CO Emission and Absorption toward V1647 Orionis (McNeil’s Nebula)

Abstract

We present high-resolution infrared spectra of V1647 Ori, the illuminating star of McNeil's Nebula, which reveal the presence of hot and cold gas-phase CO and ices of CO and H 2 O. The emission lines of 12 CO (1-0), (2-1), and (3-2) likely originate from $2500 K gas in an inner accretion disk region, where substantial clearing of dust has occurred. The width of the emission lines increases with increasing J-value, suggesting that the hottest CO gas we detect is located closest to the central star. The narrower widths of the low-J CO emission lines are indicative of more distant, cooler material in the inner disk. Superposed on the low-J emission lines are narrow 12 CO absorption components, which are typical of cold interstellar cloud material at a temperature of $18 K. The very low column density and very cold temperature for the absorbing gas suggest that we are viewing the central star through intervening material within the L1630 cloud and that the disk is oriented nearly face-on. The Doppler shift of the cold CO is offset from the hot gas by 6 AE 2 km s À1 , so it is likely that the very cold CO originates in a foreground cloud rather than the circumstellar material surrounding V1647 Ori. Model fits to the strong H 2 O and CO ice absorption bands are consistent with cold (<20 K) amorphous water ice ( ¼ 0:65) and predominantly apolar CO ice ( ¼ 0:58). The CO and H 2 O ices are unprocessed (unannealed), similar to the ices in dense clouds.

Post‐Outburst Infrared Spectra of V1647 Ori, the Illuminating Star of McNeil’s Nebula

“…2). The narrow, cold ($18 K ) CO gas absorption lines and cold ice, coupled with the Doppler-shifted gas absorption ($6 km s À1 relative to V1647 Ori), are consistent with the claim (Rettig et al 2005) that the cold ice and gas are due to intervening interstellar material that is not directly associated with V1647 Ori.…”

“…An analysis of the H 2 O and CO ices was presented by Rettig et al (2005) using the 2004 March SpeX data by Vacca et al (2004) and the 2004 February NIRSPEC spectra. The ice absorption features were fit to laboratory spectra and found to be consistent with cold ($20 K ) amorphous ice.…”

“…The CO ice was primarily apolar (CO mixed with a nonpolar species like CO 2 or O 2 ) rather than polar (mixed with H 2 O). Figure 3 shows the optical depth profiles of water and CO ice on three nights with the laboratory spectrum from Rettig et al (2005) ( Fig. 2).…”

“…The CO emission can be divided into two regions: a hot ($2500 K ) inner disk traced by the high-J lines and a warm ($400 K ) inner disk traced by the low-J lines (Rettig et al 2005). The hot emission provides an excellent probe for measuring the physical and structural changes due to, for example, rapid replenishment of material in the inner disk.…”

“…This is consistent with the idea that previous outbursts have cleared most of the ambient cloud material in the close vicinity of V1647 Ori. Emission lines of 12 CO (1-0), (2-1), and (3-2) were shown to originate from $2500 K gas in an inner accretion disk region ( Rettig et al 2005). The narrower widths of the low-J CO emission lines are indicative of more distant, warm ($400 K ) material in the inner disk.…”