Abstract by De Marchi et al.
Using observations obtained with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST), we have studied the properties of the stellar populations in the central regions of 30 Dor, in the Large Magellanic Cloud. The observations clearly reveal the presence of considerable differential extinction across the field. We characterise and quantify this effect using young massive main sequence stars to derive a statistical reddening correction for most objects in the field. We then search for pre-main sequence (PMS) stars by looking for objects with a strong (> 4 sigma) Halpha excess emission and find about 1150 of them over the entire field. Comparison of their location in the Hertzsprung-Russell diagram with theoretical PMS evolutionary tracks for the appropriate metallicity reveals that about one third of these objects are younger than ~4Myr, compatible with the age of the massive stars in the central ionising cluster R136, whereas the rest have ages up to ~30Myr, with a median age of ~12Myr. This indicates that star formation has proceeded over an extended period of time, although we cannot discriminate between an extended episode and a series of short and frequent bursts that are not resolved in time. While the younger PMS population preferentially occupies the central regions of the cluster, older PMS objects are more uniformly distributed across the field and are remarkably few at the very centre of the cluster. We attribute this latter effect to photoevaporation of the older circumstellar discs caused by the massive ionising members of R136.
Relevant Papers
"Star formation in 30 Doradus"
De Marchi et al.
Talk Slides
pdf
From Claus:
ReplyDeleteHow strong is the PMS Ha emission compared with the HII region Ha? Is the background subtraction an issue?
From Claus:
ReplyDeleteWhy is there a gap in the CMD in slide #28?
@Claus:
ReplyDeleteQ. How strong is the PMS Ha emission compared with the HII region Ha? Is the background subtraction an issue?
A. The Ha intensity from the diffuse medium (HII region) can be rather substantial, say, from just comparable to about 20 times the one emitted by the PMS star. As long as the nebular emission is essentially uniform over the stellar PSF (say, a 2 pixel radius circle) and the reference region (say, an annulus 3 to 5 pixel radius), the subtraction can safely be done and can provide an accurate measurement of the stellar Ha emission. On the other hand, when filaments are crossing the stellar PSF and/or the reference region substantial errors may occur and the measurement is not reliable anymore. In practice one has to check ALL Ha emission candidate stars individually to make sure than such filaments (or bright star spikes) are not present. This is done by a clever application of un-sharp masking that enhances the contrast of narrow features and make these checks much easier and reliable. This procedure is described and discussed in some detail by De Marchi et al 2010 (ApJ 715, 1), Beccari et al 2010 (ApJ 720, 1108), as well as De Marchi et al 2011 (ApJ 739, 27).
Q. Why is there a gap in the CMD in slide #28?
A. The gap seen in slide #28 (see Fig. 7 in De Marchi et al 2011 (ApJ 739, 27) is due to an age effect and indicates a lower rate of star formation between about 10 and 16Myr ago. This feature is clearly seen because Figure 7 shows only stars that are unambiguously young, i.e. stars with significant Ha excess: younger than 5Myr are shown as green dots, and older than 5Myr as red crosses. What we actually see there, are numerous stars both above an ~10Myr isochrone and below an ~16Myr isochrone and a paucity in the middle, thus indicating a lull in star formation during the time interval 10-16 Myr ago.
How do you correct H_alpha for the [N II] lines?
ReplyDelete@Brad:
ReplyDeleteQ. How do you correct H_alpha for the [N II] lines?
A. Spectroscopy of PMS stars shows that [NII] lines are rather weak (because of high electron density and consequent collisional deexcitation) , from virtually absent to about 12% the intensity of Ha, thus representing a minor contamination. Moreover, the narrow Ha filter of WFC3 (18A wide) does not extend to either of the [NII] doublet lines, and therefore, there is no need to apply any correction. For observations made with the ACS (its "narrow" Ha filter is 80A wide and includes the [NII] 6584A line) we apply a "statistical" correction of 6+/-6% to the emission line intensities measured with Ha filter photometry.
Q. Could kink in isochrone seen in slide #7 explain gap in CMD seen in slide #28?
A. Yes, see my answer to 2nd Claus' question above.
Q. How do you get V-I color if you have an H_alpha excess?
ReplyDelete@ Derck:
ReplyDeleteQ. How do you get V-I color if you have an H_alpha excess?
A. The locus of the stars with no Ha excess in the (V-Ha) vs (V-I) diagram is provided by a multitude of stars present in the field (both young and old) as well as model atmosphere SED: we find excellent agreement between observations and theory, so that we can proceed safely along this route.
Is it possible that H_alpha depends on metallicity and not M_acc?
ReplyDeleteWhat is the reason why metallicity should favor M_acc?
@ Danny:
ReplyDeleteQ. Is it possible that H_alpha depends on metallicity and not M_acc?
A. I doubt because both processes of ionization and recombination of hydrogen can hardly be affected by metals. Therefore, the intensity of Ha is expected to be simply proportional to the power released by accretion.
Q. What is the reason why metallicity should favor M_acc?
A. Good question! A "possible" reason could be that a lower metallicity implies lower dust absorption in the falling material that is forming a star, with the result that a larger amount of material can accumulate into a circumstellar disk.
Q. Can gap in CMD of slide #28 could be explained in terms of the removal of older background stars?
ReplyDelete@ Elena:
ReplyDeleteQ. Can gap in CMD of slide #28 could be explained in terms of the removal of older background stars?
A. Yes, in the sense that slide #28 shows only young population stars. See full discussion of this point in the answer to 2nd Claus' question.