Transcript AAS Poster, NM 2002: "The Discovery of New

The Discovery
of
New Wolf-Rayet Star Candidates
Purpose
IC10 is a Local Group dwarf irregular galaxy described by
Hubble (1936) as ``one of the most curious objects in the sky".
Massey and Armandoff (1995) proposed that it is currently
undergoing a starburst: despite its small size, it contains 15
spectroscopically confirmed Wolf-Rayet stars, which is a galaxyaveraged surface density that is as high as that found in young,
massive OB associations. This is consistent with a comparison
of the Hα luminosity to HI mass or blue light luminosity (Hunter
and Gallagher 1986, Hunter 1993), which suggests a starformation rate comparable to that of NGC 1569, a classical
starburst irregular. However, the relative number of WC-type
and WN-type Wolf-Rayet stars is surprising high given the
galaxy's low metallicity (Figure 1). Does this suggest a peculiar
initial mass function, or an extremely short burst of star
formation? Or could previous studies simply missed some of the
weaker-lined WNs?
Galaxy IC10
S. B. Holmes, P. Massey (Lowell Observatory)
in the
Starburst
Analysis
Photometry of the galaxy was done using the IRAF/daophot
package and our own scripts and code. In order to preserve
the photometric integrity of the individual chips within a
Mosaic frame, chips were analyzed separately. In the case of
IC10, the galaxy was confined to a single chip. An additional
chip was used as a “control field”.
We selected candidates based upon the magnitude
differences between the continuum and emission-line filters.
We used the photometric errors to judge if a magnitude
difference was significant or not. Multiple detections were also
considered a plus in constructing our final candidate list.
All spectroscopically confirmed Wolf-Rayet stars were found.
In addition, we have confirmed the candidacy of several stars
proposed to be Wolf-Rayets by Royer et al (2001 A&A 366,
L1) based upon their own interference filter imaging but never
confirmed spectroscopically. The exceptions are their “WC9"
candidates; none of these were detected in our survey,
causing us to question their surprising result that late-type
WCs were to be found in such a low metallicity system.
IC10 stacked image: Hα is denoted by red, V is denoted by green, B is denoted by blue.
Comparing our final numbers against the control field, we
have found a minimum of 6 & a maximum of 41 new WolfRayet stars (Figures 2 & 3).
Figure 1. The WC/WN ratios for Local Group Galaxies: IC10 has a high
WC/WN ratio based on the survey done by Massey & Armandoff (1995).
Future Work
Observations
Using data obtainedDetections
with the MOSAIC 8k x 8k CCD at the 4-m
Mayall telescope at KPNO, we have been able to sample the
entire galaxy much deeper than earlier studies. New candidates,
once spectroscopically confirmed, might then alter the WC/WN
ratio.
Figure 2. Potential WR candidates: Asterisks indicate new detections, squares are
spectroscopically confirmed WR stars and diamonds are the “control field”. The smallest
ellipse contains likely non-WR stars. Progressing in size, the next region contains 25
questionable candidates, followed by a larger region containing an additional 10 probable
WR candidates. The largest region contains 6 high significance candidates.
The IC10 field was observed using three different filters in order
to determine Wolf-Rayet candidacy. Filters used include: the
WC filter (4650), the WN filter (4690), & the CT filter (4750).
Respectively, the filters detect the CIII 4650 line, the HeII
4686 line, & a clean continuum.
We plan to spectroscopically observe our new Wolf-Rayet
candidates. Spectra will indicate which candidates are
definite WC or WN stars, possibly changing the WC/WN ratio,
and either bringing it into better accord with more normal
galaxies, or telling us that the WC/WN ratio is high in
starbursts.
Acknowledgements
This research is supported by the National Science Foundation
under grant AST-0093060.
A set of three, dithered images were taken in each of the three
filters. The exposure time for each image was 30 minutes; i.e.,
1.5 hrs per filter. The seeing was XXXX
Figure 3. Spatial distribution of candidates in IC10: Based on fig. 2, red denotes
spectroscopically confirmed WR stars, light blue indicates the location of the 6 high
significance candidates, dark blue denotes the 10 probable candidates, and green represents
the 25 questionable candidates.