The pistol, the pistol star, and the quintuplet in the galactic centre: results from ISO CVF imaging and SWS sprectroscopy

A. Moneti ¹, J. Blommaert ¹, D.F. Figer ², F. Najarro ³, & S. Stolovy ⁴

¹ ISO Science Operations Centre, VILSPA, Madrid, Spain
² Univ. California, Los Angeles, USA
³ Consejo Superior Investigaciones Cientificas, Madrid, Spain
⁴ Steward Observatory, Univ. of Arizona, USA

We present ISOCAM CVF spectroscopy from 5.0 to 17 microns of the ``Pistol'' and of the ``Quintuplet'' sources near the Galactic Centre, and ISOSWS spectroscopy of the central part of the Pistol Nebula and the Pistol star, a Luminous Blue Variable (LBV) that is believed to have ejected the gas and dust that now make up the Pistol Nebula (Figer et al. 1998, ApJ, in press).

At >10  microns, the Pistol Nebula appears as a nearly spherical bubble centred on the Pistol star. Its diameter is 1/3 parsec, and it is brighter on its northern side. The CVF spectrum of the Pistol Nebula reveals a spectral energy distribution (SED) that increases with wavelength, with a deep 9.7 silicate feature, and with the fine structure lines of [ArII] 7.0 , [NeII] 12.8 , and [NeIII] 15.5 . Line images, produced by selecting the CVF channel with the line and subtracting the average of the adjacent channels. differ for the three lines: the line intensities are strongest in the northern part of the nebula, while there is little or no line emission in the southern part. The [ArII] image is the one that most resembles the 6 cm structure radiograph (Yusef-Zadeh et al. 1989, IAU Symp. 136, p. 275, bus see Figer et al. 1998, ApJ, in press, for best 6 cm image). The shape of the SED suggests a dust temperature somewhat lower than 300 K on average, but with the northern part a few tens of degrees hotter than the southern part.

The SWS spectrum of the Pistol Nebula (2.4 - 45 micron, resolving power 1500) shows deep silicate absorption at 9.7 and 18 microns, the fine structure lines discussed above, and a wealth of other lines of various species, including H (various transitions), [ArIII] 9.0 , [SIII] 18.7 and 33.5 , [SIV] 10.5 , [FeIII] 22.9 and 33.0 . The high excitation lines are all weak, but clearly detected, indicating an exciting source with T>35,000 K. There is no evidence for the broad O-H stretch absorption at 3 mic or for other absorption features, e.g. CO₂ ice (4.26 ) or CO (4.66 ), which are seen toward the Quintuplet cocoon stars (Nagata et al. 1986, AA 315, L205) and towards (Lutz et al. 1986, AA 315, L272).

The CVF 5.0 - 17  spectra of the cocoon stars in the Quintuplet do not show any emission feature, while a deep silicate feature is seen absorption. Their spectral energy distributions peak at different wavelengths, confirming a range in effective temperatures between 500 and 1,000 K (e.g. Moneti, Glass, and Moorwood 1992, MN 258, 705).

The location of the Pistol star in the centre of the Pistol Nebula corroborates the suggestion that the two objects are intimately associated, and specifically that the Pistol Nebula is formed from ejecta from the Pistol star. Dust formed within the ejecta, is heated primarily by the Pistol star, and is responsible for the continuum emission from the Nebula. The Pistol star may not be hot enough to produce a significant fraction of the ionization. The ionization is expected to come primarily from the hot Wolf-Rayet stars in the Quintuplet cluster, which are the only sources hot enough to excite the [SIV] line. This explains the line emission is seen primarily along the northern rim of the Nebula. The Quintuplet stars may be responsible for some of the dust heating and thus for the gradient in temperatures observed in the Pistol Nebula.