51 research outputs found
IRAS colors of VLA identified objects in the galaxy
Infrared Astronomy Satellite (IRAS) sources found within 4 degrees of l = 125 deg, b = 2 deg on the 3rd HCON 60 micron Sky Brightness Images were observed at the Very Large Array (VLA). Regions were to be identified where massive stars are forming by looking for small areas of radio continuum emissions. The IRAS sources could be divided into three groups by their IRAS 12 micron/25 micron and 60 micron/100 micron color. The group identified with star forming regions contained essentially all of the objects with extended radio emission. In all of these cases the extended radio emission showed a morphology consistent with the identification of these objects as HII regions. The conclusion drawn is that star formation regions can be distinguished from other objects by their infrared colors
IRAS results on outer galaxy star formation
An infrared defined (60 micron) sample of IRAS sources were systematically studied in order to investigate star formation in the outer Galaxy. Five percent of the sample are point sources with IRAS spectra that suggest the emission is from a dust shell surrounding a mature star. Ninety five percent have spectra where flux density strictly rises with wavelength. The sources are extended, and it is shown that Point Source Catalog fluxes seriously underestimate total fluxes. CO kinematic distances were reliably assigned to two thirds of the sources. Most of the infrared luminosities correspond to B spectral types. Six cm continuum emission were detected from all sources inferred to have spectral type B1 or earlier. The combined IRAS/CO/6 cm data show these sources are young, moderately massive stars that are embedded in interstellar clouds. The young embedded sources define a distinct band in an IRAS color-colar diagram. Normal IRAS galaxies fall in the same band, consistent with the interpretation that their infrared emission is due to star formation
Star formation in the giant HII regions of M101
The molecular components of three giant HII regions (NGC 5461, NGC 5462, NGC
5471) in the galaxy M101 are investigated with new observations from the James
Clerk Maxwell Telescope, the NRAO 12-meter, and the Owens Valley millimeter
array. Of the three HII regions, only NGC 5461 had previously been detected in
CO emission.
We calculate preliminary values for the molecular mass of the GMCs in NGC
5461 by assuming a CO-to-H_2 factor (X factor) and then compare these values
with the virial masses. We conclude that the data in this paper demonstrate for
the first time that the value of X may decrease in regions with intense star
formation.
The molecular mass for the association of clouds in NGC 5461 is approximately
3x10^7 Mo and is accompanied by 1-2 times as much atomic mass. The observed CO
emission in NGC 5461 is an order of magnitude stronger than in NGC 5462, while
it was not possible to detect molecular gas toward NGC 5471 with the JCMT. An
even larger ratio of atomic to molecular gas in NGC 5471 was observed, which
might be attributed to efficient conversion of molecular to atomic gas.
The masses of the individual clouds in NGC 5461, which are gravitationally
bound, cover a range of (2-8) x 10^5 Mo, comparable with the masses of Galactic
giant molecular clouds (GMCs). Higher star forming efficiencies, and not
massive clouds, appear to be the prerequisite for the formation of the large
number of stars whose radiation is required to produce the giant HII regions in
M101.Comment: 32 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Studies of Molecular Clouds associated with H II Regions: S175
We are studying the impact of HII regions on star formation in their
associated molecular clouds. In this paper we present JCMT RxA molecular line
observations of S175 and environs. This is the first within a sample of ten HII
regions and their surrounding molecular clouds selected for our study. We first
make 7'x 7' maps in CO(2-1), which are used to investigate the structure
of the cloud and to identify individual clumps. Single point observations were
made in CO(2-1) and CS(5-4) at the peak of the CO(2-1) emission
within each clump in order to measure the physical properties of the gas.
Densities, temperatures, clump masses, peak velocities, and line widths were
measured and calculated using these observations. We have identified two
condensations (S175A and S175B) in the molecular cloud associated with this HII
region. S175A is adjacent to the ionization front and is expected to be
affected by the HII region while S175B is too distant to be disturbed. We
compare the structure and gas properties of these two regions to investigate
how the molecular gas has been affected by the HII region.
S175A has been heated by the HII region and partially compressed by the
ionized gas front, but contrary to our expectation it is a quiescent region
while S175B is very turbulent and dynamically active. Our investigation for the
source of turbulence in S175B resulted in the detection of an outflow within
this region.Comment: 33 pages, 11 figures, 6 tables, accepted for publication in
Astronomical Journa
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