Astron. Astrophys. 323, 442-448 (1997)

1. Introduction

During the course of a study of flare stars (Tsikoudi, 1988) at the IRAS wavebands, we came across the bright source IRAS 18530+0817, which had not been optically identified. The IRAS Low Resolution Spectrum (IRAS LRS) of this source has strong silicate emission peaking near 9µm, which is either self-absorbed near 10µm or whose profile is affected by another spectral feature. Figure 1 shows the LRS spectra of IRAS 18530+0817 and three other sources with different types of silicate feature. Z Pup (IRAS 07304-2032), Fig. 1b, and IRAS 17050-4123, Fig. 1d, have strong silicate emission features with blue continua (LRS class 29). These spectra show the normal location of the silicate emission feature, peaking around 9.6µm to 10µm. RAFGL 5146S (IRAS 17482-2824), Fig. 1c, has a self-absorbed silicate emission feature (LRS class 42) with peaks around 8.5 and 11.5 µm . The possibly unusual silicate feature in IRAS 18530+0817 prompted us to investigate this source further. The source is so faint in the blue and red wavebands that it is not seen on the Palomar B and R plates. Using a finding chart of the area around the IRAS position we made follow-up observations of this object at red and infrared wavelengths, at the South African Astronomical Observatory (SAAO) and the Mauna Kea Observatories (Hawaii) respectively. In this paper we discuss our observations as well as those in the literature and we attempt to understand and determine the nature of this source and its dust shell.

Fig. 1a-d. IRAS LRS Spectra of: a IRAS 18530+0817, class 04 (AI class 1), b Z Pup, class 29 (AI class 1), c RAFGL 5146S, class 42 (AI class 4), d IRAS 17050-4123, class 29 (AI class 11). The unit of flux is W m-2 µm-1.

The IRAS LRS Atlas (1986) did not place the spectrum in the normal O-rich classes, but labelled it 04, "strange band shape". There are only 12 sources classed as 04; two of these look similar to IRAS 18530+0817. Cheeseman et al. (1989) classified the LRS spectra using Artificial Intelligence techniques, where spectra which were judged "similar" to each other, using Bayesian statistics, were put into the same class. They placed the spectrum of IRAS 18530+0817 in one of the less common silicate emission feature classes, namely epsilon 1 ( 1), with the spectra of 137 other objects. These stars have weak emission features in addition to silicate emission, high variability, and some members are thought to be supergiants with thick dust shells. Of these 137 objects, about 80 show a feature around 8.5 - 11.5 µm which is very similar to IRAS 18530+0817. There are also some objects in the IRAS LRS Atlas with a similar spectral shape not placed in the class. Z Pup and IRAS 17050-4123, in Fig. 1, belong to the pure silicate emission feature classes, the beta classes ( 1 and 11 respectively).

More than 40 catalogues which contain celestial objects were searched when compiling the IRAS PSC in order to identify the IRAS detections with known sources. The identification given for IRAS 18530+0817 was EIC 719. The Equatorial Infrared Catalog (EIC) contains sources which were observed at 2.7µm during an infrared survey in 1977 (Sweeney et al., 1978). EIC 719 was observed for the first time during this survey. There was a 2.7µm flux measurement for EIC 719 but no further information nor an identification given. Kastner et al. (1993) gave H, K, L photometry for IRAS 18530+0817.

IRAS 18530+0817 was included in the radio-survey of IRAS sources which searched for OH molecules at 1612 MHz with Arecibo radio telescope (Lewis et al., 1990). There was no positive detection for this object. Another survey of IRAS sources, searching for CO at mm wavelengths (Kastner et al., 1993) found a marginal detection for IRAS 18530+0817, in the (2-1) transition but not in the (1-0) transition. They also detected SiO from the source. The ROSAT database was examined, but only an upper limit could be determined for the source. Kastner et al. (1993) reported that they could not estimate a distance based on their models of kinematic distances and measured velocities because the was not consistent with its position in the sky. Loup et al. (1993) estimated the distance to be 1.9kpc, using a standard luminosity for their sample of objects. They reported a mass loss rate for IRAS 18530+0817 of 1 10^-6 yr^-1 and an expansion velocity of 12 km s^-1.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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