The Power of SOFIA/FORCAST in Estimating Internal Luminosities of Low-mass Class 0/I Protostars

Abstract

With the Stratospheric Observatory for Infrared Astronomy (SOFIA) routinely operating science flights, we demonstrate that observations with the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) can provide reliable estimates of the internal luminosities, L int , of protostars. We have developed a technique to estimate L int using a pair of FORCAST filters: one "short-wavelength" filter centered within 19.7-25.3μm, and one "long-wavelength" filter within 31.5-37.1μm. These L int estimates are reliable to within 30%-40% for 67% of protostars and to within a factor of 2.3-2.6 for 99% of protostars. The filter pair comprised of F 25.3 μm and F 37.1 μm achieves the best sensitivity and most constrained results. We evaluate several assumptions that could lead to systematic uncertainties. The OH5 dust opacity matches observational constraints for protostellar environments best, although not perfectly; we find that any improved dust model will have a small impact of 5%-10% on the L int estimates. For protostellar envelopes, the TSC84 model yields masses that are twice those of the Ulrich model, but we conclude that this mass difference does not significantly impact results at the mid-infrared wavelengths probed by FORCAST. Thus, FORCAST is a powerful instrument for luminosity studies targeting newly discovered protostars or suspected protostars lacking detections longward of 24μm. Furthermore, with its dynamic range and greater angular resolution, FORCAST may be used to characterize protostars that were either saturated or merged with other sources in previous surveys using the Spitzer Space Telescope or the Herschel Space Observatory.

Physical and Chemical Structure of the Disk and Envelope of the Class 0/I Protostar L1527

“…We adopt a dust model that is appropriate for protostellar environments, based on a comparison of models with observations (Huard & Terebey 2017). Specifically we adopt the thinly ice-mantled, coagulated dust of Ossenkopf & Henning (1994) (see their Table 1, 5th column), often referred to as "OH5" grains in the literature (e.g., Evans et al (2001); Shirley et al (2005)), augmented by the opacities of Pollack et al (1994) at wavelengths shorter than 1.25 µm, as described in Dunham et al (2010).…”

“…For both collapse models, we set the envelope radius to R env and, for the TSC model, the infall velocity is zero outside ∼ 4000 au, which is the current boundary of the infall region and it moreover has an infall rate of 3.0 × 10 −6 M yr −1 . But for the UCM model, the infall velocity in this outer region is not zero, but is given by the free-fall velocity onto the 0.22 M protostar, with a mass infall rate of 5.0×10 −6 M yr −1 ; moreover, UCM neglects any acceleration from the massive envelope (see Huard & Terebey (2017) for additional discussion). Hence the UCM and TSC models presented here have different values for the density of the envelope, but the spatial grid, as well as the outflow and disk definitions, are the same for both models.…”

“…This led to choosing a stellar radius of 1.70R with an effective temperature of 3,300 K that gives a stellar luminosity of 0.31 L (see Table 1). One additional term L ISRF contributes to the total luminosity of the system, L tot = L int + L ISRF = 3.23 L ; the term L ISRF is due to external illumination by the Interstellar Radiation Field (ISRF) and is based on the galactic value com-puted near our solar system; see description in Huard & Terebey (2017).…”