55 research outputs found
Stellar populations in the Magellanic Clouds: looking through the dust
We present the first results of our study of stellar populations in the Large
and Small Magellanic Clouds based on multi-band WFPC2 observations of "random"
fields taken as part of the "pure parallel" programme carried out with the HST
as a service to the community.Comment: Two pages, one figure, to appear in the proceedings of IAU Symposium
241 "Stellar Populations as Building Blocks of Galaxies", eds. A. Vazdekis
and R. Peletie
Photometric determination of the mass accretion rates of pre-main sequence stars. I. Method and application to the SN1987A field
We have developed and successfully tested a new self-consistent method to
reliably identify pre-main sequence (PMS) objects actively undergoing mass
accretion in a resolved stellar population, regardless of their age. The method
does not require spectroscopy and combines broad-band V and I photometry with
narrow-band Halpha imaging to: (1) identify all stars with excess Halpha
emission; (2) derive their Halpha luminosity L(Halpha); (3) estimate the Halpha
emission equivalent width; (4) derive the accretion luminosity L_acc from
L(Halpha); and finally (5) obtain the mass accretion rate M_acc from L_acc and
the stellar parameters (mass and radius). By selecting stars with photometric
accuracy in Halpha better than 15%, the statistical uncertainty on the derived
M_acc is typically <17% and is dictated by the precision of the Halpha
photometry. Systematic uncertainties, of up to a factor of 3 on the value of
M_acc, are caused by our incomplete understanding of the physics of the
accretion process and affect all determinations of the mass accretion rate,
including those based on a spectroscopic Halpha line analysis.
As an application of our method, we study a field of 9.16 arcmin2 around
SN1987A, using existing HST photometry. We identify as bona-fide PMS stars a
total of 133 objects with a Halpha excess above the 4 sigma level and a median
age of 13.5 Myr. Their median mass accretion rate of 2.6x10-8 Msolar/yr is in
excellent agreement with previous determinations based on the U-band excess of
the stars in this field, and with the value measured for G-type PMS stars in
the Milky Way. Their L_acc shows a strong dependence on their distance from a
group of hot massive stars in the field and suggests that the UV radiation of
the latter is rapidly eroding the circumstellar discs around PMS stars.Comment: 18 pages, 18 figures, accepted for publication in the Astrophysical
Journa
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