Solid Carbon Dioxide in Regions of Low‐Mass Star Formation

Abstract

We present high-resolution (R D 1500È2000) spectra of the 4.27 km asymmetric stretching feature of solid in eight lines of sight observed with the Short Wavelength Spectrometer of the Infrared Space CO 2 Observatory. Two of the sources are Ðeld stars located behind the Taurus molecular cloud ; the others are young stellar objects (YSOs) of predominantly low-to-intermediate mass. We Ðnd a signiÐcant source-tosource variation in the solid abundance ratio in our sample : two lines of sight, Elias 18 andabundances of D 34%È37%, considerably higher than in other lines of sight CO 2 studied to date. In agreement with a previous study of Elias 16, we conÐrm a substantial (D20%) abundance of solid relative to in the quiescent intracloud medium. We compare the proÐles CO 2 H 2 O C O 2 with laboratory spectra of interstellar ice analogs from the Leiden Observatory Laboratory database. Results show that the 4.27 km proÐles toward Ðeld stars and embedded low-mass objects are remarkably similar to each other and seem to originate mostly in cold ice. In two higher mass YSOs H 2 O-rich (RAFGL 989 and S255 IRS1), the proÐles are clearly di †erent, and at least the latter source shows signs of thermal processing.

Mid‐infrared Spectra of Class I Protostars in Taurus

“…Only one of our targets ( IRAS 04108+2803B) lies near a cloud core ( L1495) dense enough that its silicate feature could be explained by a location on the far side and absorption by the intervening cloud; even if this were the case, the 15.2 m CO 2 ice feature would still be unusually strong. The foreground and molecularcloud extinguishing material can be inferred to have very weak absorption in the 15.2 m CO 2 ice feature compared to what we observe, judging from observations of the 4.3 m CO 2 feature in stars behind the Taurus clouds ( Nummelin et al 2001), and from Spitzer IRS observations of other Taurus association objects ( Forrest et al 2004). To first approximation, then, we can take the Taurus Class I objects to be unaffected by mid-infrared foreground absorption and can attribute the absorptions to the objects's disks and envelopes.…”

“…Superficially, the spectra in Figure 1 resemble those of embedded YSOs observed with ISO SWS and ISOCAM (e.g., Gibb et al 2004;Alexander et al 2003;Nummelin et al 2001;Whittet et al 1996). Strong features from amorphous silicates appear at 9.7 and 18 m. These features appear purely in absorption for all except IRAS 04181+2654B, which shows the effects of emission and absorption in similar amounts.…”

The influence of temperature on the synthesis of molecules on icy grain mantles in dense molecular clouds

“…Jenniskens et al 1993). These models based on a fixed set of physical conditions, such as a gas and dust temperature of 10 K, reproduce the abundances of solid H 2 O and CO observed towards the field star Elias16, but underestimate the abundance of solid CO 2 , finding a maximum of 1% with respect to water ice, while the observed abundance is around 23% (Nummelin et al 2001). Ruffle & Herbst (2001b) developed more complete gas-grain models with a variety of densities and temperatures to determine whether a change in these parameters could enhance the production of CO 2 , while preserving the agreement between the predicted and observed abundance of the other major ice components.…”

A survey of [HDCO]/[ H$_\mathsf{2}$CO] and [DCN]/[HCN] ratios towards low-mass protostellar cores

“…van der Tak et al 2000; Nummelin et al 2001) suggest that ice composition may varies between different environments. These differences may be due to the precise details of the surface chemistry which is occurring (as yet, not well understood), or they may depend on the degree of processing (e.g.…”