NGC 7129: SVS 13 and Other Stars

Pierre Bastien, Roger Hajjar, and Daniel Nadeau

Observatoire du Mont Mégantic et Département de physique, Université de Montréal,
C.P. 6128, Succ. Centre-ville, Montréal, QC, Canada H3C 3J7
Electronic-Mail: bastien@physcn.umontreal.ca, rhajjar@dm.net.lb, nadeaud@ere.umontreal.ca

Abstract:

Polarimetric observations carried out at the mont Mégantic Observatory have revealed that SVS 13, which we call Noor, has the highest linear polarization in NGC 7129. It also shows on polarization maps the largest disk as determined by the angular distance between the two null points, at the edges of the parallel vector pattern. At the distance of NGC 7129, 1.25 kpc, the disk has a size of $\approx$ 5600 AU in the I band. The J-band polarimetric map (from the CFHT) also shows a large disk. The emissivity index, deduced from mm and submm continuum photometric measurements at the JCMT, $\beta$ = 0.22, is compatible with that of Young Stellar Objects. We found 4 new infrared sources around Noor. A large number of infrared sources are also present around LkH$\alpha$ 234. They fall on or around a V-shaped enhanced emission ridge in the IR, also visible in H₂, giving insight into their star formation history. Based on this information, we confirm this scenario. The formation of BD+65^o 1638 triggered the formation of the other stars through shocked molecular gas by the stellar wind. It is still an ongoing process where now Noor and LkH$\alpha$ 234 are triggering star formation.

Introduction

  NGC 7129 is a region of star formation containing two well-known Herbig Ae/Be stars (Herbig 1960), LkH$\alpha$ 234 and BD+65^o 1637. The other two stars surrounded by nebulosity in the region are BD+65^o 1638 and another one identified as SVS 13 (Strom et al. 1976)¹⁾, which we call hereafter Noor. The estimated age of this group is, according to Strom et al (1976), not more than a few times 10⁵ years. To study the circumstellar material around these young stars, we carried out the following observations.

Observations and Results

  We observed NGC 7129 in September 1991 at the mont Mégantic Observatory. Direct images were obtained in the B, V, and R bands. We also took polarimetric images in the I band using a polarizer positioned at three different orientations, namely 0^o, 60^o, and 120^o. All these images were taken with a 512$\times$320 RCA CCD with a scale of 0.48'' per pixel at f/8.

Additional observations were obtained at the f/8 focus of the CFHT in June 1996 with MONICA (MONtreal Infrared CAmera) (Nadeau et al. 1994). A rotating half-wave plate and a linear polarizer were used in front of the HgCdTe 256$\times$256 pixels detector in the J band. Observations not discussed here were also obtained on the JCMT (see Hajjar et al. 1998 for details).

We discuss here briefly the results for Noor and for the general environment of NGC 7129. Noor and its infrared companions can be seen in Figure 1. The polarization map in the I band is shown in Figure 2 . It shows a pattern of aligned polarization vectors near the central source. The size of the disk is determined from the position of the two null points which delimit the parallel vector pattern. A special algorithm was developed to obtain those positions with more accuracy, as explained by Hajjar et al. 1998). The sizes are 5.6'' and 3.6'' in the I and J bands respectively. We also estimated the disk inclination by comparing with the models of Bastien & Ménard (1990) and found an inclination angle > 80^o for both maps.

 

Figure 1:  Contour plots of Noor in all 6 bands observed. The letter in the corner gives the bandpass. Offsets are from Noor. North is up and East to the left.

 

Figure 2:   I-band polarization map of Noor. Polarization vectors are overlayed on the intensity contours. The polarization scale is indicated on the map.

The number of sources that can be seen on the images of the area increases significantly in going from the B band to the K band. The spatial distribution of the stars and their infrared companions, and the presence of a shock seen in H₂, confirm a scenario where BD+65^o 1638 formed first and then triggered the formation of other stars with its strong stellar wind. NGC 7129 is still a very active site of star formation and a prime candidate to study shock-triggered star formation.

This work is part of the Ph. D. Thesis by R. Hajjar at Université de Montréal and will be published in full in the near future (Hajjar et al. 1998).

This research was financed by the Conseil de recherche en sciences naturelles et génie du Canada.

1)

Another star labelled SVS 13 is known in the Serpens dark cloud. Since the name often refers to the Serpens star, we chose to change it for the sake of clarity.