Universität zu Köln

Mackey, Jonathan ORCID: 0000-0002-5449-6131, Castro, Norberto ORCID: 0000-0003-0521-473X, Fossati, Luca and Langer, Norbert (2015). Cold gas in hot star clusters: the wind from the red supergiant W26 in Westerlund 1. Astron. Astrophys., 582. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

The massive red supergiant W26 in Westerlund 1 is one of a growing number of red supergiants shown to have winds that are ionized from the outside in. The fate of this dense wind material is important for models of second generation star formation in massive star clusters. Mackey et al. (2014, Nature, 512, 282) showed that external photoionization can stall the wind of red supergiants and accumulate mass in a dense static shell. We use spherically symmetric radiation-hydrodynamic simulations of an externally photoionized wind to predict the brightness distribution of H alpha and [N II] emission arising from photoionized winds both with and without a dense shell. We analyse spectra of the H alpha and [N II] emission lines in the circumstellar environment around W26 and compare them with simulations to investigate whether W26 has a wind that is confined by external photoionization. Simulations of slow winds that are decelerated into a dense shell show strongly limb-brightened line emission, with line radial velocities that are independent of the wind speed. Faster winds (greater than or similar to 22 km s(-1)) do not form a dense shell, have less limb-brightening, and the line radial velocity is a good tracer of the wind speed. The brightness of the [N II] and H alpha lines as a function of distance from W26 agrees reasonably well with observations when only the line flux is considered. The radial velocity of the simulated winds disagrees with observations, however: the brightest observed emission is blueshifted by approximate to 25 km s(-1) relative to the radial velocity of the star, whereas a spherically symmetric wind has the brightest emission at zero radial velocity because of limb brightening. Our results show that the bright nebula surrounding W26 must be asymmetric, and we suggest that it is confined by external ram pressure from the extreme wind of the nearby supergiant W9. We obtain a lower limit on the nitrogen abundance within the nebula of 2.35 times solar. The line ratio strongly favours photoionization over shock ionization, and so even if the observed nebula is pressure confined there should still be an ionization front and a photoionization-confined shell closer to the star that is not resolved by the current observations, which could be tested with better spectral resolution and spatial coverage.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Mackey, Jonathan UNSPECIFIED orcid.org/0000-0002-5449-6131 UNSPECIFIED Castro, Norberto UNSPECIFIED orcid.org/0000-0003-0521-473X UNSPECIFIED Fossati, Luca UNSPECIFIED UNSPECIFIED UNSPECIFIED Langer, Norbert UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-390375
DOI:	10.1051/0004-6361/201526159
Journal or Publication Title:	Astron. Astrophys.
Volume:	582
Date:	2015
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language MASSIVE STELLAR POPULATION; X-RAY-EMISSION; STARBURST CLUSTER; BOW SHOCKS; MULTIPLE POPULATIONS; EVOLUTIONARY MODELS; PHYSICAL-PROPERTIES; REDDENED CLUSTER; SELF-ENRICHMENT; RADIO-EMISSION Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/39037