Warren R Brown | Smithsonian Institution (original) (raw)
Papers by Warren R Brown
Ground-based and Airborne Instrumentation for Astronomy IV, 2012
The Astrophysical Journal, 2007
Hypervelocity stars (HVSs) are stars ejected completely out of the Milky Way by three-body intera... more Hypervelocity stars (HVSs) are stars ejected completely out of the Milky Way by three-body interactions with the massive black hole in the Galactic center. We describe 643 new spectroscopic observations from our targeted survey for HVSs. We find a significant (3.5 � ) excess of B-type stars with large velocities +275 km s � 1 10 kpc that are most plausibly explained as a new class of HVSs: stars ejected from the Galactic center on bound orbits. If a Galactic center ejection origin is correct, the distribution of HVSs on the sky should be anisotropic for a survey complete to a fixed limiting apparent magnitude. The unbound HVSs in our survey have a marginally anisotropic distribution on the sky, consistent with the Galactic center ejection picture. Subject headingg Galaxy: center — Galaxy: halo — Galaxy: kinematics and dynamics — Galaxy: stellar content — stars: early-type
The Astrophysical Journal, 2012
We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so... more We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Our survey, now half complete, has successfully identified a total of four probable HVSs plus a number of other unusual objects. Here we report the most recently discovered two HVSs: SDSS J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least +508±12 and +418±10 km s, respectively. The other late B-type objects in our survey are consistent with a population of post-main sequence stars or blue stragglers in the Galactic halo, with mean metallicity [Fe/H]Wk = −1.3 and velocity dispersion 108 ± 5 km s . Interestingly, the velocity distribution shows a tail of objects with large positive velocities that may be a mix of low-velocity HVSs and high-velocity runaway stars. Our survey also includes a number of DA white dwarfs with unusually red colors...
We describe new methods of analysis that have been developed in order to make optimal use of the ... more We describe new methods of analysis that have been developed in order to make optimal use of the stellar spectral data obtained during the course of follow-up medium-resolution observations of color-selected stars from the ``Century Survey.'' The initial selection of stars in the stellar component of the Century Survey is based on broadband V and R photometry covering a strip
The Smithsonian Widefield Infrared Camera (SWIRC) is a Y -, J-, and H-band imager for the f/5 MMT... more The Smithsonian Widefield Infrared Camera (SWIRC) is a Y -, J-, and H-band imager for the f/5 MMT. Proposed in May 2003 and commissioned in June 2004, the goal of the instrument was to deliver quickly a wide field-of-view instrument with minimal optical elements and hence high throughput. The trade-off was to sacrifice K-band capability by not having an internal, cold Lyot stop. We describe SWIRC’s design and capabilities, and discuss lessons learned from the thermal design and the detector mount, all of which have been incorporated into the upcoming MMT & Magellan Infrared Spectrograph.
We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 03... more We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and supernova contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week which are unrelated to the presence of a supernova. The early afterglow light curve also shows deviations from the typical power-law decay. A supernova spectrum is first detectable ∼ 7 days after the burst and dominates the light after ∼ 11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the supernova explosion is very close to the observed time of the G...
The Astrophysical Journal Letters
We report the discovery of the brightest detached binary white dwarfs with periods less than an h... more 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 min 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 +0.05 −0.03 M companion at an inclination of 69 ± 9 •. The second system, SDSS J063449.92+380352.2 (hereafter J0634), is a 26.5 min orbital period, g = 17.0 mag detached double white dwarf binary at a distance of 435 pc. J0634 contains a 0.452 +0.070 −0.062 M white dwarf with a 0.209 +0.034 −0.021 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.
Nature
Astronomers have discovered thousands of planets outside the solar system 1 , most of which orbit... more Astronomers have discovered thousands of planets outside the solar system 1 , most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star 2 , but more distant planets can survive this phase and remain in orbit around the white dwarf 3, 4. Some white dwarfs show evidence for rocky material floating in their atmospheres 5 , in warm debris disks 6-9 , or orbiting very closely 10-12 , which has been interpreted as the debris of rocky planets that were scattered inward and tidally disrupted 13. Recently, the discovery of a gaseous debris disk with a composition similar to ice giant planets 14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether the planets can survive the journey. So far, the detection of intact planets in close orbits around white dwarfs has remained elusive. Here, we report the discovery of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95% confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red-giant phase and shrinks due to friction. In this case, though, the low mass and relatively long orbital period of the planet candidate make common-envelope evolution less likely. Instead, the WD 1856+534 system seems to demonstrate that giant planets can be scattered into tight orbits without being tidally disrupted, and motivates searches for smaller transiting planets around white dwarfs. WD 1856+534 (hereafter, WD 1856 for brevity) is located 25 parsecs away in a visual triple star system. It has an effective temperature of 4710 ± 60 Kelvin and became a white dwarf 5.9 ± 0.5 billion years ago, based on theoretical models for how white dwarfs cool over time. The total system age, including the star's main sequence lifetime, must be older. Table 1 gives the other key parameters of the star. WD 1856 is one of thousands of white dwarfs that was targeted for observations with NASA's Transiting Exoplanet Survey Satellite (TESS), in order to search for any periodic dimming events caused by planetary transits. A statistically significant transit-like event was detected by the TESS Science Processing Operations Center (SPOC) pipeline based
The Astrophysical Journal
We begin the search for extremely-low mass (M ≤ 0.3M , ELM) white dwarfs (WDs) in the southern sk... more We begin the search for extremely-low mass (M ≤ 0.3M , ELM) white dwarfs (WDs) in the southern sky based on photometry from the VST ATLAS and SkyMapper surveys. We use a similar colorselection 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.
The Astrophysical Journal
We present the final sample of 98 detached double white dwarf (WD) binaries found in the Extremel... more We present the final sample of 98 detached double white dwarf (WD) binaries found in the Extremely Low Mass (ELM) Survey, a spectroscopic survey targeting <0.3 M ⊙ He-core WDs completed in the Sloan Digital Sky Survey footprint. Over the course of the survey we observed ancillary low mass WD candidates like GD 278, which we show is a P = 0.19 d double WD binary, as well as candidates that turn out to be field blue straggler/subdwarf A-type stars with luminosities too large to be WDs given their Gaia parallaxes. Here, we define a clean sample of ELM WDs that is complete within our target selection and magnitude range 15 < g 0 < 20 mag. The measurements are consistent with 100% of ELM WDs being 0.0089 < P < 1.5 d double WD binaries, 35% of which belong to the Galactic halo. We infer these are mostly He+CO WD binaries given the measurement constraints. The merger rate of the observed He+CO WD binaries exceeds the formation rate of stable mass transfer AM CVn binaries by a factor of 25, and so the majority of He+CO WD binaries must experience unstable mass transfer and merge. The shortest-period systems like J0651+2844 are signature LISA verification binaries that can be studied with gravitational waves and light.
Monthly Notices of the Royal Astronomical Society
We present minute cadence photometry of 31 732 point sources observed in one 3 rmdeg2\rm deg^{2}rmdeg2 DECa... more We present minute cadence photometry of 31 732 point sources observed in one 3 rmdeg2\rm deg^{2}rmdeg2 DECam pointing centred at RA = 09:03:02 and Dec. = −04:35:00 over eight consecutive half-nights. We use these data to search for eclipse-like events consistent with a planetary transit of a white dwarf and other sources of stellar variability within the field. We do not find any significant evidence for minute-long transits around our targets, hence we rule out planetary transits around ∼370 white dwarfs that should be present in this field. Additionally, we identify 49 variables, including 40 new systems. These include 23 detached or contact stellar binaries, one eclipsing white dwarf + M dwarf binary, 16 δ Scuti, three RR Lyrae, and two ZZ Ceti pulsators. Results from the remaining two fields in our survey will allow us to place more stringent constraints on the frequency of planets orbiting white dwarfs in the habitable zone.
The Astrophysical Journal
We consider how the gravity of the Galactic disk and the Large Magellanic Cloud (LMC) modifies th... more We consider how the gravity of the Galactic disk and the Large Magellanic Cloud (LMC) modifies the radial motions of hypervelocity stars (HVSs) ejected from the Galactic Center. For typical HVSs ejected towards low (high) Galactic latitudes, the disk bends trajectories by up to 30 • (3 • to 10 •). For many linesof-sight through the Galaxy, the LMC produces similar and sometimes larger deflections. Bound HVSs suffer larger deflections than unbound HVSs. Gravitational focusing by the LMC also generates a factor of two overdensity along the line-of-sight towards the LMC. With large enough samples, observations can detect the non-radial orbits and the overdensity of HVSs towards the LMC. For any Galactic potential model, the Galactic rest-frame tangential velocity provides
The Astrophysical Journal
We use new Gaia measurements to explore the origin of the highest velocity stars in the Hypervelo... more We use new Gaia measurements to explore the origin of the highest velocity stars in the Hypervelocity Star Survey. The measurements reveal a clear pattern in the B-type stars. Halo stars dominate the sample at speeds ≃100 km s −1 below Galactic escape velocity. Disk runaway stars have speeds up to ≃100 km s −1 above Galactic escape velocity, but most disk runaways are bound. Stars with speeds 100 km s −1 above Galactic escape velocity originate from the Galactic center. Two bound stars may also originate from the Galactic center. Future Gaia measurements will enable a large, clean sample of Galactic center ejections for measuring the massive black hole ejection rate of hypervelocity stars, and for constraining the mass distribution of the Milky Way dark matter halo.
The Astrophysical Journal
We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremel... more We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ∼1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M ⊙ main sequence stars with ≃0.8 M ⊙ companions. While WDs must exist at sdA temperatures, only ∼1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A-F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.
Monthly Notices of the Royal Astronomical Society
We present the results from a Gemini snapshot radial-velocity survey of 44 low-mass whitedwarf ca... more We present the results from a Gemini snapshot radial-velocity survey of 44 low-mass whitedwarf candidates selected from the Sloan Digital Sky Survey (SDSS) spectroscopy. To find sub-hour orbital period binary systems, our time-series spectroscopy had cadences of 2-8 min over a period of 20-30 min. Through follow-up observations at Gemini and the MMT, we identify four double-degenerate binary systems with periods ranging from 53 min to 7 h. The shortest period system, SDSS J123549.88+154319.3, was recently identified as a sub-hour period detached binary by Breedt and collaborators. Here, we refine the orbital and physical parameters of this system. High-speed and time-domain survey photometry observations do not reveal eclipses or other photometric effects in any of our targets. We compare the period distribution of these four systems with the orbital period distribution of known double white dwarfs; the median period decreases from 0.64 to 0.24 d for M = 0.3-0.5 M to M < 0.3 M white dwarfs. However, we do not find a statistically significant correlation between the orbital period and white-dwarf mass.
The Astrophysical Journal
We report the discovery of two detached double white dwarf (WD) binaries, SDSS J082239.546+304857... more We report the discovery of two detached double white dwarf (WD) binaries, SDSS J082239.546+304857.19 and SDSS J104336.275+055149.90, with orbital periods of 40 and 46 min, respectively. The 40 min system is eclipsing; it is composed of a 0.30 M ⊙ and a 0.52 M ⊙ WD. The 46 min system is a likely LISA verification binary. The short 20 ± 2 Myr and ∼34 Myr gravitational wave merger times of the two binaries imply that many more such systems have formed and merged over the age of the Milky Way. We update the estimated Milky Way He+CO WD binary merger rate and affirm our previously published result: He+CO WD binaries merge at a rate at least 40 times greater than the formation rate of stable mass-transfer AM CVn binaries, and so the majority must have unstable mass-transfer. The implication is that spin-orbit coupling in He+CO WD mergers is weak, or perhaps nova-like outbursts drive He+CO WDs into merger as proposed by Shen.
The Astrophysical Journal, 2016
We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM) < 0... more We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM) < 0.3 M ⊙ white dwarfs in the Galaxy. Such white dwarfs are detectable for timescales of 0.1 Gyr-1 Gyr in the ELM Survey; the binaries they reside in have gravitational wave merger times of 0.001 Gyr-100 Gyr. To explain the observed distribution requires that most ELM white dwarf binary progenitors detach from the common envelope phase with <1 hr orbital periods. We calculate the local space density of ELM white dwarf binaries and estimate a merger rate of 3 × 10 −3 yr −1 over the entire disk of the Milky Way; the merger rate in the halo is 10 times smaller. The ELM white dwarf binary merger rate exceeds by a factor of 40 the formation rate of stable mass transfer AM CVn binaries, marginally exceeds the rate of underluminous supernovae, and is identical to the formation rate of R CrB stars. On this basis, we conclude that ELM white dwarf binaries can be the progenitors of all observed AM CVn and possibly underluminous supernovae, however the majority of He+CO white dwarf binaries go through unstable mass transfer and merge, e.g. into single massive ∼1 M ⊙ white dwarfs.
Ground-based and Airborne Instrumentation for Astronomy IV, 2012
The Astrophysical Journal, 2007
Hypervelocity stars (HVSs) are stars ejected completely out of the Milky Way by three-body intera... more Hypervelocity stars (HVSs) are stars ejected completely out of the Milky Way by three-body interactions with the massive black hole in the Galactic center. We describe 643 new spectroscopic observations from our targeted survey for HVSs. We find a significant (3.5 � ) excess of B-type stars with large velocities +275 km s � 1 10 kpc that are most plausibly explained as a new class of HVSs: stars ejected from the Galactic center on bound orbits. If a Galactic center ejection origin is correct, the distribution of HVSs on the sky should be anisotropic for a survey complete to a fixed limiting apparent magnitude. The unbound HVSs in our survey have a marginally anisotropic distribution on the sky, consistent with the Galactic center ejection picture. Subject headingg Galaxy: center — Galaxy: halo — Galaxy: kinematics and dynamics — Galaxy: stellar content — stars: early-type
The Astrophysical Journal, 2012
We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so... more We discuss our targeted search for hypervelocity stars (HVSs), stars traveling with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Our survey, now half complete, has successfully identified a total of four probable HVSs plus a number of other unusual objects. Here we report the most recently discovered two HVSs: SDSS J110557.45+093439.5 and possibly SDSS J113312.12+010824, traveling with Galactic rest-frame velocities at least +508±12 and +418±10 km s, respectively. The other late B-type objects in our survey are consistent with a population of post-main sequence stars or blue stragglers in the Galactic halo, with mean metallicity [Fe/H]Wk = −1.3 and velocity dispersion 108 ± 5 km s . Interestingly, the velocity distribution shows a tail of objects with large positive velocities that may be a mix of low-velocity HVSs and high-velocity runaway stars. Our survey also includes a number of DA white dwarfs with unusually red colors...
We describe new methods of analysis that have been developed in order to make optimal use of the ... more We describe new methods of analysis that have been developed in order to make optimal use of the stellar spectral data obtained during the course of follow-up medium-resolution observations of color-selected stars from the ``Century Survey.'' The initial selection of stars in the stellar component of the Century Survey is based on broadband V and R photometry covering a strip
The Smithsonian Widefield Infrared Camera (SWIRC) is a Y -, J-, and H-band imager for the f/5 MMT... more The Smithsonian Widefield Infrared Camera (SWIRC) is a Y -, J-, and H-band imager for the f/5 MMT. Proposed in May 2003 and commissioned in June 2004, the goal of the instrument was to deliver quickly a wide field-of-view instrument with minimal optical elements and hence high throughput. The trade-off was to sacrifice K-band capability by not having an internal, cold Lyot stop. We describe SWIRC’s design and capabilities, and discuss lessons learned from the thermal design and the detector mount, all of which have been incorporated into the upcoming MMT & Magellan Infrared Spectrograph.
We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 03... more We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and supernova contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week which are unrelated to the presence of a supernova. The early afterglow light curve also shows deviations from the typical power-law decay. A supernova spectrum is first detectable ∼ 7 days after the burst and dominates the light after ∼ 11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the supernova explosion is very close to the observed time of the G...
The Astrophysical Journal Letters
We report the discovery of the brightest detached binary white dwarfs with periods less than an h... more 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 min 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 +0.05 −0.03 M companion at an inclination of 69 ± 9 •. The second system, SDSS J063449.92+380352.2 (hereafter J0634), is a 26.5 min orbital period, g = 17.0 mag detached double white dwarf binary at a distance of 435 pc. J0634 contains a 0.452 +0.070 −0.062 M white dwarf with a 0.209 +0.034 −0.021 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.
Nature
Astronomers have discovered thousands of planets outside the solar system 1 , most of which orbit... more Astronomers have discovered thousands of planets outside the solar system 1 , most of which orbit stars that will eventually evolve into red giants and then into white dwarfs. During the red giant phase, any close-orbiting planets will be engulfed by the star 2 , but more distant planets can survive this phase and remain in orbit around the white dwarf 3, 4. Some white dwarfs show evidence for rocky material floating in their atmospheres 5 , in warm debris disks 6-9 , or orbiting very closely 10-12 , which has been interpreted as the debris of rocky planets that were scattered inward and tidally disrupted 13. Recently, the discovery of a gaseous debris disk with a composition similar to ice giant planets 14 demonstrated that massive planets might also find their way into tight orbits around white dwarfs, but it is unclear whether the planets can survive the journey. So far, the detection of intact planets in close orbits around white dwarfs has remained elusive. Here, we report the discovery of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95% confidence). Other cases of white dwarfs with close brown dwarf or stellar companions are explained as the consequence of common-envelope evolution, wherein the original orbit is enveloped during the red-giant phase and shrinks due to friction. In this case, though, the low mass and relatively long orbital period of the planet candidate make common-envelope evolution less likely. Instead, the WD 1856+534 system seems to demonstrate that giant planets can be scattered into tight orbits without being tidally disrupted, and motivates searches for smaller transiting planets around white dwarfs. WD 1856+534 (hereafter, WD 1856 for brevity) is located 25 parsecs away in a visual triple star system. It has an effective temperature of 4710 ± 60 Kelvin and became a white dwarf 5.9 ± 0.5 billion years ago, based on theoretical models for how white dwarfs cool over time. The total system age, including the star's main sequence lifetime, must be older. Table 1 gives the other key parameters of the star. WD 1856 is one of thousands of white dwarfs that was targeted for observations with NASA's Transiting Exoplanet Survey Satellite (TESS), in order to search for any periodic dimming events caused by planetary transits. A statistically significant transit-like event was detected by the TESS Science Processing Operations Center (SPOC) pipeline based
The Astrophysical Journal
We begin the search for extremely-low mass (M ≤ 0.3M , ELM) white dwarfs (WDs) in the southern sk... more We begin the search for extremely-low mass (M ≤ 0.3M , ELM) white dwarfs (WDs) in the southern sky based on photometry from the VST ATLAS and SkyMapper surveys. We use a similar colorselection 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.
The Astrophysical Journal
We present the final sample of 98 detached double white dwarf (WD) binaries found in the Extremel... more We present the final sample of 98 detached double white dwarf (WD) binaries found in the Extremely Low Mass (ELM) Survey, a spectroscopic survey targeting <0.3 M ⊙ He-core WDs completed in the Sloan Digital Sky Survey footprint. Over the course of the survey we observed ancillary low mass WD candidates like GD 278, which we show is a P = 0.19 d double WD binary, as well as candidates that turn out to be field blue straggler/subdwarf A-type stars with luminosities too large to be WDs given their Gaia parallaxes. Here, we define a clean sample of ELM WDs that is complete within our target selection and magnitude range 15 < g 0 < 20 mag. The measurements are consistent with 100% of ELM WDs being 0.0089 < P < 1.5 d double WD binaries, 35% of which belong to the Galactic halo. We infer these are mostly He+CO WD binaries given the measurement constraints. The merger rate of the observed He+CO WD binaries exceeds the formation rate of stable mass transfer AM CVn binaries by a factor of 25, and so the majority of He+CO WD binaries must experience unstable mass transfer and merge. The shortest-period systems like J0651+2844 are signature LISA verification binaries that can be studied with gravitational waves and light.
Monthly Notices of the Royal Astronomical Society
We present minute cadence photometry of 31 732 point sources observed in one 3 rmdeg2\rm deg^{2}rmdeg2 DECa... more We present minute cadence photometry of 31 732 point sources observed in one 3 rmdeg2\rm deg^{2}rmdeg2 DECam pointing centred at RA = 09:03:02 and Dec. = −04:35:00 over eight consecutive half-nights. We use these data to search for eclipse-like events consistent with a planetary transit of a white dwarf and other sources of stellar variability within the field. We do not find any significant evidence for minute-long transits around our targets, hence we rule out planetary transits around ∼370 white dwarfs that should be present in this field. Additionally, we identify 49 variables, including 40 new systems. These include 23 detached or contact stellar binaries, one eclipsing white dwarf + M dwarf binary, 16 δ Scuti, three RR Lyrae, and two ZZ Ceti pulsators. Results from the remaining two fields in our survey will allow us to place more stringent constraints on the frequency of planets orbiting white dwarfs in the habitable zone.
The Astrophysical Journal
We consider how the gravity of the Galactic disk and the Large Magellanic Cloud (LMC) modifies th... more We consider how the gravity of the Galactic disk and the Large Magellanic Cloud (LMC) modifies the radial motions of hypervelocity stars (HVSs) ejected from the Galactic Center. For typical HVSs ejected towards low (high) Galactic latitudes, the disk bends trajectories by up to 30 • (3 • to 10 •). For many linesof-sight through the Galaxy, the LMC produces similar and sometimes larger deflections. Bound HVSs suffer larger deflections than unbound HVSs. Gravitational focusing by the LMC also generates a factor of two overdensity along the line-of-sight towards the LMC. With large enough samples, observations can detect the non-radial orbits and the overdensity of HVSs towards the LMC. For any Galactic potential model, the Galactic rest-frame tangential velocity provides
The Astrophysical Journal
We use new Gaia measurements to explore the origin of the highest velocity stars in the Hypervelo... more We use new Gaia measurements to explore the origin of the highest velocity stars in the Hypervelocity Star Survey. The measurements reveal a clear pattern in the B-type stars. Halo stars dominate the sample at speeds ≃100 km s −1 below Galactic escape velocity. Disk runaway stars have speeds up to ≃100 km s −1 above Galactic escape velocity, but most disk runaways are bound. Stars with speeds 100 km s −1 above Galactic escape velocity originate from the Galactic center. Two bound stars may also originate from the Galactic center. Future Gaia measurements will enable a large, clean sample of Galactic center ejections for measuring the massive black hole ejection rate of hypervelocity stars, and for constraining the mass distribution of the Milky Way dark matter halo.
The Astrophysical Journal
We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremel... more We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ∼1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ≃1.2 M ⊙ main sequence stars with ≃0.8 M ⊙ companions. While WDs must exist at sdA temperatures, only ∼1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A-F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.
Monthly Notices of the Royal Astronomical Society
We present the results from a Gemini snapshot radial-velocity survey of 44 low-mass whitedwarf ca... more We present the results from a Gemini snapshot radial-velocity survey of 44 low-mass whitedwarf candidates selected from the Sloan Digital Sky Survey (SDSS) spectroscopy. To find sub-hour orbital period binary systems, our time-series spectroscopy had cadences of 2-8 min over a period of 20-30 min. Through follow-up observations at Gemini and the MMT, we identify four double-degenerate binary systems with periods ranging from 53 min to 7 h. The shortest period system, SDSS J123549.88+154319.3, was recently identified as a sub-hour period detached binary by Breedt and collaborators. Here, we refine the orbital and physical parameters of this system. High-speed and time-domain survey photometry observations do not reveal eclipses or other photometric effects in any of our targets. We compare the period distribution of these four systems with the orbital period distribution of known double white dwarfs; the median period decreases from 0.64 to 0.24 d for M = 0.3-0.5 M to M < 0.3 M white dwarfs. However, we do not find a statistically significant correlation between the orbital period and white-dwarf mass.
The Astrophysical Journal
We report the discovery of two detached double white dwarf (WD) binaries, SDSS J082239.546+304857... more We report the discovery of two detached double white dwarf (WD) binaries, SDSS J082239.546+304857.19 and SDSS J104336.275+055149.90, with orbital periods of 40 and 46 min, respectively. The 40 min system is eclipsing; it is composed of a 0.30 M ⊙ and a 0.52 M ⊙ WD. The 46 min system is a likely LISA verification binary. The short 20 ± 2 Myr and ∼34 Myr gravitational wave merger times of the two binaries imply that many more such systems have formed and merged over the age of the Milky Way. We update the estimated Milky Way He+CO WD binary merger rate and affirm our previously published result: He+CO WD binaries merge at a rate at least 40 times greater than the formation rate of stable mass-transfer AM CVn binaries, and so the majority must have unstable mass-transfer. The implication is that spin-orbit coupling in He+CO WD mergers is weak, or perhaps nova-like outbursts drive He+CO WDs into merger as proposed by Shen.
The Astrophysical Journal, 2016
We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM) < 0... more We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM) < 0.3 M ⊙ white dwarfs in the Galaxy. Such white dwarfs are detectable for timescales of 0.1 Gyr-1 Gyr in the ELM Survey; the binaries they reside in have gravitational wave merger times of 0.001 Gyr-100 Gyr. To explain the observed distribution requires that most ELM white dwarf binary progenitors detach from the common envelope phase with <1 hr orbital periods. We calculate the local space density of ELM white dwarf binaries and estimate a merger rate of 3 × 10 −3 yr −1 over the entire disk of the Milky Way; the merger rate in the halo is 10 times smaller. The ELM white dwarf binary merger rate exceeds by a factor of 40 the formation rate of stable mass transfer AM CVn binaries, marginally exceeds the rate of underluminous supernovae, and is identical to the formation rate of R CrB stars. On this basis, we conclude that ELM white dwarf binaries can be the progenitors of all observed AM CVn and possibly underluminous supernovae, however the majority of He+CO white dwarf binaries go through unstable mass transfer and merge, e.g. into single massive ∼1 M ⊙ white dwarfs.