Dust content in compact HII regions (NGC 7538-IRS1, IRS2, and IRS3)

Free access article

Issue		A&A Volume 431, Number 1, February III 2005


Page(s)		183 - 195
Section		Interstellar and circumstellar matter
DOI		http://dx.doi.org/10.1051/0004-6361:200400116

A&A 431, 183-195 (2005)
DOI: 10.1051/0004-6361:200400116

Dust content in compact HII regions (NGC 7538-IRS1, IRS2, and IRS3)

K. Akabane and N. Kuno

Nobeyama Radio Observatory (Nobeyama Radio Observatory (NRO) is a division of the National Astronomical Observatory of Japan (NAOJ) under the National Institutes of Natural Sciences (NINS).) , Minamimaki, Minamisaku, Nagano 384-1305, Japan
e-mail: kuno@nro.nao.ac.jp

(Received 4 February 2003 / Accepted 13 September 2004)

Abstract
The luminosity of the central star of the compact HII regions of NGC 7538 was estimated from the solid angle of the IR sources subtended relative to the central star, and was found to be $5\sim 10$ times as intense as that of IR sources. Under the single central star approximation, the luminosity gives a stellar UV photon rate $N_{{\rm U}}(*) ({\rm s}^{-1})$ of ~ $3.0 \times 10^{48}$ , ~ $1.5 \times 10^{49}$ , ~ $5.1 \times 10^{49}$ , and ~ $1.7 \times 10^{47}$ for the compact HII regions of NGC 7538-IRS1(A/2), B, IRS2, and IRS3, respectively. $N_{{\rm U}} (*)$ and the observed electron density, $n_{{\rm e}}$ , provide the dust opacity of the ionizing photon, $\tau_{{\rm Sd}}$ , for the optical path out to the Strömgren sphere radius $r_{{\rm S}}$ , assuming a gas with standard dust content. Ionizing photon opacity over the same optical path but with the actual dust content $\tau_{{\rm Sda}}$ is also derived from $r_{{\rm i}} / r_{{\rm S}}$ , where $r_{{\rm i}}$ is the radius of the ionized sphere, which is estimated from $N_{{\rm U}} (*)$ and the observed volume emission measure $n_{{\rm e}}^{2} (4 \pi r_{{\rm i}}^3/3)$ (Spitzer 1978). An observational trend of $\gamma \{ N_{{\rm U}}(*) / 4\pi r_{{\rm i}}^2 \}^{1/2} \sim \mbox{constant}$ , where $\gamma = \tau_{{\rm Sda}} / \tau_{{\rm Sd}}$ , was obtained for the 4 compact HII regions of the NGC 7538(N). Fourteen selected compact HII regions from data catalogued by VLA observations were examined for this trend, and a similar result was obtained. A limit of $\gamma$ as $15 \geq \gamma \geq 0.1$ was given for the 14 selected sources. The size of the dust-depleted cavity of the NGC 7538(N) suggested by Chini et al. (1986) coincides with that of the ionized sphere of the IRS2 of the region.

Key words: HII regions -- infrared: ISM -- radio continuum: ISM -- dust, extinction

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