3 research outputs found
A Survey of Local Group Galaxies Currently Forming Stars: III. A Search for Luminous Blue Variables and Other H-alpha Emission-Lined Stars
We describe a search for H-alpha emission-lined stars in M31, M33, and seven
dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, Sextans B, Sextans A,
Pegasus and the Phoenix dwarf) using interference filter imaging with the KPNO
and CTIO 4-m telescope and Mosaic cameras. The survey is aimed primarily at
identifying new Luminous Blue Variables (LBVs) from their spectroscopic
similarity to known LBVs, avoiding the bias towards photometric variability,
which may require centuries to manifest itself if LBVs go through long
quiescent periods. Followup spectroscopy with WIYN confirms that our survey
detected a wealth of stars whose spectra are similar to the known LBVs. We
"classify" the spectra of known LBVs, and compare these to the spectra of the
new LBV candidates. We demonstrate spectacular spectral variability for several
of the new LBV candidates, such as AM2, previously classified as a Wolf-Rayet
star, which now shows FeI, FeII and Balmer emission lines but neither the NIII
4634,42 nor HeII 4686 emission that it did in 1982. Profound spectral changes
are also noted for other suspected and known LBVs. Several of the LBV
candidates also show >0.5 mag changes in V over the past 10-20 years. The
number of known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822,
and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33 may be
several hundred, in contrast to the 8 known historically through large-scale
photometric variability. This has significant implications for the time scale
of the LBV phase. We also identify a few new WRs and peculiar emission-lined
objects.Comment: Accepted by the Astronomical Journal. Version with higher quality
figures may be downloaded from http://www.lowell.edu/users/massey/has.pdf.g
A Survey of Local Group Galaxies Currently Forming Stars: II. UBVRI Photometry of Stars in Seven Dwarfs and a Comparison of the Entire Sample
We have obtained UBVRI images with the Kitt Peak and Cerro Tololo 4-m
telescopes and Mosaic cameras of seven dwarfs in (or near) the Local Group, all
of which have known evidence of recent star formation: IC10, NGC 6822, WLM,
Sextans B, Sextans A, Pegasus,and Phoenix. We construct color-magnitude
diagrams (CMDs) of these systems, as well as neighboring regions that can be
used to evaluate the degree of foreground contamination by stars in the Milky
Way. Inter-comparison of these CMDs with those of M31, M33, the LMC, and the
SMC permits us to determine improved reddening values for a typical OB star
found within these galaxies. All of the CMDs reveal a strong or modest number
of blue supergiants. All but Pegasus and Phoenix also show the clear presence
of red supergiants in the CMD, although IC10 appears to be deficient in these
objects given its large WR population. The bright stars of intermediate color
in the CMD are badly contaminated by foreground stars (30-100%), and
considerable spectroscopy is needed before statistics on the yellow supergiants
in these systems will be known. This study is intended to serve both as the
impetus and "finding charts" for further space-based imaging, and for many
spectroscopic programs at large aperture.Comment: Added Erratum. Revised versions of Tables 12 and 13 can be found at
http://www.lowell.edu/users/massey/Table12Revised.txt and
http://www.lowell.edu/users/massey/Table13Revised.tx