On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud^*

¹ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy e-mail: dewit@obs.ujf-grenoble.fr
² Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France
³ Astronomical Institute, University of Utrecht, Princetonplein 5, 3584 CC, Utrecht, The Netherlands
⁴ SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁵ CEA, DSM, DAPNIA, Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
⁶ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 22 April 2004
Accepted: 7 November 2004

Abstract

We investigate a sample of 18 Large Magellanic Cloud Herbig Ae/Be candidate stars looking at their (1) spectral types (2) brightness variability mechanism and (3) near infra-red JHK emission. We find that the majority of the target stars have Hα emission, are of spectral type early- to mid-B and lack strong JHK excess emission. Their Balmer decrements are found to be similar to that of Galactic Be stars in general. Their erratic brightness variability is evaluated by using the observed optical color excess and the color gradient from the light curves and is subsequently interpreted as being due to variable dust obscuration or variable bf-ff emission from circumstellar ionized gas. For approximately half of the target stars in our sample the type of variability seems to be dissimilar to the mechanism involving bound-free and free-free emission, but could be interpreted as caused by variable dust obscuration, as we have proposed in earlier studies. It is therefore suggested that they are pre-main sequence objects, despite the fact that they nearly all lack thermal dust emission in the near infra-red; mid/far infra-red observations for these objects are warranted. One star is observed to have JHK excess emission and an inspection of its 7.5 year MACHO light curve confirms its erratic photometric behavior. The object displays deep photometric minima with a quasi-period of 191.3 days, as generally seen in the Galactic pre-main sequence subgroup of the UX Orionis stars.