Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evidence of Short Timescale Flux Density Variations of UC HII regions in
  Sgr B2 Main and North   We have recently published observations of significant flux density
variations at 1.3 cm in HII regions in the star forming regions Sgr B2 Main and
North (De Pree et al. 2014). To further study these variations, we have made
new 7 mm continuum and recombination line observations of Sgr B2 at the highest
possible angular resolution of the Karl G. Jansky Very Large Array (VLA). We
have observed Sgr B2 Main and North at 42.9 GHz and at 45.4 GHz in the BnA
configuration (Main) and the A configuration (North). We compare these new data
to archival VLA 7 mm continuum data of Sgr B2 Main observed in 2003 and Sgr B2
North observed in 2001. We find that one of the 41 known ultracompact and
hypercompact HII regions in Sgr B2 (K2-North) has decreased $\sim$27% in flux
density from 142$\pm$14 mJy to 103$\pm$10 mJy (2.3$\sigma$) between 2001 and
2012. A second source, F3c-Main has increased $\sim$30% in flux density from
82$\pm$8 mJy to 107 $\pm$11 mJy (1.8$\sigma$) between 2003 and 2012. F3c-Main
was previously observed to increase in flux density at 1.3 cm over a longer
time period between 1989 and 2012 (De Pree et al. 2014). An observation of
decreasing flux density, such as that observed in K2-North, is particularly
significant since such a change is not predicted by the classical hypothesis of
steady expansion of HII regions during massive star accretion. Our new
observations at 7 mm, along with others in the literature, suggest that the
formation of massive stars occurs through time-variable and violent accretion.

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The categories of this paper are: ao-sci
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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: q-bio
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The categories of this paper are: hep-th
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The categories of this paper are: q-fin
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The categories of this paper are: alg-geom
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The categories of this paper are: solv-int
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The categories of this paper are: bayes-an
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The categories of this paper are: math-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: nlin
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The categories of this paper are: supr-con