1. Probing the mass accretion process in the neighbourhood of SN1987A
Loredana Spezzi (ESA), Guido De Marchi (ESA), Nino Panagia (STScI/INAF-CT), Giacomo Beccari (ESA), Martino Romaniello (ESO)
Introduction: The knowledge of the mass accretion rate (Macc) of pre-main sequence (PMS) stars is of paramount importance to constraint the models of star formation and early stellar evolution. The determination of Macc (via the UV excess, veiling or emission line profile methods) has so far mostly relied on spectroscopy, which is often very hard or impossible to obtain in dense, distant star-forming regions. De Marchi et al. (ApJ, in press) have recently presented a novel method to determine the PMS mass accretion rate that relies solely on photometry. We have started a pilot project aiming at applying this technique to the existing and extensive HST photometry of star-forming regions in the Milky Way and Magellanic Clouds. This will allow us to increase by more than two orders of magnitude the current sample of PMS stars with a measured mass accretion rate. In this contribution, we present some of the first results of this study in a 2.7’×2.7’ area in the Large Magellanic Cloud (LMC) centered at RA=05h34m36s and DEC=-69d13m57s, i.e. about 6 arcmin NW of the supernova SN1987A (Fig.1). The area was observed as a parallel field while taking spectroscopy of SN1987 in 1999 February with HST/WFPC2 in the f300w (U), f450w (B), f606w (V), f656n (Hα) and f814w (I) filters. The region appears still embedded in gas and it is then expected to be site of a more recent star formation event than the field of SN1987A itself. ●
Method: We select PMS stars in the field and determine their mass accretion rates using narrow-band Hα and broad-band V and I dereddend photometry. We proceed as follows:
● Reddening: Since we expect differential reddening, we divided our field in 22 regions each 32”×32” in size and obtained the V vs. (V-I) diagram for each sub-image (Fig.2). The reddening in each of these snapshots was then determined applying a method similar to that described in Piotto et al. (1999, AJ 118, 1727).
Fig.1 SDSS image of a 40’×40’ area in the LMC. The blue polygon displays the area observed with HST/WFPC2.
● Selection of PMS stars: We use the median (V–Hα) dereddened colour of stars with small (≤0.1) photometric errors in the three (V, I and Hα) bands, as a function of (V–I), to define the reference template with respect to which the excess Hα emission is sought (Fig.3a). We select a first sample of stars with excess Hα emission by considering all those with a (V–Hα) colour above that of the reference line by 4 times their intrinsic photometric uncertainty. Then we derive the equivalent width of the Hα emission (EWHa) from the measured colour excess (Hα = Hαref–Hαstar). We finally consider as bona fide PMS stars those objects with EWHa>20Å and V-I>0 (Fig.3b); this allows us to clean our sample from possible contaminants (i.e. older star with chromospheric activity and Ae/Be star, respectively).
● Stellar parameters: We derive the effective temperature (Teff), radius (R*) and luminosity (L*) of our bona fide PMS stars from their dereddened (V-I) colour and V magnitude. We adopt the distance of 51.4 Kpc to SN1987A and use the tabulation of HST/WFPC2 photometry as a function of Teff by Bessel et al. (1998, A&A 333, 231). Mass (M*) and age of each star are then derived by comparing its L* and Teff with expectations from the theoretical evolutionary models for PMS stars (Fig.4) by Siess et al. (2000, A&A 358, 593) for the metallicity of the LMC (Z=0.008).
Fig.2 The WFPC2 field divided into 22 sub-images and relative V vs. V-I diagrams. The blue line is the 4 Myr isochrone by Girardi et al. 2004, A&A 422, 205) for the metallicity of the LMC (Z=0.008), while the red dots are the stars used for estimating the reddening. The minimum and maximum E(V-I) measured in each snapshot are also reported.
● Mass accretion rate: The mass accretion rate is finally derived from the free-fall equation as:
log Macc (M/yr) = log Lacc/L  + log R*/R - log M*/M
where Lacc is the energy released by the accretion process, directly proportional to the Hα luminosity (Dahm 2008, AJ 136, 521). ● a b
Fig.5 Histogram of mass accretion rate for the bona fide PMS stars.
Fig.4 HR diagram for the bona fide PMS stars.
Fig.3 V-Hα vs. V-I (a) and EWHα vs. V-I (b) diagrams for stars in our field. Stars with photometric errors smaller than 0.1 mag in V, I and Hα (grey dots) define the reference template of normal stars (i.e. with no Hα emission), shown in panel “a” as a dashed line. Stars with emission exceeding 4 times their photometric uncertainty are highlighted in red. The green triangles are the stars with EWHa<20Å or V-I<0.
Fig.6 Mass accretion rate vs. age for the bona fide PMS stars (red dots). Other symbols are as in the legend.
Results: We have determined the mass accretion rates of 390 PMS stars in a 2.7’×2.7’ field in the SN1987A neighbourhood in the LMC, which we identified by means of their measured excess Hα emission. We find that the mass accretion rates considerably increase for younger PMS stars, fully in line with the theory of stellar evolution. The bona fide PMS stars in our sample have masses between 1 and 2 M (median mass  1.3 M) and ages between 0.3 and 30 Myr (median age  12 Myr). The median mass accretion rate is 5.2x10-8 M/yr (Fig.5).
The PMS stars in our field appear slightly younger and faster accreting relative to the population in the field of SN1987A itself, for which De Marchi et al. (ApJ, in press) found ages in the range 8-30 Myr and a typical mass accretion rate of 2.9 x10-8 M/yr. Our results further confirm that the accretion rates in the LMC are systematically higher, by up to a factor of ten (De Marchi et al., ApJ, in press; De Marchi et al., in prep.), than those reported for Galactic PMS stars of the same age (Muzerolle et al. 2000, ApJ 535, L47; 2005, ApJ 625, 906), but agree with those measured for G type stars (Sicilia-Aguilar et al. 2006, AJ 132, 2135) indicating that the mass accretion rate is a strong function of stellar mass and metallicity (Fig.6).