1 Introduction

Subluminous B stars (sdB) dominate the populations of faint blue stars of our own Galaxy and are found in both the old disk (field sdBs) and in halo populations as blue tails to the horizontal branches of globular clusters (Ferraro et al. 1997). Observations of elliptical galaxies with the Ultraviolet Imaging Telescope (Brown et al. 1997) and the HST (Brown et al. 2000) have shown that these stars are sufficiently common to be the dominant source for the "UV upturn phenomenon'' observed in elliptical galaxies and galaxy bulges (see also Greggio & Renzini 1999). However, important questions remain over the exact evolutionary paths and the appropriate timescales.

It is now generally accepted that the sdB stars can be identified with models for extreme Horizontal Branch (EHB) stars burning He in their core, but with a very tiny (<1% by mass) inert hydrogen envelope (Heber 1986; Saffer et al. 1994). An EHB star bears great resemblance to a helium main-sequence star of half a solar mass and its further evolution should proceed similarly (i.e. directly to the white dwarf graveyard), as confirmed by recent calculations (Dorman et al. 1993). Therefore sdB stars are certainly important as the immediate progenitors of low mass white dwarfs.

How they evolve to the EHB configuration is controversial. The problem is how the mass loss mechanism in the progenitor manages to remove all but a tiny fraction of the hydrogen envelope at precisely the same time as the He core has attained the mass ( ${\sim}0.5~M_\odot$) required for the He flash. Considerable evidence is accumulating that many sdB stars reside in close binaries (Maxted et al. 2001) and therefore mass transfer in close binary evolution is important.

Figure 1: Model fits to the Balmer and He line profiles in our time-averaged optical spectra of the four programme stars using NLTE model atmospheres (see text).

The discovery of multimode pulsators among the sdB stars has opened a new attractive possibility of probing their interiors using seismological methods. The properties of sdB pulsators (sdB Variables=sdBVs or EC14026 stars, Kilkenny et al. 1997), are characterised by relatively short pulsation periods ranging between ${\sim}1$ and 10 min and low pulsation amplitudes. PG1605+072 has the highest detected amplitude; the strongest observed peak has been found at 64 milli modulation amplitudes (mma), and more than 50 oscillation frequencies have been detected (Koen et al. 1998; Kilkenny et al. 1999). Most sdBVs have been found with a main peak pulsation amplitude of around 10mma, while the lowest level pulsator discovered up to now is PG1618+563B (Silvotti et al. 2000), with a main peak at only 2mma.

 

 

Table 1: Details of spectroscopic observations. The last coloumn gives the size and identifier of the telescope used, with the spectrographs reciprocal dispersion in parentheses. (See text for details.) The B-band magnitudes given here have been estimated from the Schmidt plates.

Object	B	Date	Exp	$\lambda$ range	Dispersion	Observatory$^{\rm a}$
	mag		s	Å	Å/mm
HS0039+4302	15.1	3.10.98	3000	4010-6720	100	CA
HS0444+0458	15.2	5.9.93	2392	4010-6720	120	CA
		15.10.00	2400	3900-6930$^{\rm b}$	110	DK
		16.10.00	2400	3390-5620$^{\rm b}$	110	DK
HS1824+5745	15.6	5.9.93	4000	4010-6720	120	CA
HS2151+0857	16.5	5.9.93	4000	4010-6720	120	CA

$^{\rm a}$  CA: Calar Alto 2.2 m, Spain, DK: Danish 1.5 m, Chile.
$^{\rm b}$  Not flux calibrated.

 

 

Table 2: Results of the spectroscopic analysis.

Object	$T_{\rm eff}$	log g	log ($n({\rm He})\over{n({\rm H})}$)	M(V)	Distance
	K			mag	kpc
HS0039+4302	32400$\pm$700	5.70$\pm$0.10	-2.20$\pm$0.20	4.2	1.5
HS0444+0458	33800$\pm$1000	5.60$\pm$0.15	-1.85$\pm$0.20	3.9	1.8
HS1824+5745	33100$\pm$1700	6.0$\pm$0.25	-1.52$\pm$0.20	4.9	1.4
HS2151+0857	34500$\pm$1300	6.1$\pm$0.25	-1.37$\pm$0.20	5.1	1.9

The pulsations of the sdB stars are driven by an opacity bump associated with iron ionization (Charpinet et al. 1996, 1997). Both radial and nonradial modes are expected to have about the same range of frequencies, corresponding to what is observed (see also Kawaler 1999). The instability is predicted to occur in the temperature range between about 29000K and 37000K. However, most stars in this temperature range do not vary. Our ongoing survey indicates that only one tenth of the sdB stars in this temperature interval is pulsating (Østensen 2000).

Today 19 sdBVs are known in the literature. 13 are summarised in the review by O'Donoghue et al. (1999), and six have recently been published (Piccioni et al. 2000; Billères et al. 2000; Silvotti et al. 2000; Østensen et al. 2001; Ulla et al. 2001). In this paper we present spectroscopy and photometry on another four, increasing the number of class members to 23.

The four new pulsators reported in this paper have been identified based on a list of candidates drawn from the Hamburg Schmidt survey (Hagen et al. 1995). Follow-up spectroscopy (Heber et al. 1999; Edelmann et al. 2001) revealed that their effective temperatures lie in the domain predicted for the pulsational instability. 13 objects were monitored with continuous photoelectric photometry with the Nordic Optical Telescope (NOT) in July 1999 and one pulsator, PG1618+563B, was discovered (Silvotti et al. 2000). In October 1999, in a second run at the NOT, we used a CCD camera with our own controller software (Østensen & Solheim 2000) to observe 31 additional candidates, of which three were identified as pulsators: HS0815+4243, HS2149+0847 and HS2201+2610 (Østensen et al. 2001). The third run was completed in July 2000, when 24 candidates were observed during four nights, resulting in two of the discoveries presented in this paper. During the fourth run, three months later, 31 new candidates were observed and another 8 reobserved, revealing another two new pulsators. A more detailed description of the technique used, and upper limits to the pulsation amplitudes of the sdBs for which we do not see any pulsations above the $3\sigma$ detection limit, will be reported in a subsequent paper when our search programme will be completed.

 

 

Table 3: Detected pulsation frequencies and amplitudes.

Object	Date	F1	F2	F3	F4	A1	A2	A3	A4
	(2000)	(mHz)				(mma)
HS0039+4302	Oct. 4-5	5.17		4.28		10.3		2.6
	Oct. 5-6	5.14	5.50	4.27		8.1	6.8	2.7
	Oct. 7	5.13	5.46	4.27	5.29	8.0	5.4	2.6	3.0
	Combined	5.14	5.48	4.27	5.21	8.0	5.4	2.6	1.9
HS0444+0458	Oct. 6	7.31	5.91			11.9	2.6
	Oct. 7	7.32	5.87			11.5	2.6
	Oct. 8	7.31	5.90			11.1	2.4
	Combined	7.31	5.86			12.1	2.5
HS1824+5745	Jul. 9	7.27				2.6
	Jul. 10	7.21				4.5
	Jul. 11	7.19				4.8
	Combined	7.21				4.0
HS2151+0857	Jul. 9	7.40	7.74	6.59	7.01	4.8	2.1	2.8	2.5
	Jul. 10	7.39	7.80	6.58	6.88	4.8	2.6	3.1	3.1
	Jul. 11	7.42	7.73	6.62	6.85	4.8	4.6	3.6	2.5
	Combined	7.43	7.72	6.59	6.86	5.0	3.3	3.3	2.1