A New OB association in the HII region N 50 in the LMC

Pablo G. Ostrov, Virpi S. Niemela, Nidia I. Morrell

Facultad de Ciencias Astronómicas y Geofísicas, Paseo del Bosque S/N (1900) La Plata, Argentina

ostrov@fcaglp.fcaglp.unlp.edu.ar

virpi@fcaglp.fcaglp.unlp.edu.ar

nidia@fcaglp.fcaglp.unlp.edu.ar

INTRODUCTION

Sk-67$^{\circ }$105 is one of the most luminous objects in the Large Magellanic Cloud and the exiting star of the H II region N 50 (DEM 193).

Niemelä & Morrell (1986) have found that Sk -67$^{\circ }$105 is indeed a double lined spectroscopic binary (O4f + O6:V), with a period of 3.301 days. Later, Haefner et al. (1994) have reported light variability of this system and analyzed its light curve.

As the luminous OB stars are usually found in young stellar groups, we have searched for such aggregate in the vicinity of Sk -67$^{\circ }$105.

Based on B and V CCD photometry of the field surrounding
Sk-67$^{\circ }$105 and spectroscopic data of several selected stars, we report the discovery of a new OB association in the LMC, being Sk-67$^{\circ }$105 its more notorious member.

We have also found that Sk-67$^{\circ }$105 is the most luminous star of a small compact cluster, composed by at least 8 stars (detected both in B and V filters) inside a circle of 10 arcsec radius.

THE OBSERVATIONS

The data presented here were obtained at the 2.15m Telescope of the Complejo Astronómico El Leoncito (CASLEO), San Juan, Argentina, along with observations of other interesting Magellanic Clouds objects.

PHOTOMETRY

$$\times$$ Detector

Readout Noise 10.4 e^-

Gain 7.98 e^-/ADU

Filters Johnson's B and V

Date 7 nights in October 1993

Exposures 8 best V frames + 7 best B frames (slightly non photometric nights)

$$\sim \times$$ Field

Scale 0.54 arcsecs/px (after binning)

$$\sim$$ Seeing

Date November 19, 1997

Exposures 4 V frames, amounting 110 secs exposure time 3 B frames, amounting 140 secs exposure time

Field Circle of 9 arcmins of diameter (focal reducer)

Scale 0.813 arcsecs/px

$$\sim$$ Seeing

The profile fitting photometry of these frames was carried out with DAOPHOT II.

SPECTROSCOPY

Date February 1998

Spectrograph Boller & Chivens Cassegrain

$$\times$$ Detector

Resolution 2.3 Å/px

The spectroscopic observations were processed and analysed using IRAF routines.

RESULTS

In Figure 1 we show a composite image made with the B and V combined CCD frames, obtained in November 19, 1997. This image reveals a concentration of blue stars surrounding Sk-67$^{\circ }$105.

In Figure 2 we show in detail the immediate neighborhood of Sk-67$^{\circ }$105, which has been removed from this frame, in order to show fainter stars in a compact cluster.

A PSF fitting photometry carried out with DAOPHOT (Stetson, 1987) reveals the existence in this compact star cluster of, at least, 8 stars inside a circle of 10 arcsec radius surrounding Sk-67$^{\circ }$105.

In Figure 3 we show the CMD based on the photometric frames obtained in 1997. This diagram reveals the presence of an OB association within the field of about 9 arcmin of diameter.

We obtained spectra of some of the brightest and bluest stars in the field of the new OB Association. A sample of them is shown in Figure 4 and Figure 5.

Figure 1a is a finding chart of the region showing the stars observed spectroscopically.

Spectral types for these stars were obtained following classification criteria from Walborn & Fitzpatrick (1990). The spectra were also measured for radial velocity determinations.

Spectral types and radial velocities (listed in Table 1) confirm that these stars belong to a young association in LMC.

TABLE 1

ID V B-V Sp.T. R.V. (km/s)

$$\pm$$ 5 13.80 -0.01 B5 III

$$\pm$$ 7 14.40 -0.13 B0.5 II-III

$$\pm$$ 9 14.58 -0.13 B0.5 III

$$\pm$$ 15 15.17 -0.14 O7 V

$$\pm$$ 41 16.16 -0.08 B1 IV-V (e?)

Spectrophotmetric data for stars in the new OB association

REFERENCES

Haefner R., Simon K. P. & Fiedler A., 1994, A&A, 288, L9

Niemelä V. S. & Morrell N. I., 1985, ApJ, 310, 175

Stetson P. B., 1987, PASP, 99, 191

Walborn, N.R. & Fitzpatrick, E., 1990, PASP, 102, 379