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Showing 1-20 of about 90 results.
A 1201 s Orbital Period Detached Binary: The First Double Helium Core White Dwarf LISA Verification BinaryBrown, Warren R.Kilic, MukreminBédard, A.Kosakowski, AlekzanderBergeron, P.DOI: info:10.3847/2041-8213/ab8228v. 892L35
Brown, Warren R., Kilic, Mukremin, Bédard, A., Kosakowski, Alekzander, and Bergeron, P. 2020. "A 1201 s Orbital Period Detached Binary: The First Double Helium Core White Dwarf LISA Verification Binary." The Astrophysical Journal 892:L35.
ID: 157296
Type: article
Authors: Brown, Warren R.; Kilic, Mukremin; Bédard, A.; Kosakowski, Alekzander; Bergeron, P.
Abstract: We report the discovery of a 1201 s orbital period binary, the third shortest-period detached binary known. Sloan Digital Sky Survey J232230.20 + 050942.06 contains two He-core white dwarfs orbiting with a 27° inclination. Located 0.76 kpc from the Sun, the binary has an estimated Laser Interferometer Space Antenna (LISA) 4 yr signal-to-noise ratio of 40. J2322 + 0509 is the first He + He white dwarf LISA verification binary, a source class that is predicted to account for one-third of resolved LISA ultra-compact binary detections.
The ELM Survey. VIII. Ninety-eight Double White Dwarf BinariesBrown, Warren R.Kilic, MukreminKosakowski, AlekzanderAndrews, Jeff J.Heinke, Craig O.Agüeros, Marcel A.Camilo, FernandoGianninas, A.Hermes, J. J.Kenyon, Scott J.DOI: info:10.3847/1538-4357/ab63cdv. 88949
Brown, Warren R., Kilic, Mukremin, Kosakowski, Alekzander, Andrews, Jeff J., Heinke, Craig O., Agüeros, Marcel A., Camilo, Fernando, Gianninas, A., Hermes, J. J., and Kenyon, Scott J. 2020. "The ELM Survey. VIII. Ninety-eight Double White Dwarf Binaries." The Astrophysical Journal 889:49.
ID: 155706
Type: article
Authors: Brown, Warren R.; Kilic, Mukremin; Kosakowski, Alekzander; Andrews, Jeff J.; Heinke, Craig O.; Agüeros, Marcel A.; Camilo, Fernando; Gianninas, A.; Hermes, J. J.; Kenyon, Scott J.
Abstract: 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 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 day double WD binary, as well as candidates that turn out to be field blue straggler/subdwarf A-type stars with luminosities too high 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 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 day double WD binary, as well as candidates that turn out to be field blue straggler/subdwarf A-type stars with luminosities too high 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 0 < 20 mag. The measurements are consistent with 100% of ELM WDs being 0.0089 < P < 1.5 day double WD binaries, 35% of which belong to the Galactic halo. We infer that 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 systems with the shortest periods, such as J0651+2844, are signature LISA verification binaries that can be studied with gravitational waves and light.
The 100 pc White Dwarf Sample in the SDSS FootprintKilic, MukreminBergeron, P.Kosakowski, AlekzanderBrown, Warren R.Agüeros, Marcel A.Blouin, SimonDOI: info:10.3847/1538-4357/ab9b8dv. 89884
Kilic, Mukremin, Bergeron, P., Kosakowski, Alekzander, Brown, Warren R., Agüeros, Marcel A., and Blouin, Simon. 2020. "The 100 pc White Dwarf Sample in the SDSS Footprint." The Astrophysical Journal 898:84.
ID: 157791
Type: article
Authors: Kilic, Mukremin; Bergeron, P.; Kosakowski, Alekzander; Brown, Warren R.; Agüeros, Marcel A.; Blouin, Simon
Abstract: We present follow-up spectroscopy of 711 white dwarfs within 100 pc, and we present a detailed model atmosphere analysis of the 100 pc white dwarf sample in the Sloan Digital Sky Survey footprint. Our spectroscopic follow-up is complete for 83% of the white dwarfs hotter than 6000 K, where the atmospheric composition can be constrained reliably. We identify 1508 DA white dwarfs with pure hydrogen atmospheres. The DA mass distribution has an extremely narrow peak at 0.59 M and reveals a shoulder from relatively massive white dwarfs with M = 0.7-0.9 M. Comparing this distribution with binary population synthesis models, we find that the contribution from single stars that form through mergers cannot explain the overabundance of massive white dwarfs. In addition, the mass distribution of cool DAs shows a near absence of M > 1 M white dwarfs. The pile-up of 0.7-0.9 M and the disappearance of M > 1 M white dwarfs is consistent with the effects of core crystallization. Even though the evolutionary models predict the location of the pile-up correctly, the delay from the latent heat of crystallization by itself is insufficient to create a significant pile-up, and additional cooling delays from related effects like phase separation are necessary. We also discuss the population of infrared-faint (ultracool) white dwarfs and demonstrate for the first time the existence of a well-defined sequence in color and magnitude. Curiously, this sequence is connected to a region in the color-magnitude diagrams where the number of white dwarfs with a helium-dominated atmosphere is low. This suggests that the infrared-faint white dwarfs likely have mixed H/He atmospheres.
The ELM Survey South. I. An Effective Search for Extremely Low Mass White DwarfsKosakowski, AlekzanderKilic, MukreminBrown, Warren R.Gianninas, AlexandrosDOI: info:10.3847/1538-4357/ab8300v. 89453
Kosakowski, Alekzander, Kilic, Mukremin, Brown, Warren R., and Gianninas, Alexandros. 2020. "The ELM Survey South. I. An Effective Search for Extremely Low Mass White Dwarfs." The Astrophysical Journal 894:53.
ID: 156910
Type: article
Authors: Kosakowski, Alekzander; Kilic, Mukremin; Brown, Warren R.; Gianninas, Alexandros
Abstract: 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 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 4.1 m telescope to test the Gaia-based selection, and identify more than two dozen low mass white dwarfs, including six 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 be 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 hr), 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.
Gaia white dwarfs within 40 pc - I. Spectroscopic observations of new candidatesTremblay, P. -EHollands, M. A.Gentile Fusillo, N. P.McCleery, J.Izquierdo, P.Gänsicke, B. T.Cukanovaite, E.Koester, D.Brown, Warren R.Charpinet, S.Cunningham, T.Farihi, J.Giammichele, N.Van Grootel, V.Hermes, J. J.Hoskin, M. J.Jordan, S.Kepler, S. O.Kleinman, S. J.Manser, C. J.Marsh, T. R.De Martino, D.Nitta, A.Parsons, S. G.Pelisoli, I.Raddi, R.Rebassa-Mansergas, A.Ren, J. -JSchreiber, M. R.Silvotti, R.Toloza, O.Toonen, S.Torres, S.DOI: info:10.1093/mnras/staa1892v. 497130–145
Tremblay, P. -E, Hollands, M. A., Gentile Fusillo, N. P., McCleery, J., Izquierdo, P., Gänsicke, B. T., Cukanovaite, E., Koester, D., Brown, Warren R., Charpinet, S., Cunningham, T., Farihi, J., Giammichele, N., Van Grootel, V., Hermes, J. J., Hoskin, M. J., Jordan, S., Kepler, S. O., Kleinman, S. J., Manser, C. J., Marsh, T. R., De Martino, D., Nitta, A., Parsons, S. G., Pelisoli, I. et al. 2020. "Gaia white dwarfs within 40 pc - I. Spectroscopic observations of new candidates." Monthly Notices of the Royal Astronomical Society 497:130– 145.
ID: 157792
Type: article
Authors: Tremblay, P. -E; Hollands, M. A.; Gentile Fusillo, N. P.; McCleery, J.; Izquierdo, P.; Gänsicke, B. T.; Cukanovaite, E.; Koester, D.; Brown, Warren R.; Charpinet, S.; Cunningham, T.; Farihi, J.; Giammichele, N.; Van Grootel, V.; Hermes, J. J.; Hoskin, M. J.; Jordan, S.; Kepler, S. O.; Kleinman, S. J.; Manser, C. J.; Marsh, T. R.; De Martino, D.; Nitta, A.; Parsons, S. G.; Pelisoli, I.; Raddi, R.; Rebassa-Mansergas, A.; Ren, J. -J; Schreiber, M. R.; Silvotti, R.; Toloza, O.; Toonen, S.; Torres, S.
Abstract: We present a spectroscopic survey of 230 white dwarf candidates within 40 pc of the Sun from the William Herschel Telescope and Gran Telescopio Canarias. All candidates were selected from Gaia Data Release 2 (DR2) and in almost all cases, had no prior spectroscopic classifications. We find a total of 191 confirmed white dwarfs and 39 main-sequence star contaminants. The majority of stellar remnants in the sample are relatively cool (eff> = 6200 K), showing either hydrogen Balmer lines or a featureless spectrum, corresponding to 89 DA and 76 DC white dwarfs, respectively. We also recover two DBA white dwarfs and 9-10 magnetic remnants. We find two carbon-bearing DQ stars and 14 new metal-rich white dwarfs. This includes the possible detection of the first ultra-cool white dwarf with metal lines. We describe three DZ stars for which we find at least four different metal species, including one that is strongly Fe- and Ni-rich, indicative of the accretion of a planetesimal with core-Earth composition. We find one extremely massive (1.31 ± 0.01 M) DA white dwarf showing weak Balmer lines, possibly indicating stellar magnetism. Another white dwarf shows strong Balmer line emission but no infrared excess, suggesting a low-mass sub-stellar companion. A high spectroscopic completeness (>99 per cent) has now been reached for Gaia DR2 sources within 40-pc sample, in the Northern hemisphere (δ > 0°) and located on the white dwarf cooling track in the Hertzsprung-Russell diagram. A statistical study of the full northern sample is presented in a companion paper.
A giant planet candidate transiting a white dwarfVanderburg, AndrewRappaport, Saul A.Xu, SiyiCrossfield, Ian J. M.Becker, Juliette C.Gary, BruceMurgas, FelipeBlouin, SimonKaye, Thomas G.Palle, EnricMelis, CarlMorris, Brett M.Kreidberg, LauraGorjian, VaroujanMorley, Caroline V.Mann, Andrew W.Parviainen, HannuPearce, Logan A.Newton, Elisabeth R.Carrillo, AndreiaZuckerman, BenNelson, LorneZeimann, GregBrown, Warren R.Tronsgaard, RenéKlein, BethRicker, George R.Vanderspek, Roland K.Latham, David W.Seager, SaraWinn, Joshua N.Jenkins, Jon M.Adams, Fred C.Benneke, BjörnBerardo, DavidBuchhave, Lars A.Caldwell, Douglas A.Christiansen, Jessie L.Collins, Karen A.Colón, Knicole D.Daylan, TansuDoty, JohnDoyle, Alexandra E.Dragomir, DianaDressing, CourtneyDufour, PatrickFukui, AkihikoGlidden, AnaGuerrero, Natalia M.Guo, XueyingHeng, KevinHenriksen, Andreea I.Huang, Chelsea X.Kaltenegger, LisaKane, Stephen R.Lewis, John A.Lissauer, Jack J.Morales, FarisaNarita, NorioPepper, JoshuaRose, Mark E.Smith, Jeffrey C.Stassun, Keivan G.Yu, LiangDOI: info:10.1038/s41586-020-2713-yv. 585No. 7825363–367
Vanderburg, Andrew, Rappaport, Saul A., Xu, Siyi, Crossfield, Ian J. M., Becker, Juliette C., Gary, Bruce, Murgas, Felipe, Blouin, Simon, Kaye, Thomas G., Palle, Enric, Melis, Carl, Morris, Brett M., Kreidberg, Laura, Gorjian, Varoujan, Morley, Caroline V., Mann, Andrew W., Parviainen, Hannu, Pearce, Logan A., Newton, Elisabeth R., Carrillo, Andreia, Zuckerman, Ben, Nelson, Lorne, Zeimann, Greg, Brown, Warren R., Tronsgaard, René et al. 2020. "A giant planet candidate transiting a white dwarf." Nature 585 (7825):363– 367.
ID: 157058
Type: article
Authors: Vanderburg, Andrew; Rappaport, Saul A.; Xu, Siyi; Crossfield, Ian J. M.; Becker, Juliette C.; Gary, Bruce; Murgas, Felipe; Blouin, Simon; Kaye, Thomas G.; Palle, Enric; Melis, Carl; Morris, Brett M.; Kreidberg, Laura; Gorjian, Varoujan; Morley, Caroline V.; Mann, Andrew W.; Parviainen, Hannu; Pearce, Logan A.; Newton, Elisabeth R.; Carrillo, Andreia; Zuckerman, Ben; Nelson, Lorne; Zeimann, Greg; Brown, Warren R.; Tronsgaard, René; Klein, Beth; Ricker, George R.; Vanderspek, Roland K.; Latham, David W.; Seager, Sara; Winn, Joshua N.; Jenkins, Jon M.; Adams, Fred C.; Benneke, Björn; Berardo, David; Buchhave, Lars A.; Caldwell, Douglas A.; Christiansen, Jessie L.; Collins, Karen A.; Colón, Knicole D.; Daylan, Tansu; Doty, John; Doyle, Alexandra E.; Dragomir, Diana; Dressing, Courtney; Dufour, Patrick; Fukui, Akihiko; Glidden, Ana; Guerrero, Natalia M.; Guo, Xueying; Heng, Kevin; Henriksen, Andreea I.; Huang, Chelsea X.; Kaltenegger, Lisa; Kane, Stephen R.; Lewis, John A.; Lissauer, Jack J.; Morales, Farisa; Narita, Norio; Pepper, Joshua; Rose, Mark E.; Smith, Jeffrey C.; Stassun, Keivan G.; Yu, Liang
The DECam minute cadence survey - II. 49 variables but no planetary transits of a white dwarfDame, KyraBelardi, ClaudiaKilic, MukreminRest, ArminGianninas, A.Barber, SaraBrown, Warren R.DOI: info:10.1093/mnras/stz398v. 4901066–1075
Dame, Kyra, Belardi, Claudia, Kilic, Mukremin, Rest, Armin, Gianninas, A., Barber, Sara, and Brown, Warren R. 2019. "The DECam minute cadence survey - II. 49 variables but no planetary transits of a white dwarf." Monthly Notices of the Royal Astronomical Society 490:1066– 1075.
ID: 154589
Type: article
Authors: Dame, Kyra; Belardi, Claudia; Kilic, Mukremin; Rest, Armin; Gianninas, A.; Barber, Sara; Brown, Warren R.
Abstract: We present minute cadence photometry of 31 732 point sources observed in one 3 deg2 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.
Binospec: A Wide-field Imaging Spectrograph for the MMTFabricant, DanielFata, RobertEpps, HarlandGauron, ThomasMueller, MarkZajac, JosephAmato, StephenBarberis, JackBergner, HenryBrennan, PatriciaBrown, WarrenChilingarian, IgorGeary, JohnKradinov, VladimirMcLeod, BrianSmith, Matthew C.Woods, DeborahDOI: info:10.1088/1538-3873/ab1d78v. 131075004
Fabricant, Daniel, Fata, Robert, Epps, Harland, Gauron, Thomas, Mueller, Mark, Zajac, Joseph, Amato, Stephen, Barberis, Jack, Bergner, Henry, Brennan, Patricia, Brown, Warren, Chilingarian, Igor, Geary, John, Kradinov, Vladimir, McLeod, Brian, Smith, Matthew C., and Woods, Deborah. 2019. "Binospec: A Wide-field Imaging Spectrograph for the MMT." Publications of the Astronomical Society of the Pacific 131:075004.
ID: 154168
Type: article
Authors: Fabricant, Daniel; Fata, Robert; Epps, Harland; Gauron, Thomas; Mueller, Mark; Zajac, Joseph; Amato, Stephen; Barberis, Jack; Bergner, Henry; Brennan, Patricia; Brown, Warren; Chilingarian, Igor; Geary, John; Kradinov, Vladimir; McLeod, Brian; Smith, Matthew C.; Woods, Deborah
Abstract: Binospec is a high-throughput, 370 to 1000 nm, imaging spectrograph that addresses two adjacent 8′ by 15′ fields of view. Binospec was commissioned in late 2017 at the f/5 focus of the 6.5 m MMT and is now available to all MMT observers. Aperture masks cut from stainless steel with a laser cutter are used to define the entrance apertures that range from 15′ long slits to hundreds of 2″ slitlets. System throughputs, including the MMT's mirrors and the f/5 wide-field corrector peak at ̃30%. Three reflection gratings, duplicated for the two beams, provide resolutions (λ/∆λ) between 1300 and >5000 with a 1″ wide slit. Two through-the-mask guiders are used for target acquisition, mask alignment, guiding, and precision offsets. A full-time Shack-Hartmann wavefront sensor allows continuous adjustment of primary mirror support forces, telescope collimation and focus. Active flexure control maintains spectrograph alignment and focus under varying gravity and thermal conditions.
The McDonald Observatory search for pulsating sdA stars. Asteroseismic support for multiple populationsBell, K. J.Pelisoli, I.Kepler, S. O.Brown, Warren R.Winget, D. E.Winget, K. I.Vanderbosch, Z.Castanheira, B. G.Hermes, J. J.Montgomery, M. H.Koester, D.DOI: info:10.1051/0004-6361/201833279v. 617A6
Bell, K. J., Pelisoli, I., Kepler, S. O., Brown, Warren R., Winget, D. E., Winget, K. I., Vanderbosch, Z., Castanheira, B. G., Hermes, J. J., Montgomery, M. H., and Koester, D. 2018. "The McDonald Observatory search for pulsating sdA stars. Asteroseismic support for multiple populations." Astronomy and Astrophysics 617:A6.
ID: 149189
Type: article
Authors: Bell, K. J.; Pelisoli, I.; Kepler, S. O.; Brown, Warren R.; Winget, D. E.; Winget, K. I.; Vanderbosch, Z.; Castanheira, B. G.; Hermes, J. J.; Montgomery, M. H.; Koester, D.
Abstract: Context. The nature of the recently identified "sdA" spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values between main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors or blue stragglers in the halo.
Aims: Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by sdA stars would contribute substantially to our developing understanding of this class.
Methods: We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from the McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signatures from four new pulsators.
Results: Out of 23 sdA stars observed, we clearly detect stellar pulsations in 7. Dominant pulsation periods range from 4.6 min to 12.3 h, with most on timescales of approximately one hour. We argue specific classifications for some of the new variables, identifying both compact and likely main sequence dwarf pulsators, along with a candidate low-mass RR Lyrae star.
Conclusions: With dominant pulsation periods spanning orders of magnitude, the pulsational evidence supports the emerging narrative that the sdA class consists of multiple stellar populations. Since multiple types of sdA exhibit stellar pulsations, follow-up asteroseismic analysis can be used to probe the precise evolutionary natures and stellar structures of these individual subpopulations. Light curves of seven new pulsating variable stars are only available at the CDS via anonymous ftp to ( or via
Gaia and the Galactic Center Origin of Hypervelocity StarsBrown, Warren R.Lattanzi, Mario G.Kenyon, Scott J.Geller, Margaret J.DOI: info:10.3847/1538-4357/aadb8ev. 86639
Brown, Warren R., Lattanzi, Mario G., Kenyon, Scott J., and Geller, Margaret J. 2018. "Gaia and the Galactic Center Origin of Hypervelocity Stars." The Astrophysical Journal 866:39.
ID: 149385
Type: article
Authors: Brown, Warren R.; Lattanzi, Mario G.; Kenyon, Scott J.; Geller, Margaret J.
Abstract: We use new Gaia measurements to explore the origin of the highest velocity stars in the hypervelocity star (HVS) survey. The measurements reveal a clear pattern in B-type stars. Halo stars dominate the sample at speeds of ≃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 HVSs, and for constraining the mass distribution of the Milky Way dark matter halo.
A quantitative spectral analysis of 14 hypervelocity stars from the MMT surveyIrrgang, A.Kreuzer, S.Heber, U.Brown, Warren R.DOI: info:10.1051/0004-6361/201833315v. 615L5
Irrgang, A., Kreuzer, S., Heber, U., and Brown, Warren R. 2018. "A quantitative spectral analysis of 14 hypervelocity stars from the MMT survey." Astronomy and Astrophysics 615:L5.
ID: 147924
Type: article
Authors: Irrgang, A.; Kreuzer, S.; Heber, U.; Brown, Warren R.
Abstract: Context. Hypervelocity stars (HVSs) travel so fast that they may leave the Galaxy. The tidal disruption of a binary system by the supermassive black hole in the Galactic center is widely assumed to be their ejection mechanism.
Aims: To test the hypothesis of an origin in the Galactic center using kinematic investigations, the current space velocities of the HVSs need to be determined. With the advent of Gaia's second data release, accurate radial velocities from spectroscopy are complemented by proper motion measurements of unprecedented quality. Based on a new spectroscopic analysis method, we provide revised distances and stellar ages, both of which are crucial to unravel the nature of the HVSs.
Methods: We reanalyzed low-resolution optical spectra of 14 HVSs from the MMT HVS survey using a new grid of synthetic spectra, which account for deviations from local thermodynamic equilibrium, to derive effective temperatures, surface gravities, radial velocities, and projected rotational velocities. Stellar masses, radii, and ages were then determined by comparison with stellar evolutionary models that account for rotation. Finally, these results were combined with photometric measurements to obtain spectroscopic distances.
Results: The resulting atmospheric parameters are consistent with those of main sequence stars with masses in the range 2.5-5.0 M&sun;. The majority of the stars rotate at fast speeds, providing further evidence for their main sequence nature. Stellar ages range from 90 to 400 Myr and distances (with typical 1sigma-uncertainties of about 10-15%) from 30 to 100 kpc. Except for one object (B 711), which we reclassify as A-type star, all stars are of spectral type B.
Conclusions: The spectroscopic distances and stellar ages derived here are key ingredients for upcoming kinematic studies of HVSs based on Gaia proper motions.
Impact of the Galactic Disk and Large Magellanic Cloud on the Trajectories of Hypervelocity Stars Ejected from the Galactic CenterKenyon, Scott J.Bromley, Benjamin C.Brown, Warren R.Geller, Margaret J.DOI: info:10.3847/1538-4357/aada04v. 864130
Kenyon, Scott J., Bromley, Benjamin C., Brown, Warren R., and Geller, Margaret J. 2018. "Impact of the Galactic Disk and Large Magellanic Cloud on the Trajectories of Hypervelocity Stars Ejected from the Galactic Center." The Astrophysical Journal 864:130.
ID: 149170
Type: article
Authors: Kenyon, Scott J.; Bromley, Benjamin C.; Brown, Warren R.; Geller, Margaret J.
Abstract: 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 (GC). For typical HVSs ejected toward low (high) Galactic latitudes, the disk bends trajectories by up to 30° (3°–10°). For many lines of 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 an overdensity of a factor of two along the line of sight toward the LMC. With large enough samples, observations can detect the non-radial orbits and the overdensity of HVSs toward the LMC. For any Galactic potential model, the tangential velocity in the Galactic rest frame provides an excellent way to detect unbound and nearly bound HVSs within 10 kpc of the Sun. Similarly, the radial velocity in the rest frame isolates unbound HVSs beyond 10–15 kpc from the Sun. Among samples of unbound HVSs, measurements of the radial and tangential velocities serve to distinguish GC ejections from other types of high-velocity stars.
A refined search for pulsations in white dwarf companions to millisecond pulsarsKilic, MukreminHermes, J. J.Córsico, A. H.Kosakowski, AlekzanderBrown, Warren R.Antoniadis, JohnCalcaferro, Leila M.Gianninas, A.Althaus, Leandro G.Green, M. J.DOI: info:10.1093/mnras/sty1546v. 4791267–1272
Kilic, Mukremin, Hermes, J. J., Córsico, A. H., Kosakowski, Alekzander, Brown, Warren R., Antoniadis, John, Calcaferro, Leila M., Gianninas, A., Althaus, Leandro G., and Green, M. J. 2018. "A refined search for pulsations in white dwarf companions to millisecond pulsars." Monthly Notices of the Royal Astronomical Society 479:1267– 1272.
ID: 149156
Type: article
Authors: Kilic, Mukremin; Hermes, J. J.; Córsico, A. H.; Kosakowski, Alekzander; Brown, Warren R.; Antoniadis, John; Calcaferro, Leila M.; Gianninas, A.; Althaus, Leandro G.; Green, M. J.
Abstract: We present optical high-speed photometry of three millisecond pulsars with low-mass () white dwarf companions, bringing the total number of such systems with follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is underconstrained, but the mode periods we observe are consistent with expectations for an M_{\star }=0.16 - 0.19 M_{⊙} white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars.
Pruning The ELM Survey: Characterizing Candidate Low-mass White Dwarfs through Photometric VariabilityBell, Keaton J.Gianninas, A.Hermes, J. J.Winget, D. E.Kilic, MukreminMontgomery, M. H.Castanheira, B. G.Vanderbosch, Z.Winget, K. I.Brown, Warren R.DOI: info:10.3847/1538-4357/835/2/180v. 835180
Bell, Keaton J., Gianninas, A., Hermes, J. J., Winget, D. E., Kilic, Mukremin, Montgomery, M. H., Castanheira, B. G., Vanderbosch, Z., Winget, K. I., and Brown, Warren R. 2017. "Pruning The ELM Survey: Characterizing Candidate Low-mass White Dwarfs through Photometric Variability." The Astrophysical Journal 835:180.
ID: 142817
Type: article
Authors: Bell, Keaton J.; Gianninas, A.; Hermes, J. J.; Winget, D. E.; Kilic, Mukremin; Montgomery, M. H.; Castanheira, B. G.; Vanderbosch, Z.; Winget, K. I.; Brown, Warren R.
Abstract: 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 ? 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.
The Physical Nature of Subdwarf A Stars: White Dwarf ImpostorsBrown, Warren R.Kilic, MukreminGianninas, A.DOI: info:10.3847/1538-4357/aa67e4v. 83923
Brown, Warren R., Kilic, Mukremin, and Gianninas, A. 2017. "The Physical Nature of Subdwarf A Stars: White Dwarf Impostors." The Astrophysical Journal 839:23.
ID: 143281
Type: article
Authors: Brown, Warren R.; Kilic, Mukremin; Gianninas, A.
Abstract: 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.
Discovery of a Detached, Eclipsing 40 Minute Period Double White Dwarf Binary and a Friend: Implications for He+CO White Dwarf MergersBrown, Warren R.Kilic, MukreminKosakowski, AlekzanderGianninas, A.DOI: info:10.3847/1538-4357/aa8724v. 84710
Brown, Warren R., Kilic, Mukremin, Kosakowski, Alekzander, and Gianninas, A. 2017. "Discovery of a Detached, Eclipsing 40 Minute Period Double White Dwarf Binary and a Friend: Implications for He+CO White Dwarf Mergers." The Astrophysical Journal 847:10.
ID: 144708
Type: article
Authors: Brown, Warren R.; Kilic, Mukremin; Kosakowski, Alekzander; Gianninas, A.
Abstract: 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 minutes, respectively. The 40 minute system is eclipsing; it is composed of a 0.30 M ? and a 0.52 M ? WD. The 46 minute 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
A Gemini snapshot survey for double degeneratesKilic, MukreminBrown, Warren R.Gianninas, A.Curd, BrandonBell, Keaton J.Allende Prieto, CarlosDOI: info:10.1093/mnras/stx1886v. 4714218–4227
Kilic, Mukremin, Brown, Warren R., Gianninas, A., Curd, Brandon, Bell, Keaton J., and Allende Prieto, Carlos. 2017. "A Gemini snapshot survey for double degenerates." Monthly Notices of the Royal Astronomical Society 471:4218– 4227.
ID: 144798
Type: article
Authors: Kilic, Mukremin; Brown, Warren R.; Gianninas, A.; Curd, Brandon; Bell, Keaton J.; Allende Prieto, Carlos
Abstract: We present the results from a Gemini snapshot radial-velocity survey of 44 low-mass white-dwarf 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 to M ? white dwarfs. However, we do not find a statistically significant correlation between the orbital period and white-dwarf mass.
A Deep Proper Motion Catalog Within the Sloan Digital Sky Survey Footprint. II. The White Dwarf Luminosity FunctionMunn, Jeffrey A.Harris, Hugh C.von Hippel, TedKilic, MukreminLiebert, James W.Williams, Kurtis A.DeGennaro, StevenJeffery, ElizabethDame, KyraGianninas, A.Brown, Warren R.DOI: info:10.3847/1538-3881/153/1/10v. 15310
Munn, Jeffrey A., Harris, Hugh C., von Hippel, Ted, Kilic, Mukremin, Liebert, James W., Williams, Kurtis A., DeGennaro, Steven, Jeffery, Elizabeth, Dame, Kyra, Gianninas, A., and Brown, Warren R. 2017. "A Deep Proper Motion Catalog Within the Sloan Digital Sky Survey Footprint. II. The White Dwarf Luminosity Function." The Astronomical Journal 153:10.
ID: 142334
Type: article
Authors: Munn, Jeffrey A.; Harris, Hugh C.; von Hippel, Ted; Kilic, Mukremin; Liebert, James W.; Williams, Kurtis A.; DeGennaro, Steven; Jeffery, Elizabeth; Dame, Kyra; Gianninas, A.; Brown, Warren R.
Abstract: A catalog of 8472 white dwarf (WD) candidates is presented, selected using reduced proper motions from the deep proper motion catalog of Munn et al. Candidates are selected in the magnitude range 16{bol}}}{bol}}-3 {{pc}}-3 and 3.5+/- 0.7x {10}-5 {{pc}}-3, respectively. We resolve the bump in the disk WDLF due to the onset of fully convective envelopes in WDs, and see indications of it in the halo WDLF as well.
The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar EjectaNicholl, M.Berger, E.Kasen, D.Metzger, B. D.Elias, J.Briceño, C.Alexander, K. D.Blanchard, P. K.Chornock, R.Cowperthwaite, P. S.Eftekhari, T.Fong, W.Margutti, R.Villar, V. A.Williams, P. K. G.Brown, Warren R.Annis, J.Bahramian, A.Brout, D.Brown, D. A.Chen, H. -YClemens, J. C.Dennihy, E.Dunlap, B.Holz, D. E.Marchesini, E.Massaro, F.Moskowitz, N.Pelisoli, I.Rest, A.Ricci, F.Sako, M.Soares-Santos, M.Strader, J.DOI: info:10.3847/2041-8213/aa9029v. 848L18
Nicholl, M., Berger, E., Kasen, D., Metzger, B. D., Elias, J., Briceño, C., Alexander, K. D., Blanchard, P. K., Chornock, R., Cowperthwaite, P. S., Eftekhari, T., Fong, W., Margutti, R., Villar, V. A., Williams, P. K. G., Brown, Warren R., Annis, J., Bahramian, A., Brout, D., Brown, D. A., Chen, H. -Y, Clemens, J. C., Dennihy, E., Dunlap, B., Holz, D. E. et al. 2017. "The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. III. Optical and UV Spectra of a Blue Kilonova from Fast Polar Ejecta." Astrophysical Journal Letters 848:L18.
ID: 144759
Type: article
Authors: Nicholl, M.; Berger, E.; Kasen, D.; Metzger, B. D.; Elias, J.; Briceño, C.; Alexander, K. D.; Blanchard, P. K.; Chornock, R.; Cowperthwaite, P. S.; Eftekhari, T.; Fong, W.; Margutti, R.; Villar, V. A.; Williams, P. K. G.; Brown, Warren R.; Annis, J.; Bahramian, A.; Brout, D.; Brown, D. A.; Chen, H. -Y; Clemens, J. C.; Dennihy, E.; Dunlap, B.; Holz, D. E.; Marchesini, E.; Massaro, F.; Moskowitz, N.; Pelisoli, I.; Rest, A.; Ricci, F.; Sako, M.; Soares-Santos, M.; Strader, J.
Abstract: We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational-wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the Southern Astrophysical Research and Magellan telescopes; the UV spectrum was obtained with the Hubble Space Telescope at 5.5 days. Our data reveal a rapidly fading blue component (T≈ 5500 K at 1.5 days) that quickly reddens; spectra later than ≳ 4.5 days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at ˜7900 Å at t≲ 4.5 days. The colors, rapid evolution, and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light r-process nuclei with atomic mass number A≲ 140. This indicates a sightline within {θ }{obs}≲ 45^\circ of the orbital axis. Comparison to models suggests ˜0.03 M of blue ejecta, with a velocity of ˜ 0.3c. The required lanthanide fraction is ˜ {10}-4, but this drops to , but this drops to -5 in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of ≲ 12 km. This mass also supports the idea that neutron star mergers are a major contributor to r-process nucleosynthesis.
Today a Duo, but Once a Trio? The Double White Dwarf HS 2220+2146 May Be a Post-blue Straggler BinaryAndrews, Jeff J.Agüeros, MarcelBrown, Warren R.Gosnell, Natalie M.Gianninas, A.Kilic, MukreminKoester, DetlevDOI: info:10.3847/0004-637X/828/1/38v. 82838
Andrews, Jeff J., Agüeros, Marcel, Brown, Warren R., Gosnell, Natalie M., Gianninas, A., Kilic, Mukremin, and Koester, Detlev. 2016. "Today a Duo, but Once a Trio? The Double White Dwarf HS 2220+2146 May Be a Post-blue Straggler Binary." The Astrophysical Journal 828:38.
ID: 140789
Type: article
Authors: Andrews, Jeff J.; Agüeros, Marcel; Brown, Warren R.; Gosnell, Natalie M.; Gianninas, A.; Kilic, Mukremin; Koester, Detlev
Abstract: For sufficiently wide orbital separations a, the two members of a stellar binary evolve independently. This implies that in a wide double white dwarf (DWD), the more massive WD should always be produced first, when its more massive progenitor ends its main sequence (MS) life, and should therefore be older and cooler than its companion. The bound, wide DWD HS 2220+2146 (a˜ 500 au) does not conform to this picture: the more massive WD is the younger and hotter of the pair. We show that this discrepancy is unlikely to be due to past mass-transfer phases or to the presence of an unresolved companion. Instead, we propose that HS 2220+2146 formed through a new wide DWD evolutionary channel involving the merger of the inner binary in a hierarchical triple system. The resulting blue straggler and its wide companion then evolved independently, forming the WD pair seen today. Although we cannot rule out other scenarios, the most likely formation channel has the inner binary merging while both stars are still on the MS. This provides us with the tantalizing possibility that Kozai-Lidov oscillations may have played a role in the inner binary’s merger. Gaia may uncover hundreds more wide DWDs, leading to the identification of other systems like HS 2220+2146. There are already indications that other WD systems may have been formed through different, but related, hierarchical triple evolutionary scenarios. Characterizing these populations may allow for thorough testing of the efficiency with which Kozai-Lidov oscillations induce stellar mergers.