Because of their proximity, the Magellanic Clouds provide the opportunity to conduct a detailed study of the history and current state of star formation in dwarf irregular galaxies. There is considerable evidence that star formation in the Clouds was and is proceeding in a manner different from that found in a typical well-ordered spiral galaxy. Star formation in both Clouds appears to have undergone a number of relatively intense bursts. There exist a number of similarities and differences in the current state of star formation in the Magellanic Clouds and the Milky Way. Examination of Infrared Astronomy Satellite (IRAS) sources with ground based telescopes allows identification of highly evolved massive stars with circumstellar shells as well as several types of compact emission line objects

Frogel, Jay A.

English

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STAR FORMATION IN THE MAGELLANIC CLOUDS
N87- 24288
Jay A. Frogel
Kitt Peak National Observatory
National Optical Astronomy Observatories
P. O. Box 26732
950 North Cherry Avenue
Tucson, AZ 85726
ABSTRACT. Because of their proximity, the Magellanic Clouds provide us
viith the opportunity to conduct a detailed study of the history and current
state of star formation in dwarf irregular galaxies. There is considerable
evidence that star formation in the Clouds has been and is proceeding in a
manner different from that found in a typical well-ordered spiral galaxy.
Star formation in both Clouds appears to have undergone a number of
relatively intense bursts. There exist a number of similarities and differences
in the current state of star formation in the Magellanic Clouds and the
Milky Way. Examination of IRAS sources with ground based telescopes
allows identification of highly evolved massive stars with circumstellar shells
as well as several types of compact emission line objects.
1. A BRIEF OVERVIEW OF STAR FORMATION IN THE CLOUDS
We are all familiar with the well-worn cliche concerning the inability to
see forests because of the trees. In astronomy the converse is often true:
while global properties of galaxies can easily be studied, the extent to which
the individual constituents can be observed is severely limited. But it is
these individual constituents - stars and clouds of gas - that determine the
appearance and evolution of a galaxy. The Magellanic Clouds present us
with a unique opportunity to study the stellar content of two galaxies in
considerable detail and to provide some insight to the problems of star
formation and evolution in a galaxy-wide context. In this first section I will
briefly review some ideas of the history of star formation in the Magellanic
Clouds.
1.1 Old Stars
There is a population of stars in both Clouds which is the equivalent
of the halo population of the Milky Way. These stars are old, ___101° yr,
and metal poor. The two most obvious examples of this stellar population
are RR Lyrae variables, found throughout both Clouds, and star clusters
similar to metal poor globular clusters in the Milky Way. These stars have
a metallicity between 30 and 100 times less than solar. The SMC's old
population is more metal poor than the LMC's. These old stars constitute
about 6% by mass of both Clouds, the same as that for the Galaxy's halo
population interior to the solar circle (Frogel 1984).
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 161
https://ntrs.nasa.gov/search.jsp?R=19870014855 2020-03-20T10:57:46+00:00Z
J. A. FROGEL
1.2 Intermediate Age Stars
About 5 Gyr ago there appears to have been a major burst of star
forming activity in the Clouds. Strong evidence for this comes from optical
color-magnitude diagrams and luminosity functions constructed for a number
of fields in both Clouds (e.g. Butcher 1977; Stryker 1984); from the
discovery of large numbers of luminous asymptotic giant branch (AGB) M
and C stars located throughout both Clouds (Blanco, McCarthy, and Blanco
1980); and, finally, from the relative number of clusters of intermediate age
(e.g. Bica, Dottori, and Pastoriza 1986), although this line of argument has
been questioned by El_on and Fall (1985).
As is the case for the oldest stars, the intermediate age population in
the LMC has a higher mean metallicity than that of the SMC. On the
whole, metal enrichment of the interstellar medium in the LMC appears to
have occurred at a higher rate over time than that in the SMC. But even
in the LMC, enrichment has proceeded much more slowly than in the Milky
Way (e.g. Twarog 1980). References to discussions of these topics may be
found in a more lengthy review article (Frogel 1984).
1.3 Young Stars
There is a significant population of objects in the Magellanic Clouds
with an age of order 108 yrs. It is best typified by the so-called blue
globular clusters and the Cepheid variables. Morphologically, the blue
globulars resemble Galactic globular clusters. However, their turn-off mass is
about 5-7 M o and their metallicity is only a factor of two down from solar.
They have no obvious Galactic counterparts and appear to have formed in
regions of very low gas density (Freeman 1980). The conditions which
brought about their formation seem to be quite different from those which
occur in the Galactic disk.
The blue globulars and AGB stars of similar age in _the field of the
Magellanic Clouds appear to be the result of an enhanced epoch of star
formation with a production about 10% of that in the intermediate age
stellar component (Frogel and Blanco 1983).
2. CURRENT STAR FORMATION
Scoville pointed out in his review talk this morning that essentially all
star formation in our galaxy takes place in giant molecular clouds (GMC)
which are delineated by their CO line emission. In the Magellanic Clouds
there is clearly a great deal of current star forming activity. For example
there are large numbers of OB stars and late type supergiants in the field of
both Clouds as well as many young stellar associations and HII regions.
However, the preliminary evidence from the Columbia CO survey of both the
LMC and SMC pointed to a low molecular abundance.
Results of a CO survey of the LMC reported by Cohen, et o2. (1984)
indicated that H2/HII is 5-10 times lower than in the Milky Way. A
similar survey of the SMC by Rubio, et al. indicated an even lower ratio
there. These results of course depend critically on the assumption that the
CO to H2 ratio is the same as in the Galaxy. With the possible exception
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STAR FORMATION IN THE MAGELLANIC CLOUDS
of the N159 region in the LMC, Israel (1984) found no unambiguous
evidence on the basis of CO line strengths for GMC complexes in the Clouds
as exist in the Milky Way.
More recently, Israel, et aL (1986) have argued that the CO abundance
of both clouds is low not just, or even primarily, because of the low overall
abundance of heavy elements, but because of the high destruction rate of
molecules due to the strong interstellar UV radiation field and lower gas-to-
dust ratios; the latter results in less shielding of the CO molecules. Hence,
use of a Galactic H2/CO ratio will result in gross underestimation of the H2
abundance since this molecule is much less affected by the interstellar UV
field than CO. Their model also qualitatively accounts for the relative CO
strengths between the Large and Small Cloud. A search for H2 from HII
regions in the Clouds has in fact revealed its presence in quite a few objects
(Israel 1986). Its discovery supports Israel, et a/.'s contention that its true
abundance cannot be inferred from observations of CO.
As reported in the poster paper at this conference by Jones, Hyland,
and Harvey (hereafter PI) there appears to be a lack of high surface
brightness, compact infrared cores to HII regions in the Clouds as are found
in many Galactic HII regions where star formation is taking place. Elias
and Frogel, in another poster paper (hereafter PII), note that they have
been able to identify only a few IRAS sources associated with HII regions
that are compact. "Compact n here means that the IRAS 12 and 251zm
fluxes are only slightly greater than equivalent fluxes measured from the
ground in apertures of a few arcseconds in diameter. In any case, the point
is that such objects are rare in the Clouds. Most IRAS sources associated
with HII regions are quite extended (PII). There also is an absence of
highly obscured but IR/radio luminous HII regions in the Clouds (PI).
Gatley, et al. (1981, 1982) have found obscured compact objects in the
SMC and LMC which they identify as protostars but Jones, et at. (1986 and
PI) note a general absence of low luminosity protostellar objects. Also, they
find an apparent under luminosity of the HII regions for the number of
ionizing photons that are inferred when compared with Galactic HII regions
and the IMF of stars therein. Their interpretation of this result is that the
cause is a lack of intermediate mass stars in regions of star formation. This
conclusion is quite dependent on the assumption that all of the light from
OB stars in the HII region is absorbed by dust and subsequently reradiated
in the infrared. On the other hand, Hunter, et al. (1986) find that
L(IR/Ha) is about the same for all dwarf galaxies with IRAS data and that
this ratio is Wroughly consistent with both luminosities measuring the
integrated OB stellar luminosity output, n
There is, of course, a danger in trying to derive an IMF from a sample
of stars that represents an extremely limited slice of space and time. While
there may well be some differences within HII regions, globally the IMFs for
the LMC and the solar neighborhood appear to be quite similar, even for
the massive stars (Humphreys and McElroy 1984).
Elias and Frogel (PII) have been attempting to identify and obtain
near infrared and optical spectroscopy of IRAS sources in the Magellanic
Clouds. They have observed about 65 sources, two-thirds of which have
IRAS colors typical of stars. Two general results of this survey are: 1) The
163
J.A.FROGEL
relative scarcity of compact infrared sources in association with HII regions
(discussed above); and, 2) a generally lower level of star forming activity in
the SMC relative to the LMC.
The IRAS data by themselves cannot distinguish between luminous
stars in the Magellanic Clouds and nearby field stars. Near infrared data,
though, divides the IRAS sources up into three reasonably well separated
categories. The first of these, about half of the star-like sources found, have
JHK colors, CO, and H20 indices quite similar to those of late-type giants
in the solar neighborhood. Most of these can be identified with visually
bright stars. Their K magnitudes are brighter than seventh; with very few
exceptions no LMC or SMC supergiants are this bright (Elias, Frogel, and
Humphreys 1985).
A second category of stellar sources consists of objects with colors and
indices typical of late-type giants with circumstellar shells. CCD
spectroscopy of the ones with optical counterparts usually reveals
exceptionally strong TiO bands. At least one of these, in the SMC, is a
carbon star. A few of the scurces in this second category have no optical
counterparts and probably are extreme examples of dust-enshrouded evolved
objects. None, however, are as red or as luminous as the two LMC
supergiants found by Elias, Frogel, and Schwering (1986) in their first look
at Magellanic Cloud IRAS sources. One of these two has recently been
identified as the first OH/IR star in the Magellanic Clouds by Wood, Bessell,
and Whiteoak (1986). It should be remembered that although these cool,
luminous stars are highly evolved, they are quite young and as such can
yield important information on current star formation in the Magellanic
Clouds. In particular they must play an important role in the enrichment
and recycling of interstellar material.
The third category into which Magellanic Cloud IRAS sources can be
divided on the basis of ground based data is compact or stellar emission line
objects. Some of these are in HII regions. Others appear to be emission
line stars. Some have optical spectra - permitted emission lines from neutral
atomic species and from the calcium triplet - characteristic of pre-main
sequence objects surrounded by a relatively dense cloud of gas and dust,
presumably the remnant out of which they formed (eft. McGregor, Persson,
and Cohen 1984). An advantage of studying such objects in the Clouds is
the fact that they are all at the same distance so that the accuracy of a
derived luminosity function will be limited only by the ability to find them.
In summary, we see that the history of star formation in the
Magellanic Clouds and its current status, various aspects of which are
considered in some of the poster papers presented at this conference, differ in
a number of significant features from the situation in the Milky Way.
Globally, star formation in the Clouds would be expected to be different
from that in the Galaxy because of their smaller mass and lack of ordercd
spiral structure. "['he increasingly well-documented low metallicity of the
Clouds, particularly of the SMC, appears to have resulted from a
significantly lower rate of enrichment of the interstellar medium in these
irregular systems than in the Milky Way. A lower enrichment rate could
come about, for example, because of the inability of a low mass galaxy to
retain the metal-enriched material present in supernova-driven winds. As
pointed out by Vader (1986), such winds, even with a moderate total mass
164
STARFORMATIONI THEMAGELLANICLOUDS
loss rate, can carry away a substantial fraction of the heavy metal
production resulting in a substantially reduced yield. A low present day
abundance of heavy metals can be expected to have important effects on
current star formation.
The Magellanic Clouds present us with the opportunity to study in
great detail star formation in an environment different from that which
obtains in the Galactic disk. Let us proceed to examine the trees and the
leaves that grow on them so we may be better able to comprehend and
appreciate the beauty and complexity of the forest as a whole.
I thank Terry Jones for material based on work in progress by him
and his collaborators, Frank Israel for drawing my attention to his recent
work on molecules in the Magellanic Clouds, and Jay Elias for considerable
discussions about our own work.
REFERENCES
Bica, E., Dottori, H., and Pastoriza, M. 1986, Astr. Ap., 151}, 261.
Blanco, V. M., McCarthy, M. F., and Blanco, B. 1980, Ap. J., 242, 938.
Butcher, H. 1977, Ap. J., 216, 372.
Cohen, R., Montani, J., and Rubio, M. 1984, in L A. U. Syrup. No. 108,
Structure and Evolution of the Magellarde Clouds, ed. S. v. d. Bergh,
and K. S. de Boer (Dordrecht: Reidel), p. 401.
Rubio, M., Cohen, R., and Montani, J. 1984, in I. A. U. Syrup. No. 108,
Structure and Evolution o/ the Magellanie Clouds, ed. S. v. d. Bergh,
and K. S. de Boer (Dordrecht: Reidel), p. 399.
Elias, J. H., Frogel, J. A., and Humphreys, R. M. 1985, Ap. J. Supp., 57, 91.
Elias, J. H., Frogel, J. A., and Schwering, P. ]3. W. 1986, Ap. J., 302, 675.
Elson, R. A. W., and Fall, S. M. 1985, Ap. J., 299, 211.
Freeman, K. C. 1980, in L A. U. Syrup. No. 85, Star Clusters, ed. J. E.
Hesser (Dordrecht: Reidel), p. 317.
Frogel, J. A. 1984, Pub. A. S. P., 96, 856.
Frogel, J. A., and Blanco, V. M. 1983, Ap. J. (Letters), 274, L57.
Gallagher, J. S., and Hunter, D. A. 1984, Ann. Rev. Astr. Ap., 22, 37.
Gatley, I., Becklin, E. E., Hyland, A. R., and Jones, T. J. 1981, M.N.R.A.S.,
197, 17P.
Gatley, I., Hyland, A. R., and Jones, T. J. 1982, M.N.R.A.S., 200, 521.
Humphreys, R. M., and McElroy, D. B. 1984, Ap. J., 284, 565.
Hunter, D. A., Gillett, F. C., Gallagher, J. S., Rice, W. L., and Low, F. J.
1986, Ap. .L, SOS, 171.
Israel, F. P. 1984, in L A. U. Syrup. No. 108, Structure and Evolution of the
Magellanie Clouds, ed. S. v. d. Bergh, and K. S. de Boer (Dordrecht:
Reidel), p. 319.
Israel, F. P. 1986, Proceedings of the Haystack Workshop, Masers, Molecules,
and Mass Outflolos in Regions of Star Formation, in press.
Israel, F. P., de Graauw, Th., van de Stadt, H., and de Vries, C. P. 1986,
Ap. J., _03, 186.
Jones, T. J., Hyland, A. R., Straw, S., Harvey, P. M., Wilking, B. A..
Gatley, 1., Joy, M., and Thomas, J. A. 1986, M.N.R.A.S., 219, 603.
McGregor, P. J., Persson, S. E., and Cohen, J. G. 198/t, Ap. J., 28t}, 609.
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J. A. FROGEL
Stryker, L. L. 1984, in L A. U. Syrup. No. 108, Structure and Evolution of
the MageUanie C_ouds, ed. S. v. d. Bergh, and K. S. de Boer
(Dordrecht: Reidel), p. 79.
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DISCUSSION
GALLAGHER:
Can you say anything about the number of OH/IR stars with regard to the death of AGB stars via
'superwinds'?
FROGEL:
Not at this time because our sample is restricted to the outer region of the Clouds where source confusion
would not be a problem.
ISRAEL:
Under the extreme conditions exemplified by the Magellanic Clouds (e.g., low heavy-element abundances,
relatively strong UV radiation field) one cannot expect CO to be a reliable tracer of H2. In fact, despite the
weakness of CO, there are several indications that the clouds may contain significant amounts of H2. The
most indirect indication is that several dark cloud silhouettes can be seen in both clouds with dimensions
similar to those of Galactic GMCs.
The second indication is found in the IRAS maps, where several warm, high dust column density sources
can be seen that do not have clear HI counterparts. Unless one is willing to accept wildly fluctuating and
at times very high dust-to-gas ratios (in a low-z environment!) the obvious explanation is that these sources
contain mostly molecular gas.
Third, Koomeef and I have detected several sources of 2#m 1-12emission, in fact in a large fraction of
all sources looked at. Although it is very difficult to derive physical parameters such as mass, etc., from these
detections, the frequent occurrence of H2 emission (often at positions where CO is weak or absent) indicates
that H2 may be widespread in both Magellanic Clouds.
FROGEL:
Great! Finding large amounts of H2 will help with one of the major problems I noted with regard to star
formation in the Clouds.
ROCHE:
Did you not find any carbon stars in the Clouds?
FROGEL:
One SMC object is a C star. Without optical spectra, we cannot tell from the IR data we have whether
the blank field SMC and LMC sources are C stars or M stars.
166 C _'-2_


Star formation in the Magellanic clouds

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