Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...208.4905e&link_type=abstract
American Astronomical Society Meeting 208, #49.05; Bulletin of the American Astronomical Society, Vol. 38, p.125
Astronomy and Astrophysics
Astronomy
Scientific paper
IC 5146 is a nearby (200pc) dark cloud complex in Cygnus. The lack of star formation activity makes it an excellent laboratory for the study of the chemical complexity in the earliest stages of dense molecular cloud evolution. We have used the Spitzer Infrared Spectrometer (IRS) to probe dust along 10 sight-lines toward K-Giant background stars, sampling a range of visual extinction from 2-20mag. Here we present 5-20um spectra and correlation studies of the 6.0um water-ice band and 9.7um silicate absorption band with Av for a sample of our Spitzer program sources.Our IC5146 Spitzer data indicate grain growth and ice formation occurs early in the history of dense cloud formation. Each sight-line observed reveals the 9.7um amorphous silicate absorption band. The highest Av sightlines show clear detections of ices at 6.0um (water-ice mixture), 6.85um (processed ice) and 15.2um (CO2). However, sight-lines in the low-to-mid Av range provide intriguing variations. We may have the first example of two objects, one with ice features and one without, seen through the same cloud sightline with similar Av 6 and similar silicate band optical depths. Also, the nominally expected linear correlation of Av with silicate band depth does not appear to hold for this cloud (turnover at Av 10-12). Both trends imply complexities in the grain growth at one of the earliest stages of dust and ice interaction ever observed. The highest extinction source in our sample, Av=20, reveals the 6.0um (water), 6.85um (processed ice), 9.7um silicate and 15.2um CO2 ice bands. Until recently, the 6.85um band had only been detected towards embedded protostellar objects. Two additional quiescent dust sightlines from the C2D results now also indicate the 6.85um band (Taurus and Serpens) by Knez et al. 2005, demonstrating the role of energetic processing within pristine ices prior to the onset of star formation.
Bernstein Mira
Chiar Jean
Ennico Kimberly
Greene Thomas
Keane Jacqueline
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