EDP Sciences
Free Access
Issue
A&A
Volume 505, Number 1, October I 2009
Page(s) 441 - 462
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/200912531
Published online 03 August 2009

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

III. DA white dwarfs[*]

D. Koester1 - B. Voss2,1 - R. Napiwotzki3 - N. Christlieb4 - D. Homeier5 - T. Lisker6,8 - D. Reimers7 - U. Heber8

1 - Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
2 - Zeiss-Planetarium, LWL-Museum für Naturkunde, 48161 Münster, Germany
3 - Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
4 - Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
5 - Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
6 - Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
7 - Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
8 - Dr. Karl Remeis Observatory, University of Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany

Received 19 May 2009 / Accepted 24 July 2009

Abstract
Context. 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 $\chi^2$ 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).

Key words: stars: white dwarfs

1 Introduction

The ESO SN Ia Progenitor Survey (SPY) is a radial velocity survey that was conducted to test the double-degenerate channel of the formation of supernovae Ia. About 800 white dwarfs were observed (most of them twice) in the course of the survey, assembling a large collection of high quality white dwarf spectra. Most of the targets for the SPY were selected from the White Dwarf catalog (McCook & Sion 1999), MCS further on, from the Hamburg/ESO Survey (Wisotzki et al. 1996; Christlieb et al. 2001), HES further on, and from the Hamburg Quasar Survey (Hagen et al. 1995), HQS further on. Some additional objects come from the Montreal-Cambridge survey (Demers et al. 1990; Lamontagne et al. 2000) and from the Edinburgh-Cape survey (Kilkenny et al. 1991). A detailed discussion of the target selection is given in Napiwotzki et al. (2003,2001). While some of the input catalogs, e.g. HES and HQS, have fairly well understood selection criteria, for the combined input catalog this would be very difficult, since the only criteria used were ``spectroscopic identification (at least from objective prism spectra) and B < 16.5'' (Napiwotzki et al. 2001).

The high quality spectra of white dwarfs were employed for a number of studies beyond the original scope of the SPY, the search for double-degenerate binaries. Koester et al. (2001) derived temperatures and gravities from a preliminary sample of about 200 objects; 71 helium-rich stars of spectral type DB and DBA were studied in Voss et al. (2007), and approximately 60 objects with detected Ca II resonance lines were discussed in Koester et al. (2005). Pauli et al. (2006,2003) studied the 3D kinematics using the SPY data, new DO and PG1159 stars have been identified by Werner et al. (2004). The hot subdwarf population has been studied by Lisker et al. (2005) and Stroeer et al. (2007).

The number of newly detected white dwarfs is of course dwarfed by the results from the Sloan Digital Sky Survey (e.g. Eisenstein et al. 2006; Kleinman et al. 2004). It is also possible to extract samples with much better controlled and understood selection effects from the SDSS data base. Thus, we will not produce yet another white dwarf mass distribution or luminosity function here. However, our sample contains much brighter objects than the typical SDSS white dwarf. These objects, and in particular the magnetic stars, binaries, or new variables will be much easier to study in follow-up observations than the faint SDSS objects. Moreover, the white dwarf parameters presented here are an important ingredient of the kinematic studies mentioned above and the analysis and interpretation of the binary results, starting with the simple distinction between helium core and carbon/oxygen core white dwarfs.

2 Observations

Since several of the papers above have given a detailed description of the data properties and further references, we only briefly summarize this information here.

The spectra were obtained with UVES, a high resolution echelle spectrograph at the ESO VLT telescope. UVES was used in a dichroic mode, resulting in small gaps, ${\approx}80$ Å wide, at 4580 Å and 5640 Å in the final merged spectrum. The spectral resolution at H$\alpha $ is R=18 500 or better, and the S/N per binned pixel (0.05 Å) is S/N=15 or higher. The total wavelength range covered is $\approx$3500 to 6650 Å.

The spectra were reduced with the ESO pipeline for UVES, including the merging of the echelle orders and the wavelength calibration. Koester et al. (2001) found that the quality of these automatically extracted spectra was very good, except for a quasi-periodic wave-like pattern that occurs in some of the spectra. The reduction has since been further improved by additional processing by collaborators of the SPY at the University of Erlangen-Nürnberg. The most important step was utilizing the featureless spectra of DC white dwarfs to remove almost completely the large scale variations of the spectral response function (Napiwotzki et al. 2003,2001). Some artifacts remain in the data, but they do not significantly affect the spectral analysis.

2.1 Model atmosphere fits

The spectral analysis of these data was originally performed by Voss (2006). Since then, the models were significantly improved by including in a consistent way the Balmer line broadening due to simultaneous interactions with neutral and charged perturbers. An up-to-date description of the methods and input physics is presented in Koester (2009). We have therefore repeated the whole fitting process for all objects; the major differences to Voss (2006) appear, as expected, at the cool end of the DA sequence.

The $\chi^2$ minimization fitting routine is based on the Levenberg-Marquardt algorithm (Press et al. 1992) to derive the best fitting effective temperature and surface gravity for each spectrum. Some more details on the fitting process can be found in Homeier et al. (1998). For the present study we applied pure hydrogen models and fitted the Balmer lines H$\alpha $ to H9. To demonstrate the typical quality of the data and of the model fits, we present the results for the arbitrarily chosen entries 301 to 310 from Table 1. The header of each panel gives the name, effective temperature, and surface gravity of the fit. Shown are the six lowest Balmer lines.

3 Data and atmospheric parameters for DAs

Table 1 lists the results of the model fitting for 615 apparently normal DA white dwarfs. Of those, 187 are newly identified DAs, 111 from the HES (Christlieb et al. 2001, designation HE) and 76 from the HQS (Homeier et al. 1998, designationHS) surveys. The primary designation of the objects in the first column is WD, if the objects appear in the SIMBAD or MCS databases and were known to be spectroscopically identified white dwarfs prior to the start of the SPY. A few objects have EC or MCT designations, if they were identified in the Edinburgh-Cape (Kilkenny et al. 1991) or Montreal-Cambridge-Tololo (Demers et al. 1990; Lamontagne et al. 2000) surveys, but don't seem to have WD designations. Column 5 gives a few alternative names. This list is not intended to be complete, but additional information can easily be found in SIMBAD or MCS.

Table 1:   Fit results for hydrogen atmosphere stars.

For the remainder of the objects we use HE or HS designations, depending on the original catalog. In this case there are three different meanings of Col. 5

  • if empty, there is no entry in either the SIMBAD or the MCS database and we assume that this is a new detection of the HQS or HES;

  • if there is an entry in parentheses, this means that there is an entry in SIMBAD, but no spectroscopic identification as a white dwarf, at least not prior to identification in a publication related to HQS, HES, or SPY. This mostly concerns catalogs of blue or large proper motion objects. Some objects are also contained in more recent catalogs, e.g. Sloan Digital Sky Survey = SDSS (Adelman-McCarthy et al. 2008), or Two Micron All Sky Survey = 2MASS, (Cutri et al. 2003), but not identified as white dwarfs;

  • if there is a regular entry in Col. 5, this describes a recent spectroscopic identification, which was unknown at the time of our target selection. Catalog entries here are SDSS (Eisenstein et al. 2006; Kleinman et al. 2004), BGK (Brown et al. 2006), Kawka06 (Kawka & Vennes 2006).
The magnitude column lists the V magnitude, if available from the SIMBAD or MCS databases. If the number is followed by a B, this is either a Johnson B magnitude, or, in most cases, a photographic magnitude from the MCT, HQS, or HES surveys. mc denotes a Greenstein multichannel magnitude, obtained from the MCS catalog. The errors of the photographic magnitudes are typically much larger (0.1-0.2 mag) than suggested by the two decimal places, which are kept only to obtain a more homogeneous table.

 \begin{figure}
\par\includegraphics[width=14.1cm,clip]{12531fg1.ps}
\end{figure} Figure 1:

Typical example for observations and fit, taking arbitrarily the entries 301 to 310 from Table 1. The header of each panel gives the name, effective temperature, and surface gravity of the fit. Shown are the six lowest Balmer lines. Vertical axis is relative intensity in arbitrary units, higher lines are offset for clarity. The light grey lines are the models.

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Since most stars have more than one spectrum observed, the parameters are the weighted averages of the individual solutions, with the inverse square of the formal 1$\sigma$ uncertainties as weights. The 1$\sigma$ final uncertainties given in Table 1 are obtained from the individual values and should only be used as an indicator of the quality of the data. As is well known, with spectra of the quality used here, the systematic errors from the reduction and fitting process are usually much larger than the purely statistical uncertainties. We estimated more realistic uncertainties by comparing the differences between solutions from several spectra of the same object. For 592 objects with multiple solutions we obtain standard deviations of $\sigma(\mbox{$T_{\rm eff}$ })$ $\approx$ $2.5\%$ and $\sigma(\mbox{$\log g$ })$ $\approx$ 0.09. These should be regarded as lower limits, because the different spectra still used the same observational setup, theoretical models, and fitting procedures. The uncertainties are definitely larger than this at the high temperature end above 50 000 K, because NLTE effects are not considered in our models. They are also larger, in particular for the surface gravity, at temperatures below 8000 K, because the spectra become less sensitive to this parameter, and because the neutral broadening of the Balmer lines higher than H$\gamma$ is only approximative.

Another method to estimate the size of the uncertainties is the comparison of results by different authors for the same objects. The most interesting recent study is the work by Liebert et al. (2005) on the DA white dwarfs in the Palomar Green Survey. Eliminating DAs with  $T_{\rm eff}$ below 8000 K or above 50 000 K (see above), as well as the double degenerates, leaves 85 objects in common. Figure 2 shows the comparison for the effective temperatures, and Fig. 3 for the surface gravities. The systematic shift in $T_{\rm eff}$ for the whole sample is 1.2%, with our temperatures being slightly higher. For $\log g$ the shift is 0.08, with our values lower. We can estimate the intrinsic uncertainties of our determinations by first correcting for these systematic shifts. Comparing the corrected parameters with those of Liebert et al. (2005), the remaining standard deviations are $\sigma(\mbox{$T_{\rm eff}$ }) = 2.3\%$, and $\sigma(\mbox{$\log g$ }) = 0.08$. These values should be taken as indicative of the statistical errors of our results, and are compatible with the estimates derived above for the internal uncertainties from multiple spectra within our sample.

The surprisingly large systematic shift in surface gravity is unsatisfactory. A similar trend was also noted by Liebert et al. (2005) in their comparison with studies by other authors. In four out of five cases their $\log g$ was higher by 0.06-0.10 dex. Three of these used similar models to those used here, and the differences could arise from differences between the ``Bergeron'' and the ``Koester'' models, or from the fitting procedures used. We are, however, not aware of any obvious explanations for such differences. Systematic differences of this magnitude were also found by Napiwotzki et al. (1999) in a comparison of different studies using low-resolution spectra.

 \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg2.ps}
\end{figure} Figure 2:

Comparison of effective temperatures from this work with the results of Liebert et al. (2005), (=LBH) for 85 objects in common.

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 \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg3.ps}
\end{figure} Figure 3:

Comparison of surface gravities from this work with the results of Liebert et al. (2005), (=LBH) for 85 objects in common.

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A possible reason for systematic differences may also come from the nature of our observational data. The main purpose of the SPY was the determination of radial velocities, which needed high resolution; therefore the echelle spectrograph UVES was used. The wavelength interval per order ranges from ${\approx}30$ Å in the blue to ${\approx}50$ Å in the red region. Thus, e.g. H$\alpha $ at maximum strength will extend over six or more orders, which have to be flatfielded and merged, removing the strong sensitivity change within orders. A real flux calibration was not possible, but the quality of the spectra obtained after some reprocessing of the ESO pipeline results was still very impressive. Before the start of this project, we were not certain that the spectra would be useful for anything else except radial velocity determinations.

Nevertheless, the merging of the orders and the approximate flux calibration attempted may have left very subtle artifacts influencing the far wings of the strong lines, thus influencing in particular the surface gravity results. An indication for this is visible in Table 1 in Koester et al. (2001), which compares the results of fits to echelle vs. single-order low-resolution spectra for the same seven DAs. The average surface gravity is lower by 0.07 in the results from the echelle spectra. Another hint towards this effect can be found in the study of low-resolution SDSS spectra of brighter DA white dwarfs by Koester et al. (2009), which used the same models and fitting routines as this work. The average surface gravity for 578 DAs with magnitude g < 19, S/N > 10, and 8000 K $\le$ $T_{\rm eff}$ $\le$ 16 000 K is 8.014, while the value from our current sample for the same temperature range is 7.947 from 211 objects. These are not the same objects of course, but the samples are so large, that the difference is significant.

Also marked in Table 1 are known ZZ Ceti variables (DAV), as well as objects, which have been observed photometrically, but were not found to vary (NOV). The references for the NOV designations are: (1) Gianninas et al. (2005); (2) Kepler et al. (1995); (3) Mukadam et al. (2004); and (4) Bergeron et al. (2004). If no reference is given the classification is a result of the SPY and/or follow-up observations (Voss et al. 2006; Voss 2006; Silvotti et al. 2005; Castanheira et al. 2006). We have not marked candidates for variability studies, but obviously all objects in the range of $T_{\rm eff}$ 10 000-13 000 K are interesting in this respect.

The Balmer lines reach their maximum strength near $T_{\rm eff}$ = 13 000, the exact value depending on the surface gravity. Therefore quite often two different fitting solutions exist, which produce the same overall strength (i.e. equivalent width) of the lines. The $\chi^2$ values of the two minima are often similar, since model and observation are fitted in the far line wings and differences show up only in the inner core of the line. Visual inspection is usually sufficient to determine the correct solution. In a few cases, however, the difference was so small that we preferred to give both solutions in Table 1.

Because of the selection of the targets - as mentioned in the introduction - our sample is not well suited for a study of white dwarf population characteristics such as the mass distribution. However, it can be used to demonstrate an effect well known for many years (Kepler et al. 2007; Eisenstein et al. 2006; Kleinman et al. 2004; Koester et al. 2009; Bergeron et al. 1990a; DeGennaro et al. 2008; Bergeron 1992). This is the fact that the surface gravity seems to increase around $T_{\rm eff}$ $\approx$ 12 000 K towards lower temperatures. Figure 4 clearly shows this for 465 objects between 7500 and 25 000 K. Taking the direct averages without any weighting we find $\langle \log g \rangle$ = 7.86 for effective temperatures above 12 500 K and $\langle \log g \rangle$ = 8.06 below. This is very similar to the results in the SDSS (Data Release 4) as studied by Koester et al. (2009). A number of possible explanations is discussed in that study, and the most likely is found to be an inadequate description of convection with the mixing-length approximation. However, this problem is certainly not yet solved.

Table 2:   Magnetic or helium-contaminated hydrogen-rich stars.

4 Double-degenerate white dwarfs

Two or more spectra were taken for the vast majority of the SPY targets. Close binaries among them could be detected with a high level of confidence by checking for radial velocity variations indicating orbital motion. A total of 36 close binaries were detected among DA sample presented here from the spectra taken for the SPY. This count includes only the double degenerates, i.e. systems consisting of two white dwarfs. The DA+dM systems listed in Table 3 are not included in this count. Seventeen of the double-degenerates are double-lined, i.e. spectral lines of both white dwarfs are present in the spectra. The white dwarf companion in the single-lined systems is already so cool and faint that it does not produce a significant contribution to the combined spectrum.

Four more double-degenerates are marked in Table 1 as DD, but were found by independently obtained observations: HE 1511-0448 (Nelemans et al. 2005), WD 1241-010 (Marsh et al. 1995), WD 1022+050 and WD 2032+188 (Morales-Rueda et al. 2005).

The fitting procedure for the model atmosphere analysis is not affected by the binary nature of the single-lined binaries. The resulting fit parameters are those of the visible bright component. The situation is different for the double-lined systems. In these cases a deconvolution of simultaneous fit of both components would be necessary for accurate parameters. We have tools for this kind of analysis available (Napiwotzki et al. 2004), but in most cases more than the two spectra taken during the survey are needed to derive reliable parameters of both components. Here we present the results of a fit assuming a single star. Although these have to be taken with a pinch of salt, they are still a useful indication of the nature of properties of the binary. Double-lined systems are indicated in Table 1.

 \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg4.ps}
\end{figure} Figure 4:

Distribution of surface gravities for all objects between $T_{\rm eff}$ 7500-25 000 K.

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5 Objects with magnetic fields or helium contamination

Table 2 summarizes the data for some objects, which appear hydrogen-rich with some peculiarities, either Zeeman splitting of the Balmer lines due to a magnetic field or helium lines in addition to the stronger Balmer lines. For most of the stars we obtained fits with pure hydrogen model atmospheres. Since these are obviously not very reliable, we do not publish the parameters here, but discuss these stars individually.

Four magnetic DA stars in the SPY sample have not been published before, HS 0051+1145, HE 1233-0519, HS 1031+0343, and WD 2051-208. Two more objects were published first as DAH stars in Koester et al. (2001), WD 0058-044 and WD 0239+109. Four more magnetic DA, already described in the literature, are in the sample. Below, these ten objects and some additional white dwarfs of special interest are discussed.

HS 1031+0343.

This object is a new magnetic DA star. The only Zeeman triplet that is completely present, and for which the three components are well discernible, is that of H$\beta $. The $\sigma^-$, $\pi$, $\sigma^+$ components are found at 4771 Å, 4851 Å, and 4905 Å. The components of the higher lines are blended together, and the $\sigma^+$ component of H$\alpha $ is shifted by an amount that places it outside of the observed spectral range; thus only the $\sigma^-$ and $\pi$ components of H$\alpha $ are available in the SPY data, at 6560 Å and 6420 Å. The magnetic field is estimated as B = 6.1 $\pm$ 0.3 MG.

Table 3:   Data for DA+dM binaries. In boldface are the new SPY detections. For explanations of the magnitude column see text.

WD 0058-044.

This star has been published as a magnetic DA by Koester et al. (2001). The shifts of the $\sigma$ components with respect to the $\pi$ components are 6.2 $\pm$ 0.3 Å and 3.6 $\pm$ 0.3 Å, for H$\alpha $ and H$\beta $, respectively. The components of the higher Balmer lines are blended. The quadratic splitting of the lines is negligible here since the split is small and thus the field strength has to be low. The field is B= 330 $\pm$ 30 kG, from H$\alpha $, and B = 310 $\pm$ 20 kG, from H$\beta $.

WD 0239+109.

Greenstein & Liebert (1990) recognized an unusual line shape for this object, and suggested a magnetic field as one of the possible reasons. Bergeron et al. (1990b) interpreted the spectrum as that of an unresolved DA+DC binary. The SPY spectra in Koester et al. (2001) revealed the presence of Zeeman splitting of H$\alpha $ and thus proved the magnetic nature of the object. In the first SPY spectrum, the H$\alpha $ components are placed at 6576.6 Å, 6562.3 Å, and 6548.4 Å, and those of H$\beta $ at 4868.9 Å, 4861.0 Å, 4853.0 Å, and from that field strengths of 700 $\pm$ 20 kG, from H$\alpha $, and 720 $\pm$ 30 kG, from H$\beta $, can be derived.

WD 0257+080.

Bergeron et al. (1997) found a flat-bottom H$\alpha $ core for this object which is typical for white dwarfs with a low-strength magnetic field, but they were not able to identify a Zeeman triplet and estimated the field strength to be ${\sim}100$ kG. One of the two SPY spectra of WD 0257+080 clearly shows an H$\alpha $ triplet. In the other spectrum, which has a slightly lower S/N, only the $\sigma^-$ component is discernible. The H$\alpha $ components of the first spectrum are split by 1.8 $\pm$ 0.3 Å, from which B=90 $\pm$ 15 kG can be derived, a more precise value than the previous estimate.

WD 1953-011.

A weak Zeeman-split triplet of H$\alpha $ was found by Koester et al. (1998), from which they derived a field strength of 93 kG. Maxted et al. (2000) had noticed a variable depression in the wings of H$\alpha $ which they identified as additional Zeeman-split H$\alpha $ features, which led them to assume a non-simple field geometry with a strong spot-like field of ${\sim}500$ kG combined with a weaker 70 kG dipole field. Comparing the two SPY spectra, the H$\alpha $ wings appear deformed near 6550 Å and 6575 Å, which might allow a very rough estimate of field strengths of $\sim$500 kG up to $\sim$750 kG, if it is assumed that these features are of magnetic origin. However no variation of these features as described by Maxted et al. can be found, the line shape is very similar in both SPY spectra. The central triplet however is obvious, and the splitting of the core is 1.9 $\pm$ 0.2 Å. This corresponds to a dipole B field of 95 $\pm$ 10 kG.

WD 2105-820.

The spectrum of this star was found to show a flat-bottom H$\alpha $ core, probably due to a low magnetic field, by Koester et al. (1998), from which they derive a field strength of 43 kG. No Zeeman triplet is obvious in the SPY spectra as well, they also only exhibit the broadened, flat core; the full width of the core of 1.8 Å in the SPY spectra is consistent with the field strength derived by Koester et al. (1998).

WD 2359-434.

Koester et al. (1998) suspected that this DA could be magnetic due to its flat H$\alpha $ core, and a very low field of only 3 kG was polarimetrically found by Aznar Cuardrado et al. (2004). They selected this object as one of their program stars based on the criterion that the SPY spectra show no signs of Zeeman splitting; however they themselves note that flat Balmer line cores are present in the spectra of that object, and if these are caused by the B field, it would indicate a higher field strength than that derived from the polarimetry data. Kawka et al. (2007) measure a low field of 3.4 $\pm$ 4.4 kG.

The SPY spectra indeed do not only show a flat-bottom H$\alpha $ core, but within that core also a pronounced $\pi$ component and less clear, broad $\sigma$ components, centered at 6561.1 Å, 6563.5 Å, and 6565.8 Å. Thus a field strength of 110 $\pm$ 10 kG can be derived. This is two orders of magnitude stronger than found by Aznar Cuardrado et al. (2004) and Kawka et al. (2007). The reason for these different results is unclear.

WD 0446-789 and WD 1105-048.

These objects are the two remaining of the three for which Aznar Cuardrado et al. (2004) discovered magnetic fields of only a few kG from polarimetry data. The H$\alpha $ core of WD 0446-789 appears slightly broadened, the width is 1.8 Å, which is slightly wider than the average line core width of ${\sim}1$ Å. If we interpret this excess width as due to magnetic broadening, this would correspond to a field strength on the order of 10 kG. The line core of WD1105-048 shows no peculiarities, it has a normal width of 1 Å and thus no detectable field.

HS 0051+1145.

This object has previously been found as a blue source, PHL 886, but was not observed spectroscopically before the SPY. It is a new magnetic DA. The SPY spectra have a rather low S/N and thus only the H$\alpha $ triplet of one of the spectra is resolved. The components are placed at 6558.9 Å, 6563.6 Å, and 6568.6 Å, yielding an approximate field strength of 240 $\pm$ 10 kG.

HE 1233-0519.

This DA was published by Koester et al. (2001), but not recognized as a magnetic star. The SPY spectra have a low S/N such that only the H$\alpha $ triplet is discernible in one of the spectra. With the components at 6552.2 Å, 6564.4 Å, and 6576.6 Å, a field strength of 610 $\pm$ 10 kG results.

WD 2051-208.

Beers et al. (1992) published this object as a DA, but it was not further investigated since. It is a new magnetic DA, and shows a variable Zeeman splitting of H$\alpha $ and H$\beta $. The H$\alpha $ components in the two SPY spectra are found at 6562.0 Å, 6566.6 Å, 6570.7 Å, and at 6560.2 Å, 6566.6 Å, 6571.9 Å, respectively, and those of H$\beta $ at 4861.2 Å, 4864.0 Å, 4866.7 Å, and at 4861.0 Å, 4863.9 Å, 4866.9 Å. The resulting field strengths are 220 $\pm$ 20 kG and 290 $\pm$ 20 kG (from H$\alpha $) as well as 250 $\pm$ 30 kG and 270 $\pm$ 30 kG (from H$\beta $). The values derived from H$\alpha $ are significantly different, and each is consistent with the corresponding value from H$\beta $.

 \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg5.ps}
\end{figure} Figure 5:

Four new magnetic DAs. Top panel: H$\alpha $ in WD2051-208, HS0051+1145, HE1223-0519 (from top). Bottom: H$\beta $ in HS1031+0343. Vertical axis is relative intensity, with arbitrary offsets between spectra for clarity.

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WD 2253-081 and WD 1344+106.

Bergeron et al. (2001) suspected that shallow line cores which they found for these objects might indicate low-strength magnetic fields. The line cores of both objects have since been fitted with rotationally broadened line profiles, corresponding to projected rotation velocities of 36+14-7 km s-1 for WD 2253-081 (Karl et al. 2005) and 4.5 $\pm$ 2 km s-1 for WD 1344+106 (Berger et al. 2005). The H$\alpha $ line cores in the SPY spectra of both objects neither show Zeeman triplets nor flat bottoms that could indicate the presence of a B field. They are non-magnetic objects.

HS0209+0832.

This DAB star is well studied since it is one of very few objects that exhibit helium features at a temperature that places it in the DB gap (Jordan et al. 1993). Heber et al. (1997) found a helium abundance that is variable at timescales of a few months, which has been interpreted as a sign of helium accretion from a clumpy interstellar medium. The equivalent widths of the He I 4471 Å and 5876 Å lines show no significant differences between both SPY spectra, i.e., no variation of the abundance is found. This is however not surprising since the spectra were recorded within 3 days of each other.

Zeeman splitted H$\alpha $ or H$\beta $ for the four new magnetic DAs are displayed in Fig. 5.

6 Binaries with DA white dwarfs and dM companions

Table 3 gives the data on binaries containing a DA and a M dwarf companion, identified from molecular features in the red spectrum and/or Balmer emission components. The names in boldface indicate new detections from the SPY. The magnitude is the Johnson V magnitude, unless indicated otherwise (see description for Table 1). The type is estimated from the effective temperature obtained with a fit with hydrogen models, using only the higher Balmer lines from H$\gamma$.

7 Conclusions

We present the data - coordinates, magnitudes, and alias names - for the hydrogen-rich objects in the SPY sample. These include 615 objects with pure hydrogen spectra, for which atmospheric parameters derived from fits with hydrogen models are given in Table 1. Of these, 187 are new white dwarf detections from this survey, or the HES and HQS surveys used to define the target list. In addition to the 615 DAs, our sample also includes 46 DA+dM binaries, of which 10 are new, and 10 magnetic DA (4 new). The results show that with careful reduction even high-resolution echelle spectra can be used to determine stellar parameters through line profile fitting, although the line profiles may extend over many echelle orders. However, there is an indication that the surface gravities obtained are lower by 0.05-0.08 dex, compared to results from high S/N low-resolution spectra. The surface gravities of the normal DAs show the well known, but still unexplained, trend to a larger value (by 0.2 dex) for temperatures below approximately 12 500 K.

Acknowledgements
T.L. is supported within the framework of the Excellence Initiative by the German Research Foundation (DFG) through the Heidelberg Graduate School of Fundamental Physics (grant number GSC 129/1). Research at Bamberg in the context of the SPY project was funded by the DFG through grants Na 365/2-1/2 and He 1356/40-3/4. This research has made extensive use of the SIMBAD database, operated at CDS, Strasbourg, France.

References

Footnotes

... dwarfs[*]
Based on data obtained at the Paranal Observatory of the European Southern Observatory for programmes 165.H-0588 and 167.D-0407.

All Tables

Table 1:   Fit results for hydrogen atmosphere stars.

Table 2:   Magnetic or helium-contaminated hydrogen-rich stars.

Table 3:   Data for DA+dM binaries. In boldface are the new SPY detections. For explanations of the magnitude column see text.

All Figures

  \begin{figure}
\par\includegraphics[width=14.1cm,clip]{12531fg1.ps}
\end{figure} Figure 1:

Typical example for observations and fit, taking arbitrarily the entries 301 to 310 from Table 1. The header of each panel gives the name, effective temperature, and surface gravity of the fit. Shown are the six lowest Balmer lines. Vertical axis is relative intensity in arbitrary units, higher lines are offset for clarity. The light grey lines are the models.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg2.ps}
\end{figure} Figure 2:

Comparison of effective temperatures from this work with the results of Liebert et al. (2005), (=LBH) for 85 objects in common.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg3.ps}
\end{figure} Figure 3:

Comparison of surface gravities from this work with the results of Liebert et al. (2005), (=LBH) for 85 objects in common.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg4.ps}
\end{figure} Figure 4:

Distribution of surface gravities for all objects between $T_{\rm eff}$ 7500-25 000 K.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{12531fg5.ps}
\end{figure} Figure 5:

Four new magnetic DAs. Top panel: H$\alpha $ in WD2051-208, HS0051+1145, HE1223-0519 (from top). Bottom: H$\beta $ in HS1031+0343. Vertical axis is relative intensity, with arbitrary offsets between spectra for clarity.

Open with DEXTER
In the text


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