Документ взят из кэша поисковой машины. Адрес оригинального документа : http://star.arm.ac.uk/~csj/papers/conference/iauc185_v652.ps Дата изменения: Tue Aug 7 18:42:53 2001 Дата индексирования: Tue Oct 2 10:12:54 2012 Кодировка: Поисковые слова: вечный календарь

**TITLE**
ASP Conference Series, Vol. **VOLUME**, **PUBLICATION YEAR**
**EDITORS**
Time-resolved spectral analysis of the pulsating helium
star V652 Her
C.S. Jeery, V.M.Woolf
Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
D.L.Pollacco
School of Physical Sciences, Queen's University Belfast, Belfast BT7
9NN, N. Ireland
Abstract. We present a summary of a new time-dependent spectro-
scopic analysis of the pulsating helium star V652 Her. The instantaneous
properties of the star (eective temperature, surface gravity, radial dis-
placement, etc.) are evaluated throughout the pulsation cycle. The data
are used to establish measurements of the mean stellar radius and mass
more accurately than hitherto. The rst direct evidence of a supersonic
shock at minimum radius is also obtained.
1. Introduction
V652 Her is an early-type extreme helium star which pulsates with a period of
0.108 days. Its surface composition and location in the HR diagram have posed
a considerable challenge to the theory of stellar evolution. However, its regular
pulsation, a decreasing pulsation period and unusual surface composition have
allowed stringent tests of various evolution models. The most successful model
(Saio & Jeery 2000) involves the merger of two helium white dwarfs which,
after expanding to become a yellow giant, contracts towards the helium main
sequence (where sdB stars are found). The most poorly determined property
of V652 Her is its mass, a consequence of the diфculty of measuring surface
gravity. The most crucial measurement, however, is that of radius, which may
be measured using Baade's method. This poster presented a summary of a
detailed analysis of new observations presented by Jeery et al. (2001) aimed
at rening the radius and mass measurements of V652 Her.
A series of 59 moderate-resolution high signal-to-noise spectra of the pul-
sating helium star V652 Her covering 1.06 pulsation cycles were obtained with
the William Herschel Telescope in 1998 July. These have been supplemented by
archival ultraviolet (IUE) and visual spectrophotometry (Kilkenny & Lynas-
Gray 1982) and used to make a time-dependent study of the properties of
V652 Her throughout the pulsation cycle. Archival high-resolution ultraviolet
spectroscopy (IUE) and new infrared spectroscopy (UKIRT) have also been in-
troduced.
1

2 Jeery et al.
2. Analysis
Analysis of the new data features the following: new software for the auto-
matic measurement of eective temperature, surface gravity and projected rota-
tion velocities from moderate-resolution spectra, the most precise radial veloc-
ity curve for V652 Her measured so far, self-consistent high-precision measure-
ments of eective temperature and surface gravity around the pulsation cycle, a
demonstration of excessive line-broadening at minimum radius and evidence for
a pulsation-driven shock front, a new method for the direct measurement of the
radius of a pulsating star using radial velocity and surface gravity measurements
alone, new software for the automatic measurement of chemical abundances and
microturbulent velocity, updated chemical abundances for V652 Her compared
with previous work (Jeery et al. 1999), a reanalysis of the total ux varia-
tions (cf. Lynas-Gray et al. 1984) in good agreement with previous work, an
independent verication of the interstellar reddening using Lyman  from IUE
high-resolution spectra and spectrum synthesis, the rst infrared spectrum of an
extreme helium star, showing He i 10830  A to be unexpectedly weak and hy-
drogen Paschen lines to be unexpectedly strong relative to other He i lines, and
revised measurements of the stellar mass and radius from a number of dierent
diagnostics (surface gravity, visual magnitude, bolometric ux).
3. The mass of V652 Her
Masses measured without reference to the ultraviolet uxes turn out to be un-
physically low ( 0:25M ). The best estimate for the dimensions of V652 Her
averaged over the pulsation cycle is given by: hT e i = 22 930  10K hlog gi =
3:46  0:05 (ionization equilibrium), hT e i = 20 950  70K (total ux method),
hRi = 2:31  0:02R  , hLi = 919  14L , M = 0:59  0:18M and d =
1:70  0:02kpc.
Two signicant problems were encountered. The line-blanketed hydrogen-
decient model atmospheres used yield eective temperatures from the optical
spectrum (ionization equilibrium) and visual and UV photometry (bolometric
ux) that are inconsistent. Secondly, the IUE spectra are poorly distributed
in phase and have low signal-to-noise. These problems may introduce system-
atic errors of up to 0:1M . New models and new ultraviolet observations are
required.
References
Jeery C.S., Hill P.W., Heber U., 1999. A&A 346, 491
Jeery C.S., Woolf V.M., Pollacco D., 2001. A&A in press
Kilkenny D., Lynas-Gray A.E. 1982. MNRAS 198, 873
Lynas-Gray A.E., Schonberner D., Hill P.W., Heber U., 1984. MNRAS 209, 387
Saio H., Jeery C.S., 2000. MNRAS 313, 671