arXiv subjects

A. Kawka

Publications and source records attributed to A. Kawka.

At least 37 records · Page 2Linked to original sources

arXiv2013

The polluted atmosphere of the white dwarf NLTT 25792 and the diversity of circumstellar environments

We present an analysis of X-Shooter spectra of the polluted, hydrogen-rich white dwarf NLTT 25792. The spectra show strong lines of calcium (Ca H&K, near-infrared calcium triplet, and Ca I 4226 and numerous lines of iron along with magnesium and aluminum lines from which we draw the abundance pattern. Moreover, the photospheric Ca H&K lines are possibly blended with a circumstellar component shifted by -20 km/s relative to the photosphere. A comparison with a sample of four white dwarfs with similar parameters show considerable variations in their abundance patterns, particularly in the calcium to magnesium abundance ratio that varies by a factor of five within this sample. The observed variations, even after accounting for diffusion effects, imply similar variations in the putative accretion source. Also, we find that silicon and sodium are significantly underabundant in the atmosphere of NLTT 25792, a fact that may offer some clues on the nature of the accretion source.

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arXiv2012

The shortest period sdB plus white dwarf binary CD-30 11223 (GALEX J1411-3053)

We report on the discovery of the shortest period binary comprising a hot subdwarf star (CD-30 11223, GALEX J1411-3053) and a massive unseen companion. Photometric data from the All Sky Automated Survey show ellipsoidal variations of the hot subdwarf primary and spectroscopic series revealed an orbital period of 70.5 minutes. The large velocity amplitude suggests the presence of a massive white dwarf in the system (M_2/M_sun > 0.77) assuming a canonical mass for the hot subdwarf (0.48 M_sun), although a white dwarf mass as low as 0.75 M_sun is allowable by postulating a subdwarf mass as low as 0.44 M_sun. The amplitude of ellipsoidal variations and a high rotation velocity imposed a high-inclination to the system (i > 68 deg) and, possibly, observable secondary transits (i > 74 deg). At the lowest permissible inclination and assuming a subdwarf mass of ~0.48 M_sun, the total mass of the system reaches the Chandrasekhar mass limit at 1.35 M_sun and would exceed it for a subdwarf mass above 0.48 M_sun. The system should be considered, like its sibling KPD 1930+2752, a candidate progenitor for a Type Ia supernova. The system should become semi-detached and initiate mass transfer within ~30 Myr.

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arXiv2012

The 1.17-day orbit of the double-degenerate (DA+DQ) NLTT 16249

New spectroscopic observations show that the double degenerate system NLTT 16249 is in a close orbit (a = 5.6+/-0.3 R_sun) with a period of 1.17 d. The total mass of the system is estimated between 1.47 and 2.04 M_sun but it is not expected to merge within a Hubble time-scale (t_merge ~ 10^11 yr). Vennes & Kawka (2012, ApJ, 745, L12) originally identified the system because of the peculiar composite hydrogen (DA class) and molecular (C_2--DQ class--and CN) spectra and the new observations establish this system as the first DA plus DQ close double degenerate. Also, the DQ component was the first of its class to show nitrogen dredged-up from the core in its atmosphere. The star may be viewed as the first known DQ descendant of the born-again PG1159 stars. Alternatively, the presence of nitrogen may be the result of past interactions and truncated evolution in a close binary system.

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arXiv2011

The Core Composition of a White Dwarf in a Close Double Degenerate System

We report the identification of the double degenerate system NLTT 16249 that comprises a normal, hydrogen-rich (DA) white dwarf and a peculiar, carbon-polluted white dwarf (DQ) showing photospheric traces of nitrogen. We disentangled the observed spectra and constrained the properties of both stellar components. In the evolutionary scenario commonly applied to the sequence of DQ white dwarfs, both carbon and nitrogen would be dredged up from the core. The C/N abundance ratio (~ 50) in the atmosphere of this unique DQ white dwarf suggests the presence of unprocessed material (14N) in the core or in the envelope. Helium burning in the DQ progenitor may have terminated early on the red-giant branch after a mass-ejection event leaving unprocessed material in the core although current mass estimates do not favor the presence of a low-mass helium core. Alternatively, some nitrogen in the envelope may have survived an abridged helium-core burning phase prior to climbing the asymptotic giant-branch. Based on available data, we estimate a relatively short orbital period (P <~ 13 hrs) and on-going spectroscopic observations will help determine precise orbital parameters.

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arXiv2011

Discovery of a Bright, Extremely Low-Mass White Dwarf in a Close Double Degenerate System

We report the discovery of a bright (V ~ 13.7), extremely low-mass white dwarf in a close double degenerate system. We originally selected GALEX J171708.5+675712 for spectroscopic follow-up among a group of white dwarf candidates in an ultraviolet-optical reduced proper-motion diagram. The new white dwarf has a mass of 0.18 M_solar and is the primary component of a close double degenerate system (P=0.246137 d, K_1 = 288 km/s) comprising a fainter white dwarf secondary with M_2 ~ 0.9 M_solar. Light curves phased with the orbital ephemeris show evidence of relativistic beaming and weaker ellipsoidal variations. The light curves also reveal secondary eclipses (depth ~ 8 mmag) while the primary eclipses appear partially compensated by the secondary gravitational deflection and are below detection limits. Photospheric abundance measurements show a nearly solar composition of Si, Ca, and Fe (0.1-1 solar), while the normal kinematics suggest a relatively recent formation history. Close binary evolutionary scenarios suggest that extremely low mass white dwarfs form via a common-envelope phase and possible Roche-lobe overflow.

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arXiv2011

On the nature of the Be star HR 7409 (7 Vul)

HR 7409 (7 Vul) is a newly identified Be star possibly part of the Gould Belt and is the massive component of a 69-day spectroscopic binary. The binary parameters and properties of the Be star measured using high-dispersion spectra obtained at Ondrejov Observatory and at Rozhen Observatory imply the presence of a low mass companion (~ 0.5-0.8 M_sun). If the pair is relatively young (<50-80 Myr), then the companion is a K V star, but, following another, older evolutionary scenario, the companion is a horizontal-branch star or possibly a white dwarf star. In the latter scenario, a past episode of mass transfer from an evolved star onto a less massive dwarf star would be responsible for the peculiar nature of the present-day, fast-rotating Be star.

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arXiv2011

Pressure shifts and abundance gradients in the atmosphere of the DAZ white dwarf GALEX J193156.8+011745

We present a detailed model atmosphere analysis of high-dispersion and high signal-to-noise ratio spectra of the heavily polluted DAZ white dwarf GALEX J1931+0117. The spectra obtained with the VLT-Kueyen/UV-Visual Echelle Spectrograph show several well-resolved Si II spectral lines enabling a study of pressure effects on line profiles. We observed large Stark shifts in silicon lines in agreement with theoretical predictions and laboratory measurements. Taking into account Stark shifts in the calculation of synthetic spectra we reduced the scatter in individual line radial velocity measurements from ~ 3 to < 1 km/s. We present revised abundances of O, Mg, Si, Ca, and Fe based on a critical review of line broadening parameters and oscillator strengths. The new measurements are generally in agreement with our previous analysis with the exception of magnesium with a revised abundance a factor of two lower than previously estimated. The magnesium, silicon and iron abundances exceed solar abundances, but the oxygen and calcium abundances are below solar. Also, we compared the observed line profiles to synthetic spectra computed with variable accretion rates and vertical abundance distributions assuming diffusive steady-state. The inferred accretion rates vary from dM/dt = 2x10^6 for calcium to 2x10^9 g/s for oxygen. We find that the accretion flow must be oxygen-rich while being deficient in calcium relative to solar abundances. The lack of radial velocity variations between two measurement epochs suggests that GALEX J1931+0117 is probably not in a close binary and that the source of the accreted material resides in a debris disc.

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arXiv2010

A selection of hot subluminous stars in the GALEX survey I. Correlation with the Guide Star Catalog

We assembled a catalogue of bright, hot subdwarf and white dwarf stars extracted from a joint ultraviolet, optical, and infrared source list. The selection is secured using colour criteria that correlate well with effective temperatures T_eff ~> 12,000 K. We built a N_UV-V versus V-J diagram for more than 60,000 bright sources using the Galaxy Evolution Explorer (GALEX) N_UV magnitude (N_UV<14), and the associated Guide Star Catalog (GSC2.3.2) photographic quick-V magnitude and the Two Micron All Sky Survey (2MASS) J and H magnitudes. This distillation process delivered a catalogue of approximately 700 sources with N_UV-V<0.5 comprising ~160 known hot subdwarf stars and another ~60 known white dwarf stars. A reduced proper-motion diagram built using the proper-motion measurements extracted from the Naval Observatory Merged Astrometric Dataset allowed us to identify an additional ~120 new hot subdwarf candidates and ~10 hot white dwarf candidates. We present a spectroscopic study of a subset of 52 subdwarfs, 48 of them analysed here for the first time, and with nine objects brighter than V ~ 12. Our sample of spectroscopically confirmed hot subdwarfs comprises ten sdO stars and 42 sdB stars suitable for pulsation and binary studies. We also present a study of 50 known white dwarfs selected in the GALEX survey and six new white dwarfs from our catalogue of subluminous candidates. Ultraviolet, optical, and infrared synthetic magnitudes employed in the selection and analysis of white dwarf stars are listed in appendix.

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arXiv2010

Spectroscopic analysis of the B/Be visual binary HR 1847

We studied both components of a slightly overlooked visual binary HR 1847 spectroscopically to determine its basic physical and orbital parameters. Basic stellar parameters were determined by comparing synthetic spectra to the observed echelle spectra, which cover both the optical and near-IR regions. New observations of this system used the Ondřejov and Rozhen 2-m telescopes and their coudé spectrographs. Radial velocities from individual spectra were measured and then analysed with the code {\FOTEL} to determine orbital parameters. The spectroscopic orbit of HR 1847A is presented for the first time. It is a single-lined spectroscopic binary with a B-type primary, a period of 719.79 days, and a highly eccentric orbit with e=0.7. We confirmed that HR 1847B is a Be star. Its Hαemission significantly decreased from 2003 to 2008. Both components have a spectral type B7-8 and luminosity class IV-V.

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arXiv2010

Two new hot subdwarf binaries in the GALEX survey

We report the discovery of two new hot, hydrogen-rich subdwarfs (sdB) in close binary systems. The hot subdwarfs, GALEX J0321+4727 and GALEX J2349+3844, were selected from a joint optical-ultraviolet catalogue of hot sub-luminous stars based on GSC2.3.2 and the Galaxy Evolution Explorer all-sky survey. Using high-dispersion spectra of the Halpha core obtained using the 2m telescope at Ondrejov Observatory we measured the radial velocities of the sdB primaries and determined orbital periods of 0.26584+/-0.00004 days and 0.46249+/-0.00007 days for GALEX J0321+4727 and GALEX J2349+3844, respectively. The time series obtained from the Northern Sky Variability Survey with an effective wavelength near the R band show that GALEX J0321+4727 is a variable star (Delta m=0.12 mag) while no significant variations are observed in GALEX J2349+3844. The period of variations in GALEX J0321+4727 coincides with the orbital period and the variability is probably caused by a reflection effect on a late-type secondary star. Lack of photometric variations in GALEX J2349+3844 probably indicates that the companion is a white dwarf star. Using all available photometry and spectroscopy, we measured the atmospheric properties of the two sdB stars and placed limits on the mass and luminosity of the companion stars.

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arXiv2010

The double degenerate system NLTT 11748

We show that the extremely low-mass white dwarf NLTT 11748 (0.17 M_sun) is in a close binary with a fainter companion. We obtained a series of radial velocity measurements of the low-mass white dwarf using the Halpha core and determined an orbital period of 5.64 hours. The velocity semi-amplitude (K=274.8 km/s) and orbital period imply that it is a degenerate star, and that the minimum mass for the companion is 0.75 M_sun (assuming a mass of 0.167 M_sun for the primary). Our analysis of Balmer line profiles shows that a 0.75 M_sun white dwarf companion does not contribute more than 2% or 5% of the flux (V-band) for helium- or hydrogen-rich surfaces, respectively. The kinematics of the system suggest that it belongs to the Galactic halo.

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arXiv2010

The heavily polluted atmosphere of the DAZ white dwarf GALEX J193156.8+011745

We report on the discovery of a new heavily polluted white dwarf. The DAZ white dwarf GALEX J193156.8+011745 was identified in a joint GALEX/GSC survey of ultraviolet-excess objects. Optical spectra obtained at ESO NTT show strong absorption lines of magnesium and silicon and a detailed abundance analysis based on VLT-Kueyen UVES spectra reveal super-solar abundances of silicon and magnesium, and near-solar abundances of oxygen, calcium, and iron. The overall abundance pattern bears the signature of ongoing accretion onto the white dwarf atmosphere. The infrared spectral energy distribution shows an excess in the H and K bands likely associated with the accretion source.

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arXiv2009

The double degenerate LP400-22 revisited

We re-examine the properties of the high-velocity and extremely low mass white dwarf LP 400-22 and its close companion. Based on an extended observation timeline we determined a binary period of P= 1.01016 d, somewhat longer than the previously published period, and a mass function f(M_2)= 0.180 M_solar implying a mass for the companion M_2 > 0.41 M_solar. We also re-appraised the mass and cooling age of the white dwarf using low-metallicity (Z=0.001) evolutionary models appropriate for an old halo member, M_1 = 0.19 M_solar and t_cool~1.8 Gyr, and we infer a mass of 0.85 - 1.0 M_solar for the progenitor of the white dwarf. We discuss the likely origin of this system.

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arXiv2009

A new extremely low-mass white dwarf in the NLTT catalogue

We report on the discovery of the extremely low-mass, hydrogen-rich white dwarf, NLTT 11748. Based on measurements of the effective temperature (8540+/-50 K) and surface gravity (log g = 6.20+/-0.15) obtained by fitting the observed Balmer line profiles with synthetic spectra, we derive a mass of 0.167+/-0.005 M_solar. This object is one of only a handful of white dwarfs with masses below 0.2 M_solar that are believed to be the product of close binary evolution with an episode of Roche lobe overflow onto a degenerate companion (neutron star or white dwarf). Assuming membership in the halo population, as suggested by the kinematics and adopting a cooling age of 4.0 - 6.3 Gyrs for the white dwarf, we infer a progenitor mass of 0.87 - 0.93 M_solar. The likely companion has yet to be identified, but a search for radial velocity variations may help constrain its nature.

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arXiv2009

Spectroscopic observations of high proper motion DA white dwarfs

We used the revised New Luyten Two-Tenths (rNLTT) catalog to select high proper motion white dwarf candidates. We studied the spectra of 70 hydrogen-rich (DA) white dwarfs, which were obtained at the Cerro Tololo Inter-American Observatory (CTIO) and extracted from the Sloan Digital Sky Survey (SDSS). We determined their effective temperature and surface gravity by fitting their Balmer line profiles to model white dwarf spectra. Using evolutionary mass-radius relations we determined their mass and cooling age. We also conducted a kinematical study of the white dwarf sample and found that most belong to the thin disk population. We have identified three magnetic white dwarfs and estimated their surface magnetic field. Finally, we have identified 6 white dwarfs that lie within 20 pc from the Sun.

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arXiv2008

On the empirical evidence for the existence of ultra-massive white dwarfs

We re-examine the evidence for the existence of ultra-massive (M > 1.1 M_sun) white dwarfs based on gravitational redshift of white dwarfs in common proper motion binaries or in clusters, on parallax measurements, on orbital solutions, and, finally, on the analysis of hydrogen line profiles. We conclude that the best evidence is largely based on the analysis of Balmer line profiles although the companion to the A8V star HR 8210 is a compelling case made initially using the large binary mass function and confirmed by an analysis of the Lyman line spectrum. The confirmation and identification of high-mass white dwarfs, more particularly non-DA white dwarfs, using parallax measurements may prove critical in establishing the population fraction of these objects and in constraining the high-end of empirical initial-mass to final-mass relations. The existence of a substantial population of ultra-massive white dwarfs supports the concept of a steeper initial-mass to final-mass relations linking 6 M_sun progenitors with approximately greater than 1.1 M_sun white dwarfs.

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arXiv2007

The H-alpha stellar and interstellar emission in the open cluster NGC 6910

We verify the nature of emission-line stars in the field of the open cluster NGC 6910. % Spectroscopy in the H-alpha region was obtained. Raw CCD frames of spectra of all stars fainter than V = 9 mag observed by us are significantly affected by nebular emission originating in the surrounding HII region IC 1318. After careful data reduction and subtraction of the nebular radiation we succeeded in obtaining reliable stellar spectra. We confirm that the star NGC 6910 37 is a Be star, and we have corrected the classification of V1973 Cyg from an Ae star to a normal A type star. Since the diffuse interstellar bands do not appear in the spectrum of this star while being present in the other stars we observed, we confirm that V1973 Cyg is a foreground object with respect to IC 1318 and NGC 6910. We also find that the H-alpha line in HD 194279 has a P Cygni profile and the H-alpha line profile is variable in HD 229196.

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arXiv2007

LP133-373: A New Chromospherically Active Eclipsing dMe Binary with a Distant, Cool White Dwarf Companion

We report the discovery of the partially eclipsing binary LP 133-373. Nearly identical eclipses along with observed photometric colors and spectroscopy indicate that it is a pair of chromospherically active dM4 stars in a circular 1.6 d orbit. Light and velocity curve modeling to our differential photometry and velocity data show that each star has a mass and radius of 0.340+/-0.014 M_sun and 0.33+/-0.02 R_sun. The binary is itself part of a common proper motion pair with LP 133-374 a cool DC or possible DA white dwarf with a mass of 0.49-0.82 M_sun, which would make the system at least 3 Gyr old.

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