Milky Way High-Mass Star Formation:
High-mass stars (stars greater than 8 times the mass of our sun) dominate the luminosity, chemistry, and energy input in galaxies. They produce and disperse heavy elements through violent supernova
explosions. However, their formation remains an open question.
High-mass star formation is much more difficult to study than low-mass star formation. Such difficulty arises
from the fact that they are: (1) distant (since they are rare); (2) highly embedded; (3) more complicated and highly disruptive (due to, e.g.,
their strong winds and ionizing radiation); and (4) formed in clusters. Many high-mass star-forming regions appear to
be specifically unique. Therefore, to have a global picture of how high-mass stars form, it is important to survey
thousands of high-mass star forming regions. Since high-mass stars dominate the energy input into galaxies, such surveys
also allow us to better understand galaxies external to the Milky Way.
I am heavily involved in several large surveys of high-mass star formation, including the
Millimetre Astronomy Legacy Team 90 GHz (MALT90) Survey survey with the ATNF Mopra 22-m telescope and
the Radio Ammonia Mid-Plane
Survey (RAMPS) and the KFPA Examinations of Young Stellar (O-star) Natal Environments(KEYSTONE)survey with the 100-m Green Bank Telescope.
Together, these surveys have used several thousand hours of telescope time to provide a better understanding of high-mass star formation.
In one MALT90 study, I investigated chemical "odd-balls," which either had unusually high or unusually low integrated intensity ratios of N2H+ and HCO+.
Those with high integrated intensity ratios of [I(N2H+)/I(HCO+)] were consider N2H+ "rich" sources, while those with
low ratios were considered N2H+ "poor" sources. We found that the N2H+ "rich" sources were actually not actually lacking HCO+; instead
the spectral lines were extremely self absorbed. N2H+ poor sources, however, were due to distinct chemistry found primarily toward H II regions or photodissociation regions. Specifically, N2H+ poor sources
were found about regions with shell-like morphologies, like in NGC 6357 below.
Figure from Stephens et al. (2015). Spitzer IRAC three-color GLIMPSE image
(red, green, and blue colors show 8, 4.5, and 3.6 μm, respectively). The bright H II complex in the top right is NGC 6357. Circles show the location of MALT90 high-mass
star-forming clumps; each color indicates a different high-mass star-formation phase: Prestellar (magenta), Protostellar (yellow), H II Region (green),
and Photo-disassociation Region (cyan). Gray circles have an unknown classification. Locations of N2H+ poor sources are shown with black and red crosses (X marks), with the
red crosses showing the more extreme N2H+ poor sources.
In another MALT90 study, I focused on trying to explain the Gao-Solomon relation using Galactic data. The Gao-Solomon relation compares the luminosity of the HCN(1-0) molecular line transition
to the star-formation rate (as probed by infrared luminosity) in external galaxies. The Gao-Solomon relation shows a tight linear relationship between LIR and LHCN.
Based on Galactic studies, it has been conjectured by Wu et al. (2005, 2010) that this relationship is due to the summation of "basic units of clustered formation" where the ratio
LIR/LHCN is constant in high-mass star-forming regions. With the large MALT90 sample, we found that such "basic units" do not exist; indeed, high-mass star-forming regions
show a significant dispersion in their LIR/LHCN, as seen in the Figure below.
Figure from Stephens et al. (2016). Gao-Solomon relation (LIR vs LHCN(1-0) ) shown for galaxies (green)
and Galactic clumps (blue and red). The black dashed line shows the original Gao–Solomon
relation fit. The location of the Central Molecular Zone (CMZ) is shown, as well as the projected location of the Milky Way.
Indeed, the Central Molecular Zone (CMZ) in our galaxy lies a factor of 4 lower than the line. To understand how the constituents of the Milky Way produces the observations seen in other
galaxies, I find that we must consider other components, such as extended infrared emission, subthermal emission from clouds, and low-mass star formation. The Gao-Solomon relation
appears to be a summation of star formation and interstellar medium at large (>1 kpc) scales.