The Low‐Mass Initial Mass Function of the Field Population in the Large Magellanic Cloud withHubble Space TelescopeWFPC2 Observations

Abstract

We present V-and I-equivalent HST WFPC2 stellar photometry of an area in the Large Magellanic Cloud (LMC), located to the west of the bar of the galaxy, which accounts for the general background field of its inner disk. The WFPC2 observations reach magnitudes as faint as V ¼ 25 mag, and the large sample of more than 80,000 stars allows us to determine in detail the present-day mass function ( PDMF) of the detected main-sequence stars, which is identical to the initial mass function ( IMF) for masses M P 1 M . The low-mass main-sequence mass function of the LMC field is found not to have a uniform slope throughout the observed mass range; i.e., the slope does not follow a single power law. This slope changes at about 1 M to become more shallow for stars with smaller masses down to the lowest observed mass of $0.7 M , giving clear indications of flattening for even smaller masses. We verified statistically that for stars with M P 1 M the IMF has a slope À around À2, with an indicative slope À ' À1:4 for 0:7 P M /M P 0:9, while for more massive stars the main-sequence mass function becomes much steeper with À ' À5. The main-sequence luminosity function ( LF) of the observed field is in very good agreement with the Galactic LF as it was previously found. Taking into account several assumptions concerning evolutionary effects, which should have changed through time the stellar content of the observed field, we reconstruct qualitatively its IMF for the whole observed mass range (0:7 P M/M P 2:3), and we find that the number of observed evolved stars is not large enough to have affected significantly the form of the IMF, which thus is found almost identical to the observed PDMF.

CHEMISTRY OF THE SAGITTARIUS DWARF GALAXY: A TOP-LIGHT INITIAL MASS FUNCTION, OUTFLOWS, AND THER-PROCESS

“…While the IMF slopes for clusters in the LMC are close to the Salpeter value seen in the MW (e.g., Massey 2003), the IMF derived from the measured current mass function of LMC field stars, a measure of the IGIMF, shows steep slopes, Γ ∼ −4, above 1 M (Gouliermis et al 2006;Massey et al 1995a), consistent with a top-light IGIMF. A caveat is that measured mass functions for field stars more massive than ∼1 M require corrections to account for evolved stars and past star formation, in order to estimate the IMF.…”

A Universal Stellar Initial Mass Function? A Critical Look at Variations

“…This led him to conclude that the IMF was environment dependent, in particular varying with density. Similarly steep mass functions have been derived by Gouliermis, Brandner & Henning (2006) using HST WFPC2 data of a field near the bar of the LMC. Parker et al (1998) used the Ultraviolet Imaging Telescope to derive the IMF of the LMC field and also found a steeper distribution (although not as extreme as the above studies) of Γ = 1.8± 0.09 for the mass range of 7− 35 M ⊙ .…”

Large Magellanic Cloud self-lensing for OGLE-II microlensing observations

“…It is worth recalling that these parametrizations are based on the analyses carried out within our Galaxy. Robust results from the analyses carried out within the LMC itself are still missing, in particular for stars with mass below 0.4 M ⊙ (in fact the most interesting mass range for our purposes) even if a few first analyses show roughly consistent, although somewhat steeper, behaviours (Gouliermis, Brandner & Henning 2006; Da Rio, Gouliermis & Henning 2009).…”