2 Observations and data analysis

 

 

Table 1: Programme stars: coordinates, observation dates, and exposure times for the WFPC2 and references for the spectroscopic classification observations.

star	$\alpha_{1950}$	$\delta_{1950}$	l	b	obs.	exp.	reference
					date	time
						[s]
PB 6107	00$^{\rm h}$39$^{\rm m}$31$^{\rm s}$	+04$^{\circ}$53$^\prime$17 $^{\prime \prime }$	118 $\stackrel{\circ}{.}$ 59	-57 $\stackrel{\circ}{.}$ 64	990627	3.5	Moehler et al. (1990)
PHL 1079	01$^{\rm h}$35$^{\rm m}$48$^{\rm s}$	+03$^{\circ}$23$^\prime$00 $^{\prime \prime }$	144 $\stackrel{\circ}{.}$ 96	-57 $\stackrel{\circ}{.}$ 22	981204	4	Theissen et al. (1995)
HE 0430-2457	04$^{\rm h}$30$^{\rm m}$59$^{\rm s}$	-24$^{\circ}$57$^\prime$37 $^{\prime \prime }$	223 $\stackrel{\circ}{.}$ 49	-40 $\stackrel{\circ}{.}$ 55	980417	8	this paper
PG 0749+658	07$^{\rm h}$49$^{\rm m}$39$^{\rm s}$	+65$^{\circ}$50$^\prime$13 $^{\prime \prime }$	150 $\stackrel{\circ}{.}$ 44	+30 $\stackrel{\circ}{.}$ 99	990329	1.8	Saffer (1991)
PG 0942+461	09$^{\rm h}$42$^{\rm m}$02$^{\rm s}$	+46$^{\circ}$08$^\prime$38 $^{\prime \prime }$	173 $\stackrel{\circ}{.}$ 11	+48 $\stackrel{\circ}{.}$ 89	980530	10	Heber et al. (1991)
TON 1281	10$^{\rm h}$40$^{\rm m}$57$^{\rm s}$	+23$^{\circ}$24$^\prime$55 $^{\prime \prime }$	213 $\stackrel{\circ}{.}$ 62	+60 $\stackrel{\circ}{.}$ 89	990623	5	Jeffery & Pollacco (1998)
TON 139	12$^{\rm h}$53$^{\rm m}$39$^{\rm s}$	+28$^{\circ}$23$^\prime$31 $^{\prime \prime }$	77 $\stackrel{\circ}{.}$ 21	+88 $\stackrel{\circ}{.}$ 57	980103	1.8	Green (1997)
PG 1309-078	13$^{\rm h}$09$^{\rm m}$09$^{\rm s}$	-07$^{\circ}$49$^\prime$18 $^{\prime \prime }$	311 $\stackrel{\circ}{.}$ 60	+54 $\stackrel{\circ}{.}$ 44	980505	8	Ferguson et al. (1984)
PG 1421+345	14$^{\rm h}$21$^{\rm m}$29$^{\rm s}$	+34$^{\circ}$27$^\prime$53 $^{\prime \prime }$	58 $\stackrel{\circ}{.}$ 36	+69 $\stackrel{\circ}{.}$ 01	990605	14	Ferguson et al. (1984)
PG 1449+653	14$^{\rm h}$49$^{\rm m}$42$^{\rm s}$	+65$^{\circ}$17$^\prime$58 $^{\prime \prime }$	104 $\stackrel{\circ}{.}$ 84	+47 $\stackrel{\circ}{.}$ 63	990619	7	Moehler et al. (1990)
PG 1511+624	15$^{\rm h}$11$^{\rm m}$25$^{\rm s}$	+62$^{\circ}$21$^\prime$00 $^{\prime \prime }$	99 $\stackrel{\circ}{.}$ 21	+47 $\stackrel{\circ}{.}$ 96	990513	14	Moehler et al. (1990)
PG 1601+145	16$^{\rm h}$01$^{\rm m}$47$^{\rm s}$	+14$^{\circ}$32$^\prime$58 $^{\prime \prime }$	27 $\stackrel{\circ}{.}$ 15	+43 $\stackrel{\circ}{.}$ 51	000613	12	Ferguson et al. (1984)
PG 1636+104	16$^{\rm h}$36$^{\rm m}$40$^{\rm s}$	+10$^{\circ}$24$^\prime$54 $^{\prime \prime }$	27 $\stackrel{\circ}{.}$ 00	+34 $\stackrel{\circ}{.}$ 04	000612	8	Ferguson et al. (1984)
TON 264	16$^{\rm h}$47$^{\rm m}$05$^{\rm s}$	+25$^{\circ}$15$^\prime$13 $^{\prime \prime }$	45 $\stackrel{\circ}{.}$ 16	+37 $\stackrel{\circ}{.}$ 12	990529	10	Theissen et al. (1993)
PG 1656+213	16$^{\rm h}$56$^{\rm m}$12$^{\rm s}$	+21$^{\circ}$15$^\prime$05 $^{\prime \prime }$	41 $\stackrel{\circ}{.}$ 25	+33 $\stackrel{\circ}{.}$ 90	980301	12	Ferguson et al. (1984)
PG 1718+519	17$^{\rm h}$18$^{\rm m}$35$^{\rm s}$	+51$^{\circ}$55$^\prime$05 $^{\prime \prime }$	79 $\stackrel{\circ}{.}$ 00	+34 $\stackrel{\circ}{.}$ 94	990427	7	Theissen et al. (1995)
PG 2148+095	21$^{\rm h}$48$^{\rm m}$41$^{\rm s}$	+09$^{\circ}$30$^\prime$39 $^{\prime \prime }$	66 $\stackrel{\circ}{.}$ 78	-32 $\stackrel{\circ}{.}$ 84	990411	4	this paper
HE 2213-2212	22$^{\rm h}$13$^{\rm m}$38$^{\rm s}$	-22$^{\circ}$12$^\prime$26 $^{\prime \prime }$	32 $\stackrel{\circ}{.}$ 63	-54 $\stackrel{\circ}{.}$ 50	981207	8	this paper
BD -7$^{\circ}$5977	23$^{\rm h}$15$^{\rm m}$12$^{\rm s}$	-06$^{\circ}$44$^\prime$56 $^{\prime \prime }$	71 $\stackrel{\circ}{.}$ 55	-59 $\stackrel{\circ}{.}$ 65	981125	0.3	Viton et al. (1991)
stars without spectroscopic evidence for a cool companion
PG 0105+276	01$^{\rm h}$05$^{\rm m}$32$^{\rm s}$	+27$^{\circ}$36$^\prime$53 $^{\prime \prime }$	127 $\stackrel{\circ}{.}$ 46	-34 $\stackrel{\circ}{.}$ 84	980226	14	this paper, new type: He-sdO
PG 1558-007	15$^{\rm h}$58$^{\rm m}$39$^{\rm s}$	-00$^{\circ}$43$^\prime$26 $^{\prime \prime }$	9 $\stackrel{\circ}{.}$ 34	+36 $\stackrel{\circ}{.}$ 51	990424	7	this paper
KPD 2215+5037	22$^{\rm h}$15$^{\rm m}$25$^{\rm s}$	+50$^{\circ}$37$^\prime$48 $^{\prime \prime }$	99 $\stackrel{\circ}{.}$ 71	-4 $\stackrel{\circ}{.}$ 91	961213	7	this paper
PG 2259+134	22$^{\rm h}$59$^{\rm m}$16$^{\rm s}$	+13$^{\circ}$22$^\prime$31 $^{\prime \prime }$	86 $\stackrel{\circ}{.}$ 36	-41 $\stackrel{\circ}{.}$ 31	000615	10	Theissen et al. (1993), this paper

  
2.1 Target selection and optical spectroscopy

For the snapshot observations a target list of fifty of the brightest sdB star binary candidates was extracted from an updated version of the Kilkenny et al. (1988) catalogue, supplemented by two stars which we discovered in the course of follow-up spectroscopy of hot stars from the Hamburg-ESO survey (see Edelmann et al. 2001a). 23 stars from this target list were actually observed with the Wide Field Planetary Camera 2 (WFPC2) onboard the HST during our snapshot project, i.e. they were scheduled for observation to fill small gaps in the HST schedule. All stars have published photometry (see Tables B.1 and B.2), but only 16 have published optical spectroscopy. Therefore additional spectra were obtained at the Calar Alto and ESO observatories (see Appendix A for details and plots of the spectra in Figs. A.1 and A.2). As can be seen from Fig. A.1 spectral features (Ca I, Ca II, Mg I and/or Fe I) indicative of a cool star are clearly present in the spectra of PG 1309-078, PG 0942+461, HE 0430-2457, HE 2213-2212, and PG 2148+095 in addition to the Balmer and helium lines of the sdB. Hence these objects are spectroscopic binaries consisting of an sdB star and a cool companion. PG 0942+461 has already been observed by Mitchell (1998), who, however, did not note the binary nature of the star. We do not find any evidence for a cool companion in the spectra of the sdB stars PG 1558-087 and KPD 2215+5037 (see Fig. A.2). We also re-analysed a published spectrum of PG 2259+134 (Theissen et al. 1993) and do not find any spectroscopic evidence for a cool companion. PG 0105+276 turns out to be not an sdB star but a helium-rich sdO star and does not show any spectroscopic evidence for a cool companion. Therefore our sample consists of 19 composite spectrum objects plus four stars showing only photometric evidence for a companion. One of these four stars (PG 0105+276) also does not belong to the programme sample since it is an sdO star.

  
2.2 WFPC2 data

We observed the candidate binary systems with the PC chip of the WFPC2. If the cool companion is a main sequence star, both components should be of comparable brightness in the R band and we therefore used the F675W filter of the WFPC2. We obtained four observations of each target, which were offset relative to the first one by (-11,-5.5), (-16.5,-16.5), (-5.5,-11) pixels. We first rebinned the data linearly to a step size of 0.5 pixels and then aligned them according to the offset pattern mentioned above. We then determined the median value of the four aligned images to avoid cosmic ray hits and hot pixels and used these median-averaged images for visual inspection. All flux measurements are performed on manually cleaned average images to ensure proper flux conservation.

The median-averaged images were first inspected by eye to see if any companion could be detected. Only 6 stars (cf. Fig. 1) showed obvious nearby stars and the angular separations and brightness differences can be found in Table 2. The brightness differences were determined using the command INTEGRATE/APERTURE from MIDAS, which performs an aperture photometry with a given radius. Aperture photometry is difficult for TON 1281, TON 139, and PG 1718+519, due to the small distance of the components. The sky background was determined in an empty region using the same aperture as for the stars.

 

 

Table 2: Separation and estimated brightness differences for the components of the 6 resolved binaries. The photometric data available for HE 0430-2457 do not allow to estimate a temperature or distance of the sdB.

system	separation		brightness
	angular	linear	difference
		[AU]	$\Delta F675W$
PG 0105+276	3 $.\!\!{\hbox{$^{\prime\prime}$ }}$37	3700	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$
	4 $.\!\!{\hbox{$^{\prime\prime}$ }}$48	4900	$\rm 1\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}6$
HE 0430-2457	1 $.\!\!{\hbox{$^{\prime\prime}$ }}$25		$\rm 2\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$
TON 1281	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$22	250	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}7$
TON 139	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$32	300	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$
PG 1558-007	2 $.\!\!{\hbox{$^{\prime\prime}$ }}$80	2500	$\rm 3\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$
PG 1718+519	0 $.\!\!{\hbox{$^{\prime\prime}$ }}$24	230	$\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}8$

Figure 1: The images of the resolved binaries. The bar in each image corresponds to 1 $^{\prime \prime }$.

To get a more quantitative estimate of possible companions we fitted two-dimensional Gaussians with variable angle of the major axis to all shifted and co-added target images and compared the results to fits obtained for archive point-spread functions (PSFs; F675W filter, PC chip). The archive PSFs define a good correlation between the length of the two axes, which is shared by most target PSFs (see Fig. 2). Besides the resolved binaries (where stray light can affect the determination of the axis ratio) four stars deviate from the main correlation between major and minor axis (see Fig. 3): PG 2148+095 (2.03/1.26), KPD 2215+5037 (2.38/1.61), TON 264 (2.35/1.83), and PG 0749+658 (2.36/1.87).

We used DAOPHOT (Stetson 1987) to obtain an average PSF from those target stars that share the axis-relation of the archive PSFs. This "target PSF'' was then used to deconvolve all systems that are either resolved by eye or show deviations from the axis-relation defined by the archive PSFs. No additional components were resolved in this process, but we could verify the brightness differences between the components of the resolved systems listed in Table 2, which were reproduced by DAOPHOT also for small separations.

Figure 2: The major and minor axes of the point spread functions for the target stars (circles, filled symbols mark brightest star of resolved binaries) and of archive point-spread functions (triangles, filled ones mark stars with positions on the PC chip close to our targets).

Figure 3: The images of the unresolved stars (PG 2148+095, KPD 2215+5037, TON 264, PG 0749+658) which show deviations from the standard PSF shape (see text). The images of PB 6107 and PG 1421+345 are well matched by the standard PSF shape and are displayed for comparison. Note that - in contrast to all other stars displayed here - there is no spectroscopic evidence for binarity of KPD 2215+5037. The bar in each image corresponds to 1 $^{\prime \prime }$.

For 13 of our target stars a homogeneous set of ground-based $R_{\rm C}$measurements exist (Allard et al. 1994, see Table B.2). Comparing those data to the instrumental F675W magnitudes integrated within an aperture of 0 $.\!\!{\hbox{$^{\prime\prime}$ }}$5 radius

$$F675W = -2.5 \log {\frac{{\rm flux}_{0\hbox{$.\!\!{\hbox{$^{... ...{$.\!\!{\hbox{$^{\prime\prime}$ }}$ }5}}{\rm exposure \ time}}$$

we find that most of the stars lie on a line with slope 1 (except KPD 2215+5037 and PG 1601+145, see Fig. 4). From the 11 stars on the line we determine a zeropoint of $\rm 21\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}21$ $\pm$ $\rm0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}02$. From the WFPC2 data handbook we determine a zeropoint of $\rm 21\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}9$ (gain 14, including an aperture correction of $\rm -0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}1$) that has to be corrected to Cousins R by adding $\rm -0\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}65$ (assuming a spectral type of A5 for the combined spectra of our binary stars), yielding a final zeropoint of $\rm 21\hspace{-0.25em}\stackrel{m}{.} \hspace{-1.0mm}25$, in agreement with our empirically determined zeropoint. Since our empirically derived zeropoint automatically takes into account the unusual flux distribution of our binary stars we decided to use it to calculate $R_{\rm HST}$given in Table B.2.

Figure 4: The instrumental F675W magnitudes compared to the $R_{\rm C}$ data from Allard et al. (1994). The open symbols are KPD 2215+5037 and PG 1601+145. The line marks the relation $R_{\rm C} = {-}2.5 \log {\frac{{\rm flux}_{0\hbox{$.\!\!{\hbox{$^{\prime\prim... ...{\rm sky}_{0\hbox{$.\!\!{\hbox{$^{\prime\prime}$ }}$ }5}}{\rm exposure \ time}}$+21.21.