The Early Evolution of Massive Stars: Radio Recombination Line Spectra

Keto, Eric; Zhang, Qizhou; Kurtz, S.

doi:10.1086/522570

Cited by 104 publications

(131 citation statements)

References 54 publications

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“…The H30α line is narrower than the H53α line, and it is redshifted by a few km s −1 . This behavior has been observed in hypercompact HII regions around embedded young massive stars (Keto et al 2008;Galván-Madrid et al 2009). Although BN is not deeply embedded in molecular gas, the same interpretation seems to apply: that the difference in linewidth is caused by the action of "pressure" broadening from particle collisions in the low-frequency line, whereas the velocity differences are caused 4 The H30α line is also detected in maps that use the complete baseline range, but bright molecular emission from the HC centered at ∼10 MHz blueshifted with respect to the RL creates sidelobes that are for the most part removed by taking only the longest baselines.…”

Section: Recombination Linessupporting

confidence: 57%

“…Finally, using Eq. (3) of Keto et al (2008), the average electron density traced by these RLs is N e = 8.4 × 10 6 cm −3 . Non-LTE effects and dust opacity may play a role for (sub)mm RLs.…”

Section: Recombination Linesmentioning

confidence: 99%

“…However, longer wavelength lines are a valuable complement because they are sensitive to the gas density. By observing multiple RL transitions, hints to the structure of the ionized nebula can be obtained even in unresolved observations (Keto et al 2008;Galván-Madrid et al 2009). …”

Section: Introductionmentioning

confidence: 99%

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ALMA and VLA observations of recombination lines and continuum toward the Becklin-Neugebauer object in Orion

Galván-Madrid

Goddi

Rodrı́guez

2012

A&A

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Compared to their centimeter-wavelength counterparts, millimeter recombination lines (RLs) are intrinsically brighter and are free of pressure broadening. We report observations of RLs (H30α at 231.9 GHz, H53α at 42.9 GHz) and the millimeter and centimeter continuum toward the Becklin-Neugebauer (BN) object in Orion, obtained from the Atacama Large Millimeter/submillimeter Array (ALMA) science verification archive and the Very Large Array (VLA). The RL emission appears to be arising from the slowly-moving, dense (N e = 8.4 × 10 6 cm −3 ) base of the ionized envelope around BN. This ionized gas has a relatively low electron temperature (T e < 4900 K) and small ( 10 km s −1 ) bulk motions. Comparing our continuum measurements with previous (non)detections, it is possible that BN has large flux variations in the millimeter. However, dedicated observations with a uniform setup are needed to confirm this. From the H30α line, the central line-of-sight LSR velocity of BN is 26.3 km s −1 .

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Section: Recombination Linessupporting

confidence: 57%

Section: Recombination Linesmentioning

confidence: 99%

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ALMA and VLA observations of recombination lines and continuum toward the Becklin-Neugebauer object in Orion

Galván-Madrid

Goddi

Rodrı́guez

2012

A&A

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“…This large difference indicates that NGC 7538IRS1 is extremely concentrated toward the central mm continuum peak whereas NGC 7538S exhibits emission on much larger scales. Since NGC 7538IRS1 has significant amounts of free-free emission (e.g., Pratap et al 1992;Keto et al 2008;Sandell et al 2009), we correct the fluxes for that contribution in Table 2. As shown by Keto et al (2008), several Hii region models can fit the data, and the exact free-free contribution is hard to isolate.…”

Section: Continuum Emissionmentioning

confidence: 99%

“…Since NGC 7538IRS1 has significant amounts of free-free emission (e.g., Pratap et al 1992;Keto et al 2008;Sandell et al 2009), we correct the fluxes for that contribution in Table 2. As shown by Keto et al (2008), several Hii region models can fit the data, and the exact free-free contribution is hard to isolate. Here, we assume ∼1000 mJy to be produced by the free-free emission, the rest is attributed to the dust emission.…”

Section: Continuum Emissionmentioning

confidence: 99%

The high-mass disk candidates NGC 7538IRS1 and NGC 7538S

Beuther

Linz

Henning

2012

A&A

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Context. The nature of embedded accretion disks around forming high-mass stars is one of the missing puzzle pieces for a general understanding of the formation of the most massive and luminous stars. Aims. We want to dissect the small-scale structure of the dust continuum and kinematic gas emission toward two of the most prominent high-mass disk candidates. Methods. Using the Plateau de Bure Interferometer at ∼1.36 mm wavelengths in its most extended configuration we probe the dust and gas emission at ∼0.3 , corresponding to linear resolution elements of ∼800 AU.Results. Even at that high spatial resolution NGC 7538IRS1 remains a single compact and massive gas core with extraordinarily high column densities, corresponding to visual extinctions on the order of 10 5 mag, and average densities within the central 2000 AU of ∼2.1 × 10 9 cm −3 that have not been measured before. We identify a velocity gradient across in northeast-southwest direction that is consistent with the mid-infrared emission, but we do not find a gradient that corresponds to the proposed CH 3 OH maser disk. The spectral line data toward NGC 7538IRS1 reveal strong blue-and red-shifted absorption toward the mm continuum peak position. While the blue-shifted absorption is consistent with an outflow along the line of sight, the red-shifted absorption allows us to estimate high infall rates on the order of 10 −2 M yr −1 . Although we cannot prove that the gas will be accreted in the end, the data are consistent with ongoing star formation activity in a scaled-up low-mass star formation scenario. Compared to that, NGC 7538S fragments in a hierarchical fashion into several sub-sources. While the kinematics of the main mm peak are dominated by the accompanying jet, we find rotational signatures from a secondary peak. Furthermore, strong spectral line differences exist between the sub-sources which is indicative of different evolutionary stages within the same large-scale gas clump. Conclusions. NGC 7538IRS1 is one of the most extreme high-mass disk candidates known today. The large concentration of mass into a small area combined with the high infall rates are unusual and likely allow continued accretion. While the absorption is interesting for the infall studies, higher-excited lines that do not suffer from the absorption are needed to better study the disk kinematics. In contrast to that, NGC 7538S appears as a more typical high-mass star formation region that fragments into several sources. Many of them will form low-to intermediate-mass stars. The strongest mm continuum peak is likely capable to form a high-mass star, however, likely of lower mass than NGC 7538IRS1.

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Recombination lines and free‐free continua formed in asymptotic ionized winds: Analytic solution for the radiative transfer

Ignace¹

2009

Astron Nachr

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matter In dense hot star winds, the infrared and radio continua are dominated by free-free opacity and recombination emission line spectra. In the case of a spherically symmetric outflow that is isothermal and expanding at constant radial speed, the radiative transfer for the continuum emission from a dense wind is analytic. Even the emission profile shape for a recombination line can be derived. Key to these derivations is that the opacity scales with only the square of the density. These results are well-known. Here an extension of the derivation is developed that also allows for line blends and the inclusion of an additional power-law dependence beyond just the density dependence. The additional power-law is promoted as a representation of a radius dependent clumping factor. It is shown that differences in the line widths and equivalent widths of the emission lines depend on the steepness of the clumping power-law. Assuming relative level populations in LTE in the upper levels of He II, an illustrative application of the model to Spitzer/IRS spectral data of the carbon-rich star WR 90 is given.

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The Early Evolution of Massive Stars: Radio Recombination Line Spectra

Cited by 104 publications

References 54 publications

ALMA and VLA observations of recombination lines and continuum toward the Becklin-Neugebauer object in Orion

ALMA and VLA observations of recombination lines and continuum toward the Becklin-Neugebauer object in Orion

The high-mass disk candidates NGC 7538IRS1 and NGC 7538S

Recombination lines and free‐free continua formed in asymptotic ionized winds: Analytic solution for the radiative transfer

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