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Register a new userThe Luminosity Functions and Timescales of MYSOs and Compact HII regions
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We present a determination of the luminosity functions of massive young stellar objects (MYSOs) and compact (C)HII regions within the Milky Way Galaxy using the large, well-selected sample of these sources identified by the Red MSX Source (RMS) survey. The MYSO luminosity function decreases monotonically such that there are few with $Lgtrsim 10^{5}$Lsol, whilst the CHII regions are detected up to ~10$^{6}Lsol. The lifetimes of these phases are also calculated as a function of luminosity by comparison with the luminosity function for local main-sequence OB stars. These indicate that the MYSO phase has a duration ranging from 4x10$^{5}$ yrs for 10$^{4}$Lsol to ~7x10$^{4}$ yrs at 10$^{5}$Lsol, whilst the CHII region phase lasts of order 3x10$^{5}$ yrs or ~3-10% of the exciting stars main-sequence lifetime. MYSOs between 10$^{4} Lsol and ~10$^{5}$ Lsol are massive but do not display the radio continuum or near-IR HI{} recombination line emission indicative of an HII region, consistent with being swollen due to high ongoing or recent accretion rates. Above ~10$^{5}$ Lsol the MYSO phase lifetime becomes comparable to the main-sequence Kelvin-Helmholtz timescale, at which point the central star can rapidly contract onto the main-sequence even if still accreting, and ionise a CHII region, thus explaining why few highly luminous MYSOs are observed.
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Ultracompact and hypercompact HII regions appear when a star with a mass larger than about 15 solar masses starts to ionize its own environment. Recent observations of time variability in these objects are one of the pieces of evidence that suggest that at least some of them harbor stars that are still accreting from an infalling neutral accretion flow that becomes ionized in its innermost part. We present an analysis of the properties of the HII regions formed in the 3D radiation-hydrodynamic simulations presented by Peters et al. as a function of time. Flickering of the HII regions is a natural outcome of this model. The radio-continuum fluxes of the simulated HII regions, as well as their flux and size variations are in agreement with the available observations. From the simulations, we estimate that a small but non-negligible fraction (~ 10 %) of observed HII regions should have detectable flux variations (larger than 10 %) on timescales of ~ 10 years, with positive variations being more likely to happen than negative variations. A novel result of these simulations is that negative flux changes do happen, in contrast to the simple expectation of ever growing HII regions. We also explore the temporal correlations between properties that are directly observed (flux and size) and other quantities like density and ionization rates.
We present radio and infrared observations of 4 hyper-compact HII regions and 4 ultra-compact HII regions in the southern Galactic plane. These objects were selected from a blind survey for UCHII regions using data from two new radio surveys of the southern sky; the Australia Telescope 20 GHz survey (AT20G) and the 2nd epoch Molonglo Galactic Plane Survey (MGPS-2) at 843 MHz. To our knowledge, this is the first blind radio survey for hyper- and ultra-compact HII regions. We have followed up these sources with the Australia Telescope Compact Array to obtain H70-alpha recombination line measurements, higher resolution images at 20 GHz and flux density measurements at 30, 40 and 95 GHz. From this we have determined sizes and recombination line temperatures as well as modeling the spectral energy distributions to determine emission measures. We have classified the sources as hyper-compact or ultra-compact on the basis of their physical parameters, in comparison with benchmark parameters from the literature. Several of these bright, compact sources are potential calibrators for the Low Frequency Instrument (30-70 GHz) and the 100-GHz channel of the High Frequency Instrument of the Planck satellite mission. They may also be useful as calibrators for the Australia Telescope Compact Array, which lacks good non-variable primary flux calibrators at higher frequencies and in the Galactic plane region. Our spectral energy distributions allow the flux densities within the Planck bands to be determined, although our high frequency observations show that several sources have excess emission at 95 GHz (3 mm) that can not be explained by current models.
The four HII regions in the Sgr A East complex: A, B, C, and D, represent evidence of recent massive star formation in the central ten parsecs. Using Paschen-alpha images taken with HST and 8.4 GHz VLA data, we construct an extinction map of A-D, and briefly discuss their morphology and location.
We have re-analyzed continuum and recombination lines radio data available in the literature in order to derive the luminosity function (LF) of Galactic HII regions. The study is performed by considering the first and fourth Galactic quadrants independently. We estimate the completeness level of the sample in the fourth quadrant at 5 Jy, and the one in the first quadrant at 2 Jy. We show that the two samples (fourth or first quadrant) include, as well as giant and super-giant HII regions, a significant number of sub-giant sources. The LF is obtained, in each Galactic quadrant, with a generalized Schmidts estimator using an effective volume derived from the observed spatial distribution of the considered HII regions. The re-analysis also takes advantage of recently published ancillary absorption data allowing to solve the distance ambiguity for several objects. A single power-law fit to the LFs retrieves a slope equal to -2.23+/-0.07 (fourth quadrant) and to -1.85+/-0.11 (first quadrant). We also find marginal evidence of a luminosity break at L_knee = 10^23.45 erg s^(-1) Hz^(-1) for the LF in the fourth quadrant. We convert radio luminosities into equivalent H_alpha and Lyman continuum luminosities to facilitate comparisons with extra-galactic studies. We obtain an average total HII regions Lyman continuum luminosity of 0.89 +/- 0.23 * 10^(53) sec^(-1), corresponding to 30% of the total ionizing luminosity of the Galaxy.
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