2 Observation, instrumentation and data reduction

Observations were made at the cassegrain focus (f/13 beam) of 1.2 m telescope of the Gurushikhar Infra-Red Telescope (GIRT; lat. = 24 $^{\circ}36'$; long. = +72 $^{\circ}43'$; altitude = 1750 m), Mount Abu, India (operated by Physical Research Laboratory, Ahmedabad, India), in two phases on the nights of Dec. 23, 24, 25, 1997 and Apr. 01, 02, 03, 1998.

From the catalogue by CB we selected a set of eight objects. Some of the clouds listed by CB were surveyed for the existence of Young stellar objects (YSO), IRAS point sources and CO outflows (Yun & Clemens 1990; Clemens et al. 1991; Yun & Clemens 1992; Yun & Clemens 1994). Accordingly the clouds can be divided into several categories (i) Y- where YSO has been detected (ii) R- where IRAS point sources have been detected and (iii) O- where CO outflows have been detected. A cloud can also belong to a category which is a combination of any of these. Further in some cases survey has been carried out, but the cloud has been found to be quiescent having no activity, we call it category Q.

Thus the selection has been made from those clouds, which have been surveyed for YSO, IRAS point sources or CO outflows. We included in our list only one cloud (CB62), where no survey has been carried out.

Also clouds were selected on the basis of their availability in the sky, during the time of our observation from our observatory site. We avoided those clouds having very bright field star(s), because this will saturate our CCD. In Table 1, we list these eight clouds, with the category of each cloud mentioned within the bracket. All the observations were taken in open filter or white light (W) with 500 seconds exposure time. Table 1 gives a detail of the observation log. We observed the cloud CB54 in two parts, from the central co-ordinates (RA = 07:04:16.8; DEC = -16:24:33) we moved into two regions and named them CB54A (same RA, DEC = 2.5' North of central DEC) and CB54B (same RA, DEC = 2.5' South of central DEC). Thus CB54A and CB54B are separated by 5' in DEC. Besides we observed a region, which is situated 5' south of CB3 and called it CB3N. Similarlry we observed another region 5' south of CB25 and called it "CB25N''.

 

 

Table 1: Obervation log of bok globules

Sr. #	Date	UT	Object Name	RA(2000)	DEC(2000)	bII	size
		h:m		h   m  s	$^{\rm o }$ ' ''	$^{\rm o }$	"$\times$''
1	Dec. 23, 97	16:13	CB3 (YR)	00:28:45.8	56:42:08	-6.03	6.7 5.6
2	Dec. 23, 97	19:33	CB25 (Q)	04:59:04.1	52:03:24	5.85	2.2 $\times$2.2
3	Dec. 25, 97	16:52	CB39 (YOR)	06:01:58.5	16:30:26	-3.04	4.5 $\times$3.4
4	Apr. 01, 98	16:30	CB52 (YR)	06:48:42.9	-16:53:27	-8.91	6.7 $\times$3.4
5	Dec. 25, 97	21:23	CB54A (YOR)	07:04:16.8	-16:22:03	-4.6	5.6 $\times$3.4
6	Dec. 25, 97	16:30	CB54B (YOR)	as above	-16:27:03	as	above
7	Apr. 03, 98	16:15	CB58 (Y)	07:18:13.2	-23:38:52	-5.04	7.8 $\times$ 3.4
8	Dec. 24, 97	00:30	CB62 (NA)	13:30:02.8	79:22:34	37.58	3.4 $\times$ 2.2
9	Dec. 24, 97	15:00	CB246 (Q)	23:56:43.6	58:34:29	-3.54	7.8 $\times$4.5
10	Dec. 24, 97	13:41	CB3N	00:28:45.8	56:37:08	-6.03	-
11	Dec. 23, 97	21:15	CB25N	04:59:04.1	51:58:24	5.85	-

 

 

Table 2: Obervation of Polarized Standard stars (Ref. 1 indicates Serkowski, 1975 and Ref. 2 indicates our present observations)

Date	UT	star	$p_{\rm max}$	$\theta _{\rm max}$	$\lambda _{\rm max}$	p	$E_{\rm p}$	$\theta$	Filter
		HD #	(%)	($^{\circ}$)	($\mu$m)	(%)	(%)	($^{\rm o }$)
			(Ref. 1)			(Ref. 2)
Dec. 23, 97	13:53	7927	3.4	94	0.51	3.36	0.01	100.2	V
Dec. 23, 97	14:33	23512	2.3	30	0.61	2.15	0.05	36.9	V
Dec. 23, 97	23:16	43384	3.0	170	0.53	2.78	0.02	177.7	W
Apr. 01, 98	16:00	43384	3.0	170	0.53	2.78	0.03	0.3	W
Apr. 02, 98	07:15	147084	4.3	32	0.68	4.23	0.02	39.1	V

The focal plane instrument used was an Imaging Polarimeter (IMPOL) recently commissioned by IUCAA, Pune, India. The instrument measures linear polarization in the wavelength band $0.45 - 0.7~\mu$m. It makes use of a Wollaston prism as the analyzer to measure simultaneously the two orthogonal polarization components that define a Stoke's parameter. An achromatic half wave plate (HWP) is used to rotate the plane of polarization with respect to the axis of analyzer so that the second Stoke's parameter is also determined. A liquid nitrogen cooled CCD (EEV CCD02-06 series) is used as detector. The CCD has $578 \times 385$ pixels of $22 \mu$ square each, with approximate spectral response (Q.E.) of 25%, 37% and 39% at the wavelengths of 500 nm, 600 nm and 700 nm respectively as per the EEV data sheet. The instrument has an built-in acquisition and guidance unit. With a field of view (FOV) of about 10 mm $\times$ 10 mm ( $6'\times 6'$), it can provide polarization information with an angular resolution of $\sim 2''$ for stellar field or extended objects. The polarimetric accuracies are limited by photon noise. (for details please see Sen & Tandon 1994; Ramaprakash et al. 1998).

For each object within the field of view, two images, called the ordinary and extraordinary, separated by about 33 pixels, are formed on the CCD. These two images correspond to the two orthogonal linear polarization components that are measured simultaneously. The ratio (R) of the difference between the fluxes collected in the two images to their sum, at different orientations ($\alpha$) of the HWP are related to the linear polarization (p) and its position angle ($\theta$) by the relation

$$% R(\alpha) = p \cos(2 \theta -4 \alpha). %$$ (1)

A polarimetric package was developed for data analysis within the IRAF environments using a mixture of standard IRAF tasks, custom- made CL scripts and FORTRAN routines (Ramaprakash 1998).