Spectra of Gw Vir Type Pulsators

Baltic Astronomy, vol.4, 340-348, 1995.

SPECTRA OF GW VIR TYPE PULSATORS

K. Werner Lehrstuhl für Astrophysik, Universität Potsdam, Germany; Institut für Astronomie und Astrophysik, Universität Kiel, D-24098, Kiel, Germany Received September 1, 1995.

Abstract. We describe our effort to confine empirically the position of the GW Vir instability strip in the HR diagram. Spectra rang- ing from the soft X-ray to the optical region of the electromagnetic spectrum are analyzed by means of non-LTE model atmospheres. We compare our results for stellar parameters with those from pulsational analyses. Generally, good agreement is found. Key words: stars: atmospheres - stars: oscillations - stars: white dwarfs - stars: individual: PG 1159-035.

1. Introduction

The PG 1159 stars represent the hottest known hydrogen- deficient post-AGB stars. They are just about to enter the white dwarf cooling sequence. Effective temperatures range from 170 000 K down to 65 000 K. Surface gravities and hence luminosities cover a wide range (log g — 5.5-8.0). The atmospheric abundances are generally dominated by carbon and helium. The oxygen abundances are high, around 20 % by mass in most stars but significant deviations from this mean value occur. The most extreme case is H 1504+65, a hydrogen- and helium-deficient star with an atmosphere essentially composed of carbon and oxygen. Mass loss and/or ingestion and burning of hydrogen are thought to have removed the hydrogen- rich surface layer. Table 1 summarizes the characteristics of the 27 presently known PG 1159 stars. About every second object is associated with a planetary nebula and eight stars are known to be variable. The table condenses many individual works, but we refrain here from detailed references. For an exhaustive review with detailed references see the recent paper by Dreizler, Werner &: Heber (1995). Spectra of G W Vir type pulsators 341

The latest detection concerns RXJ 0122.9-7521 which was found by ROSAT and identified as a PG 1159 star by Cowley etal. (1995). Spectroscopically, the PG 1159 stars are characterized by a broad absorption feature made up by He II 4686 A and C IV lines. We have introduced three subclasses which allows a rough estimate of atmospheric parameters from the spectral appearance. Generally, all the objects show absorption line spectra with more or less pro- nounced central emission reversals and are thus forming a distinct group from the Wolf-Rayet type Planetary Nebula Nuclei (PNN) which display emission line spectra as a consequence of strong mass- loss. The latter are classified as [WC], where the brackets were introduced to distinguish them from their massive Population I coun- terparts. The two PNNs, Abell 30 and Abell 78, with mixed absorption/emission line spectra represent transition objects of the [WC]- PG 1159 type. Non-radial pulsators are found within both groups, the [WC] stars and the PG 1159 stars. In this context, the term PNN variable (or PNNV) commonly in use for pulsators from both groups is unfortunate because the fact that a pulsating PG 1159 star also has a detected PN is not uniquely connected with the evolutionary state of the nucleus and hence the pulsation driving mechanism. For example, the PG 1159-type central star of K 1-16 and the [WC]-type central star of NGC 6905 are termed PNNV while other pulsating PG 1159 stars without PNs are termed PG 1159 variables. The recent detection of a very old faint PN around PG 1520+525 (Jacoby & van de Steene 1995) points at the possibility that nebulosities around other PG 1159 stars may still be detected which would mean that such pulsators move from one classification group into the other. We thus propose to clearly distinguish the variables by denoting them "[WC]-variables" and "PG 1159-variables" and to drop the "PNNV" classification.

2. Latest results from spectroscopic analyses

The vast majority of the analyses is based on optical spectra obtained with 3 m-class telescopes on Calar Alto (Spain) and La Silla (Chile). Spectral resolution amounts to 2-4 Â, typically. High S/N- ratios (near 50) are required to quantitatively analyze the shallow lines. Analyses are performed by detailed line profile fitting with line blanketed non-LTE model atmospheres. Continued effort to ob- 342 K. Werner

Table 1. A complete list of the 27 stars belonging to the PG 1159 spectral class. Spectral subtypes are given in column 2. Column 3 denotes if the star is pulsating or not, and the next column if it is associated with a planetary nebula. Effective temperatures and surface gravities are given in kK and cgs units, respectively (: =uncertain). Element abundances are in per cent mass fraction.

Star Type Var. PN Teff log g H He C N 0 H 1504+65 Ep no no 170 8.0 50 50 RXJ 2117.1+3412 lgE yes yes 170 6.1 38 56 6 PG 1144+005 Ep no no 150 6.5 39 58 1.5 1.6 RXJ 0122.9-7521 Ep no 150 6.5 : yes Jn 1 E no yes 150 6.5 30 46 24 NGC 246 lgE yes yes 150 5.7 38 56 6 PG 1520+525 E no yes 140 7.0 < 8 30 46 < 0.3 16 PG 1159-035 E yes no 140 7.0 < 8 30 46 < 0.3 16 NGC 650 E yes 140 7.0 Abell 21=Ym29 E yes 140 6.5 35 51 14 Longmore 3 lgE no yes 140 6.3 38 56 6 K 1-16 lgE yes yes 140 6.1 38 56 6 PG 1151-029 lgE no no 140 6.0 35 51 14 VV 47 E no yes 130 7.0 35 51 14 Longmore 4 IgEp yes yes 120 5.5 46 43 11 HS 0444+0453 A no 100 7.5 PG 1707+427 A yes no 100 7.0 < 8 30 46 < 0.3 16 PG 1424+535 A no no 100 7.0 < 8 30 46 < 0.3 16 IW 1 A no yes 100 7.0 MCT 0130-1937 A no no 95 7.5 50: 30 20 HS 1517+7403 A no 95 7.5 61 37 2 PG 2131+066 A yes no 80 7.5 50:30 20 PG 0122+200 A yes no 75 7.5 50 30 20 HS 0704+6153 A no 65 7.0 < 10 44 26 20 NGC 6852 lgE: yes NGC 6765 lgE: yes Sh 2-78 A yes

tain more precise results by complementary data from space-based observatories (IUE, HST, EUVE, ROSAT) has led to considerable progress. In particular, spectral features in the ultraviolet and ex- Spectra of G W Vir type pulsators 343

treme ultraviolet regions allow a much more reliable determination of the effective temperature. Primary targets were the pulsator/non- pulsator pair PG 1159-035 and PG 1520+525, which from optical spectroscopy had essentially identical parameters. PG 1159-035 was observed with HST in the UV and we could confirm our earlier result for Teff (140 000 K) but now with a high precision of ±5 000 K (Werner & Heber 1993) in contrast to the optical analysis where we faced an uncertainty of ±15 000 K (Werner, Heber & Hunger 1991). The non-pulsating twin brother PG 1520+525 was also scheduled for an HST observation but later-on dropped from the target list because of the intervening HST servicing mission. Fortunately we also had successfully applied for observations with the Extreme Ul- traviolet Explorer (EUVE) satellite which opened up a new spectral window. Most PG 1159 stars are not accessible in the EUV and soft X-ray region because they are distant and strongly attenuated by interstellar absorption. But PG 1520+525 is hot enough to have considerable flux around 100 A where the ISM absorption is less severe than at longer wavelengths. Yet, an integration time of al- most 2 days was necessary to obtain a useful spectrum. Our analysis showed that the observation can only be reproduced by models ex- ceeding Teff = 140 000 K. Our best fit is obtained at Teff = 150 000 K. This value is within the error limit of our optical analysis but it is clearly higher than that of the pulsating prototype. We thus concluded that the blue edge of the GW Vir instability strip runs right between both stars, say near 145 000 K (Werner etal. 1995b). Another pulsator/non-pulsator pair located near Teff = 100 000 K (PG 1707+427 and PG 1424+535) is suited to determine the red edge of the instability strip. HST observations of this twin are cur- rently performed and will be analyzed soon. It is already obvious from Table 1 that the edges of the instability strip are dependent on surface gravity and hence luminosity. We therefore need precise parameters of all PG 1159 pulsators to define the location of the strip within the HR diagram. Another target of high interest is the luminous RXJ 2117.1+3412, a pulsating PG 1159 central star detected in the ROSAT all sky survey (Motch, Werner & Pakull 1993). High resolution UV spectroscopy with HST was performed for two purposes. First, the origin of the optically detected super-high ionization O VIII emissions (requiring temperatures of the order of one million K) should be investigated. Second, the atmospheric parameters should be determined with high precision. We could show that Teff is even higher than previously thought, 344 K. Werner

namely 170 000 K, meaning that RXJ 2117.1+3412 is the hottest PG 1159 star at all, together with H 1504+65 (Werner et al. 1995a, Werner 1991). The question as to the hydrogen content in the atmospheres of PG 1159 pulsators is of interest not only when considering evolutionary consequences (trace hydrogen mixed in the envelope will float atop when gravitational settling sets in and will turn a PG 1159 star into a DA white dwarf). It was claimed by Starrfield et al. (1984) that traces of hydrogen in the driving regions are sufficient to poison pulsations, if indeed cyclic ionization of C and O is responsible for the variability. Interest in this question has been enhanced since the discovery of the so-called hybrid-objects, which exhibit all spectral signatures of a PG 1159 star but in addition strong Balmer lines and, hence, they have a considerable amount of hydrogen in their photo- sphere (Napiwotzki & Schonberner 1991, Dreizler et al. 1995). Un- fortunately the spectroscopic detection of hydrogen in PG 1159 stars is very difficult, because at the high temperatures in question, hydrogen is strongly ionized. In addition, every Balmer line is blended by a dominant He II line. As a consequence we could not exclude that the H/He number ratio can reach even unity without being de- tectable in our spectra. A more strict limit is only possible when high resolution spectra are obtained, which of course conflicts with the faintness of the PG 1159 stars. In a pilot project we obtained a 0.4 A resolution spectrum of PG 1159-035 in the He II 6560 A line core in order to detect a possible non-LTE emission component due to Ha. After six hours integration with the ESO 3.6 m telescope a high S/N spectrum was obtained, revealing no trace of hydrogen. The H/He ratio can now be constrained to H/He<0.2 (Werner 1995). Fig. 1 shows the location of the analyzed PG 1159 stars and other H-deflcient post-AGB objects in the Tefrvs. log g diagram, together with theoretical evolutionary tracks and the empirically derived position of the GW Vir instability strip (see Werner etal. 1995b). How does this position compare with theoretical instability strips? A com- parison with calculations of Stanghellini etal. (1991) shows that the predicted instability strips are too cool, which cast some doubt on the C/O driving mechanism to be acting. Results from more recent calculations by Saio (1995), however, are in good agreement with our observed strip position. Several objects located within the instability strip are not pulsating. The reason is unclear. But we have to face the fact that the strip boundaries are certainly depending on Spectra of G W Vir type pulsators 345

Fig. 1. Location of the GW Vir instability strip (shaded region), comprising the pulsating PG 1159 stars (filled symbols). Evolutionary tracks are labeled with the respective stellar mass. A typical error bar is shown. For object identification and other details see Dreizler et al. (1995). the chemical composition of the stellar surface layer where driving occurs.

3. Spectroscopic vs. pulsational results for stellar parameters The spectroscopic determination of effective temperature and surface gravity enables a mass determination with the help of theoretical evolutionary tracks. As a caveat, all tracks shown in Fig. 1 are, strictly speaking, not representing PG 1159 stars because all of the stellar models have H-rich envelopes. Current standard evolution theory cannot reproduce the observed exotic surface compositions. 346 K. Werner

Therefore our mass determinations can be affected by a systematic offset and an independent mass-determination is highly desirable. Asteroseismology yields stellar masses with striking accuracy, as has been demonstrated in the case of PG 1159-035 (Winget et al. 1991, Kawaler & Bradley 1994). Such independent checks are now possible for five PG 1159 variables, they are summarized in Table 2. Satisfying agreement between spectroscopic and pulsational masses is achieved in those two cases where pulsational analyses are based on WET data (PG 1159-035 and PG 2131+066). The same holds for single-site data from RXJ 2117.1+3412. Discrepant results are obtained for PG 1707+427 but we hope that the analysis of WET data will reduce the pulsational mass to the spectroscopically determined value. Even more severe is the case for PG 0122+200. We have derived the high pulsational mass given in Table 2 by taking the observed mean period spacing IIo from Vauclair et al. (1995) and inserting it into the IIo(M, L, qy) relation from Kawaler & Bradley (1994). In addition we assumed that the He-layer mass qy is similar to PG 1159-035 (0.004 M©) and used the spectroscopically determined luminosity (14 L©). How this high mass for PG 0122+200 compared to our spectroscopic result will stand up against the results of an upcoming WET observation remains to be seen. We note that the uncertainty in our spectroscopic mass determination aside from the possible systematic error mentioned above is typically of the order 0.05 to 0.1 M©.

Star -^spec MpUis Remarks Reference for pulsation result PG 1159-035 0.57 0.59 WET data (Kawaler & Bradley 1994) RXJ 2117.1+3412 0.65 0.62 Vauclair et al. 1993, WET data in analysis PG 1707+427 0.53 0.70 Fontaine etal. 1991, WET data in analysis PG 0122+200 0.56 0.90 Vauclair et al. 1995, upcoming WET target PG 2131+066 0.57 0.61 WET data (Kawaler et al. 1995) Spectra of G W Vir type pulsators 347

Another result from pulsation analyses is an independent determination of the effective temperature. It is gratifying that for the two PG 1159 pulsators covered by WET runs the temperatures agree very well with our results. For PG 1159-035 Kawaler & Bradley (1994) find Teff = 136 000 K (compared to 140 000 K) and for PG 2131+066 Kawaler et al. (1995) find Teff between 81 000 K and 88 000 K (compared to Teff = 80 000 K). It is worthwhile to note that in the latter case a completely independent pulsational mass determination was not possible, but needs additional constraints from spectroscopic results.

4. Summary We have empirically located the GW Vir instability strip in the HR diagram by means of non-LTE spectral analyses. The strip position is in agreement with recent theoretical computations. Stellar masses and effective temperatures derived from these analyses agree well with values obtained from pulsational data and models in those cases where data with high quality (from WET runs) are available. We therefore are confident that the model atmospheres together with stellar interior models derived by means of asteroseismology can ad- equately describe the structure of the PG 1159 stars.

Acknowledgments. I thank my colleagues from Kiel and Bamberg, Stefan Dreizler, Uli Heber and Thomas Rauch for their collaboration on the PG 1159 project. A DFG grant (We 1312/6- 1) is gratefully acknowledged. Travel funds were provided by the DARA (project 50 OR 9409 1) and by the organizers of the WET workshop.

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