a double white dwarf system. This essentially rules out a supernova runaway ejection mechanism. Based on its orbit, a Galactic center origin is also unlikely. However, its orbit intersects the locations of several globular clusters; dynamical interactions between LP 400-22 and other binary stars or a central black hole in a dense cluster could explain the origin of this unusual binary.

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arXiv2013

Discovery of an ultramassive pulsating white dwarf

We announce the discovery of the most massive pulsating hydrogen-atmosphere (DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) = 9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425-595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains SNe Ia progenitor systems.

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arXiv2013

The ELM Survey. V. Merging Massive White Dwarf Binaries

We present the discovery of 17 low mass white dwarfs (WDs) in short-period P<1 day binaries. Our sample includes four objects with remarkable log(g)~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 on-going accretion. Notably, six of the WDs in our sample have binary merger times <10 Gyr. Four have >=0.9 Msun 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 milli-second 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.

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arXiv2013

Variability of Hypervelocity Stars

We present time-series photometry of 11 hypervelocity stars (HVSs) to constrain their nature. Known HVSs are mostly late-B spectral type objects that may be either main-sequence (MS) or evolved blue horizontal branch (BHB) stars. Fortunately, MS stars at these effective temperatures, $T_{eff} \sim$ 12,000 K, are good candidates for being a class of variable stars known as slowly pulsating B stars (SPBs). We obtained photometry on four nights at the WIYN 3.5 m telescope, and on six nights on the 2.4 m Hiltner telescope. Using sinusoidal fits, we constrain four of our targets to have periods between $P \sim 0.2 - 2$ days, with a mean value of 0.6 days. Our amplitudes vary between $A = 0.5 - 3$%. This suggests that these four HVSs are SPBs. We discuss a possible origin for these stars, and why further observations are necessary.

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arXiv2013

Discovery of pulsations, including possible pressure modes, in two new extremely low mass, He-core white dwarfs

We report the discovery of the second and third pulsating extremely low mass white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 Msun and effective temperatures below 10,000 K, establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti stars). The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic (p-mode) pulsations in any WD, which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g-modes also present. J1112 is a Teff = 9590 +/- 140 K and log(g) = 6.36 +/- 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792-2855 s. In this star we also see short-period variability, strongest at 134.3 s, well short of expected g-modes for such a low-mass WD. The other new pulsating WD, J1518, is a Teff = 9900 +/- 140 K and log(g) = 6.80 +/- 0.05 WD. The light curve of J1518 is highly non-sinusoidal, with at least seven significant periods between 1335-3848 s. Consistent with the expectation that ELM WDs must be formed in binaries, these two new pulsating He-core WDs, in addition to the prototype SDSS J184037.78+642312.3, have close companions. However, the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars.

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