High resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey III. DA white dwarfs

Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO) DOI: 10.1051/0004-6361/200912531Context. The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Aims. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. Methods. The spectra are compared with theoretical model atmospheres using a fitting technique. Results. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).Peer reviewe

A New Version of Reimers' law of Mass Loss Based on a Physical Approach

We present a new semi-empirical relation for the mass loss of cool stellar winds, which so far has frequently been described by "Reimers' law". Originally, this relation was based solely on dimensional scaling arguments without any physical interpretation. In our approach, the wind is assumed to result from the spill-over of the extended chromosphere, possibly associated with the action of waves, especially Alfven waves, which are used as guidance in the derivation of the new formula. We obtain a relation akin to the original Reimers law, but which includes two new factors. They reflect how the chromospheric height depends on gravity and how the mechanical energy flux depends, mainly, on effective temperature. The new relation is tested and sensitively calibrated by modelling the blue end of the Horizontal Branch of globular clusters. The most significant difference from mass loss rates predicted by the Reimers relation is an increase by up to a factor of 3 for luminous late-type (super-)giants, in good agreement with observations.Comment: 12 pages, 4 figures, accepted by ApJ Letter

WIYN Open Cluster Study XI: WIYN 3.5m Deep Photometry of M35 (NGC 2168)

We present deep BVI observations of the core of M35 and a nearby comparison field obtained at the WIYN 3.5m telescope under excellent seeing. These observations display the lower main sequence in BV and VI CMDs down to V = 23.3 and 24.6, respectively. At these faint magnitudes background field stars are far more numerous than the cluster stars, yet by using a smoothing technique and CMD density distribution subtraction we recover the cluster fiducial main sequence and luminosity function to V = 24.6. We find the location of the main sequence in these CMDs to be consistent with earlier work on other open clusters, specifically NGC 188, NGC 2420, and NGC 2477. We compare these open cluster fiducial sequences to stellar models by Baraffe et al. (1998), Siess et al. (2000), Girardi et al. (2000), and Yi et al. (2001) and find that the models are too blue in both B-V and V-I for stars below ~0.4 Mo. M35 contains stars to the limit of the extracted main sequence, at M ~ 0.10-0.15 Mo, suggesting that M35 may harbor a large number of brown dwarfs, which should be easy targets for near-IR instrumentation on 8-10m telescopes. We also identify a new candidate white dwarf in M35 at V = 21.36 +- 0.01. Depending on which WD models are used to interpret this cluster candidate, it is either a very high mass WD (1.05 +- 0.05 Mo) somewhat older (0.19-0.26 Gyr, 3-4 sigma) than our best isochrone age (150 Myr), or it is a modestly massive WD (0.67-0.78 Mo) much too old (0.42-0.83 Gyr) to belong to the cluster.Comment: 28 pages + 24 figures; to be published in the Sept, 2002 A

The ESO supernovae type Ia progenitor survey (SPY) : The radial velocities of 644 DA white dwarfs

Accepted for publication in A&A. 59 pages.Close double degenerate binaries are one of the favoured progenitor channels for type Ia supernovae, but it is unclear how many suitable systems there are in the Galaxy. We report results of a large radial velocity survey for double degenerate (DD) binaries using the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY - SPY). Exposures taken at different epochs are checked for radial velocity shifts indicating close binary systems. We observed 689 targets classified as DA (displaying hydrogen-rich atmospheres), of which 46 turned out to possess a cool companion. We measured radial velocities (RV) of the remaining 643 DA white dwarfs. We managed to secure observations at two or more epochs for 625 targets, supplemented by eleven objects meeting our selection criteria from literature. The data reduction and analysis methods applied to the survey data are described in detail. The sample contains 39 double degenerate binaries, only four of which were previously known. 20 are double-lined systems, in which features from both components are visible, the other 19 are single-lined binaries. We provide absolute RVs transformed to the heliocentric system suitable for kinematic studies. Our sample is large enough to sub-divide by mass: 16 out of 44 low mass targets (0.45 Msun are double. Although the detected fraction amongst the low mass objects (36.4 +/- 7.3%) is significantly higher than for the higher-mass, carbon/oxygen-core dominated part of the sample (3.9 +/- 0.8%), it is lower than the detection efficiency based upon companion star masses >= 0.05 Msun. This suggests either companion stars of mass <0.05 Msun, or that some of the low mass white dwarfs are single.Peer reviewe

The nature of the progenitor of the Type II-P supernova 1999em

We present high quality ground-based VRI images of the site of the Type II-P SN1999em (in NGC1637) taken before explosion, which were extracted from the CFHT archive. We determine a precise position of the SN on these images to an accuracy of 0.17''. The host galaxy is close enough (7.5 +/- 0.5 Mpc) that the bright supergiants are resolved as individual objects, however we show that there is no detection of an object at the SN position before explosion that could be interpreted as the progenitor star. By determining the sensitivity limits of the VRI data, we derive bolometric luminosity limits for the progenitor. Comparing these to standard stellar evolutionary tracks which trace evolution up to the point of core carbon ignition, we initially derive an upper mass limit of approximately 12M_sol. However we present evolutionary calculations that follow 7-12M_sol stars throughout their C-burning lifetime and show that we can restrict the mass of the progenitor even further. Our calculations indicate that progenitors initially of 8-10M_sol, undergoing expected mass loss, can also be excluded because a second dredge up sends them to somewhat higher luminosities than a star of initially 12M_sol. These results limit the progenitor's initial main-sequence mass to a very narrow range of 12 +/- 1 M_sol. We discuss the similarities between the Type II-P SNe 1999em and 1999gi and their progenitor mass limits, and suggest that SN Type II-P originate only in intermediate mass stars of 8-12M_sol, which are in the red supergiant region and that higher mass stars produce the other Type II sub-types. (Abridged).Comment: Replaced with accepted version to appear in ApJ, 30 pages, inc. 6 figure

HE 0557-4840 - Ultra-Metal-Poor and Carbon-Rich

We report the discovery and high-resolution, high S/N, spectroscopic analysis of the ultra-metal-poor red giant HE 0557-4840, which is the third most heavy-element deficient star currently known. Its atmospheric parameters are T_eff = 4900 K, log g = 2.2, and [Fe/H]= -4.75. This brings the number of stars with [Fe/H] < -4.0 to three, and the discovery of HE 0557-4840 suggests that the metallicity distribution function of the Galactic halo does not have a "gap" between [Fe/H] = -4.0, where several stars are known, and the two most metal-poor stars, at [Fe/H] ~ -5.3. HE 0557-4840 is carbon rich - [C/Fe] = +1.6 - a property shared by all three objects with [Fe/H] < -4.0, suggesting that the well-known increase of carbon relative to iron with decreasing [Fe/H] reaches its logical conclusion - ubiquitous carbon richness - at lowest abundance. We also present abundances (nine) and limits (nine) for a further 18 elements. For species having well-measured abundances or strong upper limits, HE 0557-4840 is "normal" in comparison with the bulk of the stellar population at [Fe/H] ~ -4.0 - with the possible exception of Co. We discuss the implications of these results for chemical enrichment at the earliest times, in the context of single ("mixing and fallback") and two-component enrichment models. While neither offers a clear solution, the latter appears closer to the mark. Further data are required to determine the oxygen abundance and improve that of Co, and hence more strongly constrain the origin of this object.Comment: Submitted to Astrophysical Journal. 52 pages (41 text, 11 figures

Deep, Wide-field CCD Photometry for the Open Cluster NGC3532

We present the results of a deep, wide-field CCD survey for the open cluster NGC~3532. Our new $BV(RI)_{c}$ photometry effectively covers a one square degree area and reaches an unprecedented depth of $V\sim21$ to reveal that NGC~3532 is a rich open cluster that harbors a large number of faint, low-mass stars. We employ a number of methods to reduce the impact of field star contamination in the cluster color-magnitude diagrams, including supplementing our photometry with $JHK_{s}$ data from the 2MASS catalog. These efforts allow us to define a robust sample of candidate main sequence stars suitable for a purely empirical determination of the cluster's parameters by comparing them to the well-established Hyades main sequence. Our results confirm previous findings that NGC~3532 lies fairly near to the Sun [$(m-M)_0=8.46\pm0.05$; $492^{+12}_{-11}$~pc] and has an extremely low reddening for its location near the Galactic plane [$E(B-V)=0.028\pm0.006$]. Moreover, an age of $\sim300$\,Myr has been derived for the cluster by fitting a set of overshooting isochrones to the well-populated upper main-sequence. This new photometry also extends faint enough to reach the cluster white dwarf sequence, as confirmed by our photometric recovery of eight spectroscopically identified members of the cluster. Using the location of these eight members, along with the latest theoretical cooling tracks, we have identified $\sim30$ additional white dwarf stars in the $[V,~(B-V)]$ color-magnitude diagram that have a high probability of belonging to NGC~3532. The age we derive from fitting white dwarf isochrones to the locus of these stars, $300\pm100$\,Myr, is consistent with the age derived from the turnoff. Our analysis of the photometry also includes an estimation of the binary star fraction, as well as a determination of the cluster's luminosity and mass functions.Comment: Accepted for publication in AJ. 55 pages, 21 figures. High-quality version with complete data tables can be downloaded from http://www.phys.lsu.edu/~jclem/NGC3532

New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables

Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to six the total number of X-ray faint, magnetic accretion binaries that accrete at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in cataclysmic variables. This fact, coupled with donor stars that underfill their Roche lobes and very cool white dwarfs, brand the binaries as post common-envelope systems whose orbits have not yet decayed to the point of Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems exhibit spin/orbit synchronism and apparently accrete by efficient capture of the stellar wind from the secondary star, a process that has been dubbed a ``magnetic siphon''. Because of this, period evolution of the binaries will occur solely by gravitational radiation, which is very slow for periods >3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors (in addition to the asynchronous Intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG.Comment: 25 pages, 13 figures, Accepted to Ap

UV and X-Ray Monitoring of AG Draconis During the 1994/1995 Outbursts

The recent 1994-1995 active phase of AG Draconis has given us for the first time the opportunity to follow the full X-ray behaviour of a symbiotic star during two successive outbursts and to compare with its quiescence X-ray emission. With \ros observations we have discovered a remarkable decrease of the X-ray flux during both optical maxima, followed by a gradual recovering to the pre-outburst flux. In the UV the events were characterized by a large increase of the emission line and continuum fluxes, comparable to the behaviour of AG Dra during the 1980-81 active phase. The anticorrelation of X-ray/UV flux and optical brightness evolution is shown to very likely be due to a temperature decrease of the hot component. Such a temperature decrease could be produced by an increased mass transfer to the burning compact object, causing it to slowly expand to about twice its original size.Comment: 12 pages postscript incl. figures, Proc. of Workshop on Supersoft X-Ray Sources, to appear in Lecture Notes in Physics vol. 472 (1996