2 Observations

2.1 Photometry

CCD UBVRI (Cousins system) photometry for 802 stars in the region of the open cluster HM1 was obtained in May 1993 and May 1997 using the University of Toronto Southern Observatory (UTSO) telescope, Las Campanas (Chile). Four zones were measured in 1993 when the telescope was equipped with a PM $512\times 512$ METACHROME UV-coated chip (4 ${\hbox {$^\prime $ }}$ on a side, 0.45 ${\hbox{$^{\prime\prime}$ }}$/pixel). In 1997 we used a new Kodak CCD $2032\times 2048$ UV-coated (7 ${\hbox {$^\prime $ }}$ on a side, 0.40 ${\hbox{$^{\prime\prime}$ }}$/pixel) chip $2\times 2$ binned and glycol refrigerated to cover the additional large area on the tilt depicted in the finding chart in Fig. 1. On this occasion, a comparison field 15 ${\hbox {$^\prime $ }}$ north of HM1 was also exposed.

To improve the signal-noise ratio of faint stars, we combined two long exposure frames of 700, 450, 200, 110 and 110 s in the UBVRI filters respectively. Moreover, to avoid saturation among bright stars, short exposures of 40 and 50 s in the RI filters were additionally taken. The nights were photometric with seeing values ranging from 1.2 ${\hbox{$^{\prime\prime}$ }}$ to 1.4 ${\hbox{$^{\prime\prime}$ }}$.

We removed instrumental signatures with a combination of bias and flat field exposures. Using the point spread function, PSF method (DAOPHOT, Stetson 1987), instrumental magnitudes were obtained. They were matched with the standard system by means of calibration sequences in the open clusters NGC 5606 and Hogg 16 (Vázquez & Feinstein 1991a, 1991b; Vázquez et al. 1994) including over 20 stars spectrally well distributed. For extinction coefficients we used those of Grothues & Gocherman (1992).

We adopt as the external photometric error the typical calibration errors on the order of 0.02-0.03 mag in colour and magnitude. The internal accuracy of our photometry, 0.02 mag, was estimated from the mean differences of colours and magnitudes for stars with V < 17 that are located in the overlapping regions of the different frames.

Figure 2: The error (as given by DAOPHOT) of our photometry against the V magnitude

   

Table 1: Photometric catalogue of the HM1 region

#	HM-TAH	X	Y	V	B-V	U-B	V-R	V-I	LSS	ST	Rem	Note
1	1	854.5	487.0	11.02	1.48	0.40	1.18	2.14	4065	WN7/WN8-A	1/2
2	4	769.3	237.0	11.30	0.42	0.46	0.44	0.53
3	5	975.3	631.2	11.40	0.50	0.19	0.49	0.76
4	2	722.5	489.2	11.44	1.49	0.37	1.28	2.28	4067	O4If+	1	*

Comment: Table 1 is available in full in an electronic version at the CDS. A portion of it is shown here.

Figure  2 shows the magnitude and colour errors from DAOPHOT as a function of V. Up to V = 16, the U-B errors remain below 0.05 mag but quickly rise to 0.3 for V = 16-17 due to the strong absorption in this area. V = 17 is, therefore, a reliable limit for further analyses in this text. The CCD photometric catalogue containing the star identification, the x -y co-ordinates, V, U-B, B-V, V-R, V-I values, the cross correlation with other authors and the available spectral types is provided in Table 1 (available in electronic format). In the rest of this paper, the notation used by HM77 and TAH82 will be preferred; however, in those cases where ours is deemed indispensable, it will be denoted by numbers in brackets.

Figure 3: The Serkowski's (1973) law fittings for the 21 stars in our sample. Bars indicate the polarisation errors (see Table 3)

 

 

Table 2: Mean photometric differences (our data minus other authors)

	$\Delta V$	$\Delta(B-V)$	$\Delta(U-B)$	$\Delta(V-R)$	$\Delta(V-I)$	N
HM77	$-0.04\pm0.04$	$-0.04\pm0.02$	$0.03\pm0.09$	-	-	18
TAH82	$0.05\pm0.12$	-	-	$0.22\pm0.08$	$0.22\pm0.12$	4

Eighteen out of 802 stars have UBV photoelectric photometry made by HM77. Four of them have also VRI and near IR photometry made by TAH82. The mean photomeric differences given by our measures minus those of other authors, along with the standard deviations, listed in Table 2, are quite small except for stars 2, 7, 10, 14, 15, 22 and 24. We found that three of them, stars 2, 10 and 22, have close companions photometrically resolved. The rest of the stars with large differences have close neighbours (at 6 ${\hbox{$^{\prime\prime}$ }}$), therefore, contamination and even variability are possible. Surprisingly, the four TAH82 stars show VRI differences exceeding several tenths of our measures. We were unable to account for this.

 

 

Table 3: Polarimetric data and fitting results

Star	$P_{U}(\%)~~~$	$P_{B}(\%)~~~$	$P_{V}(\%)~~~$	$P_{R}(\%)~~~$	$P_{I}(\%)~~~$	$P_{\max}(\%)~~$	$\varepsilon_{\rm P}/P_{\max}$
	$\theta_{U}(^\circ)~~~$	$\theta_{B}(^\circ)~~~$	$\theta_{V}(^\circ)~~~$	$\theta_{R}(^\circ)~~~$	$\theta_{I}(^\circ)~~~$	$\lambda_{\max}({\rm {\mu m}})~$	EB-V
1	$1.83\pm0.20$	$2.02\pm0.25$	$1.91\pm0.06$	$2.00\pm0.03$	$1.66\pm0.04$	$2.04\pm0.07$	0.04
	$18.4\pm3.1$	$22.0\pm3.5$	$18.3\pm0.9$	$18.2\pm0.5$	$18.2\pm0.6$	$0.56\pm0.32$	1.84
2	$2.81\pm0.16$	$2.91\pm0.11$	$2.11\pm0.08$	$2.21\pm0.04$	$1.75\pm0.08$	$2.64\pm0.24$	0.09
	$44.4\pm1.6$	$41.5\pm1.1$	$35.5\pm1.1$	$40.5\pm0.5$	$39.1\pm1.3$	$0.45\pm0.48$	1.82
3	$4.64\pm0.84$	$5.32\pm0.48$	$5.64\pm0.09$	$6.11\pm0.05$	$4.87\pm0.08$	$6.04\pm0.26$	0.04
	$28.4\pm5.2$	$38.8\pm2.6$	$39.0\pm0.5$	$37.8\pm0.2$	$39.1\pm0.5$	$0.59\pm0.54$	1.84
4	$1.26\pm0.27$	$1.66\pm0.07$	$1.71\pm0.10$	$1.81\pm0.08$	$1.72\pm0.18$	$1.84\pm0.03$	0.02
	$156.4\pm6.2$	$8.5\pm1.3$	$12.1\pm1.7$	$9.6\pm1.3$	$16.7\pm3.0$	$0.61\pm0.20$	0.38
5	$1.69\pm0.39$	$1.46\pm0.19$	$0.99\pm0.35$	$1.39\pm0.22$	$0.86\pm0.34$	$1.49\pm0.15$	0.10
	$179.2\pm6.7$	$12.6\pm3.7$	$171.2\pm10.0$	$173.4\pm4.5$	$0.2\pm11.4$	$0.45\pm0.72$	0.59
6	$4.57\pm0.44$	$4.45\pm0.10$	$4.27\pm0.14$	$4.26\pm0.11$	$3.55\pm0.17$	$4.56\pm0.10$	0.02
	$174.3\pm2.8$	$2.9\pm0.6$	$5.4\pm0.9$	$1.7\pm0.7$	$3.9\pm1.4$	$0.52\pm0.22$	1.83
7	$2.31\pm0.24$	$2.94\pm0.07$	$2.92\pm0.13$	$3.17\pm0.09$	$3.23\pm0.19$	$3.26\pm0.09$	0.03
	$10.5\pm3.0$	$21.3\pm0.7$	$27.6\pm1.3$	$21.8\pm0.8$	$24.9\pm1.7$	$0.61\pm0.30$	0.39
8	$3.43\pm0.25$	$4.33\pm0.15$	$2.60\pm0.06$	$2.73\pm0.06$	$2.22\pm0.08$	$3.07\pm0.42$	0.14
	$36.1\pm2.1$	$34.9\pm1.0$	$33.5\pm0.7$	$37.5\pm0.6$	$37.3\pm1.0$	$0.47\pm0.88$	1.85
9	$1.84\pm0.42$	$1.47\pm0.56$	$1.68\pm0.61$	$1.63\pm0.53$	$1.64\pm0.52$	$1.87\pm0.14$	0.08
	$42.5\pm6.5$	$37.8\pm10.9$	$37.0\pm10.4$	$37.9\pm 9.4$	$35.1\pm 9.1$	$0.51\pm0.57$	1.86
12	$5.88\pm0.35$	$6.34\pm0.25$	$5.37\pm0.11$	$5.62\pm0.06$	$4.76\pm0.08$	$5.86\pm0.27$	0.05
	$13.4\pm 1.7$	$23.3\pm 1.1$	$21.8\pm 0.6$	$21.4\pm0.3$	$22.0\pm 0.5$	$0.55\pm0.41$	1.77
13	$5.16\pm0.27$	$5.43\pm0.29$	$4.87\pm0.05$	$5.08\pm0.04$	$4.23\pm0.06$	$5.10\pm0.15$	0.03
	$34.2\pm1.5$	$36.7\pm1.5$	$36.3\pm0.3$	$36.8\pm0.2$	$37.8\pm0.4$	$0.58\pm0.38$	1.78
14	$1.56\pm0.28$	$1.56\pm0.09$	$1.94\pm0.10$	$1.99\pm0.06$	$1.92\pm0.08$	$2.01\pm0.03$	0.02
	$178.4\pm5.2$	$5.1\pm1.6$	$0.5\pm1.4$	$3.8\pm0.9$	$2.4\pm1.1$	$0.68\pm0.22$	0.55
15	$3.09\pm1.53$	$1.55\pm0.15$	$0.43\pm0.55$	$0.44\pm0.38$	$0.74\pm0.52$	$10.05\pm16.85$	1.68
	$165.9\pm14.2$	$177.3\pm 2.7$	$165.4\pm37.0$	$18.1\pm24.4$	$45.6\pm20.1$	$0.12\pm0.71$
16	$6.77\pm0.80$	$6.44\pm0.18$	$4.90\pm0.06$	$5.28\pm0.05$	$4.29\pm0.05$	$5.28\pm0.30$	0.06
	$39.6\pm 3.4$	$45.3\pm 0.8$	$38.2\pm 0.3$	$41.8\pm 0.3$	$41.4\pm 0.4$	$0.56\pm0.57$	2.07
17	$0.64\pm0.19$	$1.45\pm0.12$	$1.28\pm0.05$	$1.35\pm0.06$	$1.07\pm0.15$	$1.34\pm0.06$	0.05
	$16.8\pm 8.5$	$10.0\pm 2.3$	$13.2\pm 1.0$	$13.1\pm 1.2$	$17.1\pm 3.9$	$0.61\pm0.71$	0.55
18	$1.46\pm0.70$	$2.76\pm0.72$	$1.95\pm0.18$	$1.94\pm0.26$	$1.63\pm0.24$	$2.02\pm0.11$	0.06
	$4.9\pm13.8$	$24.4\pm 7.4$	$29.1\pm 2.7$	$42.1\pm 3.8$	$38.5\pm 4.2$	$0.54\pm0.55$	1.76
19	$2.75\pm0.70$	$2.10\pm1.46$	$1.35\pm0.47$	$2.20\pm0.41$	$1.39\pm0.49$	$2.18\pm0.50$	0.23
	$35.6\pm 7.3$	$35.5\pm20.0$	$22.3\pm10.0$	$33.9\pm 5.4$	$8.4\pm10.2$	$0.45\pm1.24$	1.84
20	$4.33\pm0.28$	$5.09\pm0.27$	$3.94\pm0.10$	$4.27\pm0.06$	$3.49\pm0.08$	$4.42\pm0.27$	0.06
	$21\pm 1.8$	$31.5\pm 1.5$	$27.3\pm 0.8$	$30.3\pm 0.4$	$29.5\pm 0.7$	$0.54\pm0.51$	1.57
21	$2.21\pm0.12$	$2.46\pm0.11$	$2.42\pm0.05$	$2.52\pm0.05$	$2.10\pm0.08$	$2.54\pm0.07$	0.03
	$11.2\pm 1.6$	$10.9\pm 1.3$	$11.1\pm 0.6$	$13.3\pm 0.6$	$10.2\pm 1.1$	$0.57\pm0.30$	0.59
(29)	$2.94\pm0.58$	$3.05\pm0.26$	$3.13\pm0.06$	$3.21\pm0.08$	$2.65\pm0.10$	$3.20\pm0.06$	0.02
	$179.8\pm5.7$	$178.3\pm 2.5$	$177.9\pm 0.6$	$0.3\pm 0.7$	$178.5\pm1.1$	$0.58\pm0.24$	1.43
(44)	$3.65\pm1.04$	$2.99\pm0.43$	$2.07\pm0.17$	$2.07\pm0.15$	$2.06\pm0.35$	$2.32\pm0.27$	0.12
	$19.2\pm 8.1$	$14.9\pm 4.1$	$17.1\pm 2.3$	$18.8\pm 2.0$	$7.5\pm 4.9$	$0.50\pm0.98$	0.84

2.2 Polarimetry

On three nights of June in 1997 we carried out UBVRI polarimetric observations of 21 stars using the Torino Observatory Five-Channel Photopolarimeter attached to the 215-cm telescope of the Complejo Astronómico El Leoncito (CASLEO). A set of filters with effective wavelengths, $\lambda_{U} =0.360~\mu$m, $\lambda_{B} =0.440~\mu$m, $\lambda_{V} =0.530~\mu$m, $\lambda_{R} =0.690~\mu$m, $\lambda_{I} = 0.830~\mu$m, and a 15 ${\hbox{$^{\prime\prime}$ }}$ diameter diaphragm was used. The number of observations varied from 4 to 8 for each star and each night, three stars of null polarisation, HD 68456, 102365 and 146233, (Gliese 1969), were observed to remove instrumental polarisation. The determination of the zero point of the polarisation angle was carried out by observing three stars each night, namely, HD 111613, 147084 and 187929, (Serkowski et al. 1975). The percentage of polarisation, $P_{\lambda}$, the polarisation angle, $\theta_{\lambda}$, and their errors are included in Table 3. The uncertainties in the polarisation angles shown in this table were computed using (Hsu & Breger 1982):

$$\varepsilon_\theta = 28\hbox{$.\!\!^\circ$ }65\frac{\varepsilon_p}{P}\cdot$$

The cases of large uncertainties, especially in the U and B bands, require more observations in order to be reduced. However, since we are interested in the detection of large variations of polarisation across the cluster surface, the accuracy reached in this attempt is quite satisfactory. Assuming that polarisation is produced by interstellar dust and that the wavelength $\lambda_{\max}$ at which the maximum polarisation $P_{\max}$ occurs depends both on the optical properties and size distribution of dust grains (Greenberg 1968; McMillan 1978; Wilking et al. 1980), we computed $\lambda_{\max}$ and $P_{\max}$ for 21 stars from Serkowski's law (1973):

$$\frac{P_\lambda}{P_{\max}} = \exp \left( -k \ln^{2} \left( \frac {\lambda_{\max}}{\lambda}\right) \right)$$

where k = 1.15. The $P_{\max}$ and $\lambda_{\max}$ values, the relative fitting error, $\varepsilon_{\rm P}/P_{\max}$ are included in Table 3. Figure 3 shows the Serkowski's law fittings for all the stars observed.