The Impact of the Massive Young Star GL 2591 on Its Circumstellar Material: Temperature, Density, and Velocity Structure

Abstract

The temperature, density, and kinematics of the gas and dust surroundingthe luminous (2 x 10(4) L.) young stellar object GL 2591 areinvestigated on scales as small as similar to 100 AU, probed by 4.7 mu mabsorption spectroscopy, to over 60,000 AU, probed by single-dishsubmillimeter spectroscopy. These two scales are connected byinterferometric 86-115 and 226 GHz images of size 30,000 AU andresolution 2000 AU in continuum and molecular lines. The data are usedto constrain the physical structure of the envelope and investigate theinfluence of the young star on its immediate surroundings. The infraredspectra at lambda/Delta lambda approximate to 40,000 indicate an LSRvelocity of the (13)CO rovibrational lines of -5.7 @ 1.0 km s(-1)consistent with the velocity of the rotational lines of CO. In infraredabsorption, the (12)CO lines show wings out to much higher velocities,approximate to -200 km s(-1), than are seen in the rotational emissionlines, which have a total width of approximate to 75 km s(-1). Thisdifference suggests that the outflow seen in rotational lines consistsof envelope gas entrained by the ionized jet seen in Pry and {[}S II]emission. The outflowing gas is warm, T > 100 K, since it is brighter inCO J = 6 --> 5 than in lower-J CO transitions.The dust temperature due to heating by the young star has beencalculated self-consistently as a function of radius for a power-lawdensity distribution n = n(0)r(-alpha), with alpha = 1-2. Thetemperature is enhanced over the optically thin relation (T similar tor(-0.4)) inside a radius of 2000 AU, and reaches 120 K at r less than orsimilar to 1500 AU from the star, at which point ice mantles should haveevaporated. The corresponding dust emission can match the observedlambda greater than or equal to 50 mu m continuum spectrum for a widerange of dust optical properties and values of a. However, consistencywith the C(17)O line emission requires a large dust opacity in thesubmillimeter, providing evidence for grain coagulation. The 10-20 mu memission is better matched using bare grains than using ice-coatedgrains, consistent with evaporation of the ice mantles in the warm innerpart of the envelope. Throughout the envelope, the gas kinetictemperature as measured by H(2)CO line ratios closely follows the dusttemperature. The values of alpha and n(0) have been constrained bymodeling emission lines of CS, HCN, and HCO(+) over a large range ofcritical densities. The best fit is obtained for alpha = 1.25 @ 0.25 andn(0) = (3.5 @ 1) x 10(4) cm(-3) at r = 30,000 AU, yielding an envelopemass of (42 @ 10) M. inside that radius. The derived value of a suggeststhat part of the envelope is in freefall collapse onto the star.Abundances in the extended envelope are 5 x 10(-9) for CS, 2 x 10(-9)for H(2)CO, 2 x 10(-8) for HCN, and 1 x 10(-8) for HCO(+). The strongnear-infrared continuum emission, the Br gamma line flux, and ouranalysis of the emission-line profiles suggest small deviations fromspherical symmetry, likely an evacuated outflow cavity directed nearlyalong the line of sight. The A(V) approximate to 30 toward the centralstar is a factor of 3 lower than in the best-fit spherical model.Compared to this envelope model, the Owens Valley Radio Observatory(OVRO) continuum data show excess thermal emission, probably from dust.The dust may reside in an optically thick, compact structure, withdiameter less than or similar to 30 AU and temperature greater than orsimilar to 1000 K, or the density gradient may steepen inside 1000 AU.In contrast, the HCN line emission seen by OVRO can be satisfactorilymodeled as the innermost part of the power-law envelope, with noincrease in HCN abundance on scales where the ice mantles should havebeen evaporated. The region of hot, dense gas and enhanced HCN abundance(similar to 10(-6)) observed with the Infrared Space Observatorytherefore cannot be accommodated as an extension of the power-lawenvelope. Instead, it appears to be a compact region (r < 175 AU, whereT > 300 K), in which high-temperature reactions are affectingabundances.