R. Napiwotzki

The hot subdwarf B + white dwarf binary KPD 1930+2752 A supernova type Ia progenitor candidate ,,, †

Context. The nature of the progenitors of type Ia supernovae is still under debate. KPD 1930+2752 is one of the best SN Ia progenitor candidates known today. The object is a double degenerate system consisting of a subluminous B star (sdB) and a massive white dwarf (WD). Maxted et al. (2000) conclude that the system mass exceeds the Chandrasekhar mass. This conclusion, however, rests on the assumption that the sdB mass is 0.5 M� . However, recent binary population synthesis calculations suggest that the mass of an sdB star may range from 0.3 Mto more than 0.7 M� . Aims. It is therefore important to measure the mass of the sdB star simultaneously with that of the white dwarf. Since the rotation of the sdB star is tidally locked to the orbit, the inclination of the system can be constrained if the sdB radius and the projected rotational velocity can be measured with high precision. An analysis of the ellipsoidal variations in the light curve allows the constraints derived from spectroscopy to be tightened. Methods. We derived the mass-radius relation for the sdB star from a quantitative spectral analysis of 150 low-resolution spectra obtained with the Calar Alto 2.2 m telescope using metal-rich, line-blanketed LTE model atmospheres with and without NLTE line formation. The projected rotational velocity was determined for the first time from 200 high-resolution spectra obtained with the Keck I 10 m and with the ESO-VLT 8.2 m telescopes. In addition a reanalysis of the published light curve was performed. Results. The atmospheric and orbital parameters were measured with unprecedented accuracy. In particular the projected rotational velocity vrot sini = 92.3 ± 1. 5k m s −1 was determined. Assuming the companion to be a white dwarf, the mass of the sdB is limited between 0.45 Mand 0.64 Mand the corresponding total mass of the system ranges from 1.33 Mto 2.04 M� . This constrains the inclination to i > 68 ◦ . The photometric analysis allows the parameters to be constrained even more. A neutron star companion can be ruled out and the mass of the sdB is limited to the range between 0.45 Mand 0.52 M� . The total mass of the system ranges from 1.36 Mto 1.48 Mand hence is likely to exceed the Chandrasekhar mass. The inclination angle is 80 ◦ and the light curve shows weak and shallow signs of eclipses. A high-precision light curve is needed in order to accurately measure these eclipses. So KPD 1930+2752 qualifies as an excellent double degenerate supernova Ia progenitor candidate. Conclusions.

Rotation velocities of white dwarfs determined from the Ca II K line

Department of Physics & Astronomy and Center for Astrobiology, UCLA, Los Angeles, CA 90095-1562, USAAbstract. We determine projected rotation velocities v sin i in DAZ white dwarfs, for the first time using therotational broadening of the CaII K line. The results confirm previous findings that white dwarfs are very slowrotators, and set even more stringent upper limits of typically less than 10 km/s. The few exceptions include 3stars known or suspected to be variable ZZ Ceti stars, where the line broadening is very likely not due to rotation.The results demonstrate that the angular momentum of the core cannot be preserved completely between mainsequence and final stage.Key words. stars: white dwarfs – stars: rotation

ASTROMETRY WITH THE HUBBLE SPACE TELESCOPE: TRIGONOMETRIC PARALLAXES OF PLANETARY NEBULA NUCLEI NGC 6853, NGC 7293, ABELL 31, AND DeHt 5*

We present absolute parallaxes and relative proper motions for the central stars of the planetary nebulae NGC 6853 (The Dumbbell), NGC 7293 (The Helix), Abell 31, and DeHt 5. This paper details our reduction and analysis using DeHt 5 as an example. We obtain these planetary nebula nuclei (PNNi) parallaxes with astrometric data from Fine Guidance Sensors FGS 1r and FGS 3, white-light interferometers on the Hubble Space Telescope. Proper motions, spectral classifications and VJHKT2M and DDO51 photometry of the stars comprising the astrometric reference frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each PNN. Weighted averaging with previous independent parallax measurements yields an average parallax precision, σπ/π = 5%. Derived distances are: d NGC 6853 = 405+28 –25 pc, d NGC 7293 = 216+14 –12 pc, d Abell 31 = 621+91 –70 pc, and d DeHt 5 = 345+19 –17 pc. These PNNi distances are all smaller than previously derived from spectroscopic analyses of the central stars. To obtain absolute magnitudes from these distances requires estimates of interstellar extinction. We average extinction measurements culled from the literature, from reddening based on PNNi intrinsic colors derived from model SEDs, and an assumption that each PNN experiences the same rate of extinction as a function of distance as do the reference stars nearest (in angular separation) to each central star. We also apply Lutz-Kelker bias corrections. The absolute magnitudes and effective temperatures permit estimates of PNNi radii through both the Stefan-Boltzmann relation and Eddington fluxes. Comparing absolute magnitudes with post-AGB models provides mass estimates. Masses cluster around 0.57 , close to the peak of the white dwarf mass distribution. Adding a few more PNNi with well-determined distances and masses, we compare all the PNNi with cooler white dwarfs of similar mass, and confirm, as expected, that PNNi have larger radii than white dwarfs that have reached their final cooling tracks.

Gaia photometry for white dwarfs

Context. White dwarfs can be used to study the structure and evolution of the Galaxy by analysing their luminosity function and initial mass function. Among them, the very cool white dwarfs provide the information for the early ages of each population. Because white dwarfs are intrinsically faint only the nearby ( 20 pc) sample is reasonably complete. The Gaia space mission will drastically increase the sample of known white dwarfs through its 5‐6 years survey of the whole sky up to magnitude V = 20‐25. Aims. We provide a characterisation of Gaia photometry for white dwarfs to better prepare for the analysis of the scientific output of the mission. Transformations between some of the most common photometric systems and Gaia passbands are derived. We also give estimates of the number of white dwarfs of the di erent galactic populations that will be observed. Methods. Using synthetic spectral energy distributions and the most recent Gaia transmission curves, we computed colours of three di erent types of white dwarfs (pure hydrogen, pure helium, and mixed composition with H/He= 0:1). With these colours we derived transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey, and 2MASS). We also present numbers of white dwarfs predicted to be observed by Gaia. Results. We provide relationships and colour-colour diagrams among di erent photometric systems to allow the prediction and/or study of the Gaia white dwarf colours. We also include estimates of the number of sources expected in every galactic population and with a maximum parallax error. Gaia will increase the sample of known white dwarfs tenfold to about 200 000. Gaia will be able to observe thousands of very cool white dwarfs for the first time, which will greatly improve our understanding of these stars and early phases of star formation in our Galaxy.

The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims to find sdBs with compact companions like massive white dwarfs, neutron stars or black holes. Here we provide classifications, atmospher ic parameters and a complete radial velocity (RV) catalogue containing 1914 single measurements for an sample of 177 hot subluminous stars discovered based on SDSS DR7. 110 stars show significant RV va riability, while 67 qualify as candidates. We constrain the fraction of close massive compact companions of hydrogen-rich hot subdwarfs in our sample to be smaller than ∼ 1.3%, which is already close to the theoretical predictions. However, the sample might still contain such binaries with longer periods exceeding∼ 8 d. We detect a

Time-resolved observations of the short period CV SDSS J123813.73-033933.0

Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the Pacific

A survey for post-common-envelope binary stars using GALEX and SDSS photometry

We report the first results of our programme to obtain multi-epoch radial velocity measurements of stars with a strong far-ultraviolet excess to identify post-common-envelope binaries (PCEBs). The targets have been identified using optical photometry from Sloan Digital Sky Survey (SDSS) DR4, ultraviolet photometry from Galaxy Evolution Explorer (GALEX )G R2 and proper motion information from SDSS DR5. We have obtained spectra at two or more epochs for 36 targets. Three of our targets show large radial velocity shifts (>50 km s −1 ) on a time-scale of hours or days and are almost certainly PCEBs. For one of these targets (SDSS J104234.77+644205.4) we have obtained further spectroscopy to confirm that this is a PCEB with an orbital period of 4.74 h and semi-amplitude K = 165 km s −1 . Two targets are rapidly rotating K-dwarfs which appear to show small radial velocity shifts and have strong Ca II H&K emission lines. These may be wind-induced rapidly rotating (WIRRing) stars. These results show that we can use GALEX and SDSS photometry to identify PCEBs that cannot be identified using SDSS photometry alone, and to identify new WIRRing stars. A more comprehensive survey of stars identified using the methods developed in this paper will lead to a much improved understanding of common envelope evolution.

A new merging double degenerate binary in the solar neighborhood

Characterizing the local space density of double degenerate binary systems is a complementary approach to broad sky surveys of double degenerates to determine the expected rates of white dwarf binary mergers, in particular those that may evolve into other observable phenomena such as extreme helium stars, Am CVn systems, and supernovae Ia. However, there have been few such systems detected in local space. We report here the discovery that WD 1242

Multiband photometry and spectroscopy of an all-sky sample of bright white dwarfs

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The MUCHFUSS Project—Searching for Massive Compact Companions to Hot Subdwarf Stars

105, a nearby bright WD, is a double-line spectroscopic binary consisting of two degenerate DA white dwarfs of similar mass and temperature, despite it previously having been spectroscopically characterized as a single degenerate. Follow-up photometry, spectroscopy, and trigonometric parallax have been obtained in an effort to determine the fundamental parameters of each component of this system. The binary has a mass ratio of 0.7 and a trigonometric parallax of 25.5 mas, placing it at a distance of 39 pc. The systems total mass is 0.95 M