The VLA Nascent Disk And Multiplicity Survey of Perseus Protostars (VANDAM). III. Extended Radio Emission from Protostars in Perseus

Abstract

Centimeter continuum emission from protostars offers insight into the innermost part of the outflows, as shock-ionized gas produces free-free emission. We observed a complete population of Class 0 and I protostars in the Perseus molecular cloud at 4.1 and 6.4 cm with resolution and sensitivity superior to previous surveys. From a total of 71 detections, eight sources exhibit resolved emission at 4.1 cm and/or 6.4 cm. In this paper, we focus on this subsample, analyzing their spectral indices along the jet and their alignment with respect to the large-scale molecular outflow. Spectral indices for fluxes integrated toward the position of the protostar are consistent with free-free thermal emission. The value of the spectral index along a radio jet decreases with distance from the protostar. For six sources, emission is well aligned with the outflow central axis, showing that we observe the ionized base of the jet. This is not the case for two sources, where we note misalignment of the emission with respect to the large-scale outflow. This might indicate that the emission does not originate in the radio jet, but rather in an ionized outflow cavity wall or disk surface. For five of the sources, the spectral indices along the jet decrease well below the thermal free-free limit of −0.1 with 2s > significance. This is indicative of synchrotron emission, meaning that high-energy electrons are being produced in the outflows close to the disk. This result can have far-reaching implications for the chemical composition of the embedded disks.

The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. I. Identifying and Characterizing the Protostellar Content of the OMC-2 FIR4 and OMC-2 FIR3 Regions

“…We adopt κ 0.87mm = 1.84 cm 2 g −1 from Ossenkopf & Henning (1994) column 5 (thin ice mantles, 10 6 cm −3 density), and we extrapolate the opacity to 9 mm using the 1.3 mm opacity (0.89 cm 2 g −1 ) from Ossenkopf & Henning (1994) and adopting a dust opacity spectral index (β) of 1. Note that our adopted dust opacity at 9 mm is not from a continuous dust model, but yields masses in agreement with shorter wavelength studies (e.g., Tychoniec et al 2018;Andersen et al 2019). Otherwise dust masses from the 9 mm data are unphysically large.…”

“…We note that there is often disagreement between the continuum masses measured at 0.87 mm and 9 mm. This can be due to both the uncertainty in scaling the dust mass opacity to 9 mm, but also there is likely freefree emission contributing to the 9 mm flux density and thus inflating the mass estimates (e.g., Tychoniec et al 2018). The spectral indices determined from 8.1 mm to 10.1 mm using the full bandwidth of the VLA observations, also shown in Table 4, are evidence for free-free emission with spectral indices less than 2 found for several sources.…”

Formation and Evolution of Disks Around Young Stellar Objects

“…Furthermore, in the VANDAM survey of protostars in Perseus with the VLA at 8 mm, Tychoniec et al (2018) find a typical disk mass of 0.14 M for Class 0 protostars and 0.05 M for Class I protostars. Similar measurements were found using a different analysis technique with lower-resolution data by Andersen et al (2019) and data at 1.3 mm, indicating that the measurements from Ophiuchus are very low compared to the Perseus region.…”

Low dust emissivities and radial variations in the envelopes of Class 0 protostars: possible signature of early grain growth

“…1 https://www.iram.fr/IRAMFR/GILDAS/ 2.3. Free-free + synchrotron contribution In protostars, thermal (free-free) emission produced by a jet or a photo-evaporative wind (see Anglada et al, 1996;Lugo et al, 2004;Pascucci et al, 2012;Tychoniec et al, 2018a, for observations and theoretical models) and to a lesser extent, from nonthermal (synchrotron) emission (André, 1996) originating from the presence of magnetic fields, can contribute to the 1-3 mm emission within the central 50 au around the protostar. We estimated the contribution to the 1.3 mm and 3.2 mm fluxes not coming from thermal dust emission from radio measurements beyond 2 cm.…”