Abstract
Observations of young stellar objects (YSOs) in centimeter bands can probe the continuum emission from growing dust grains, ionized winds, and magnetospheric activity that are intimately connected to the evolution of protoplanetary disks and the formation of planets. We carried out sensitive continuum observations toward the Ophiuchus A star-forming region, using theKarl G. JanskyVery Large Array (VLA) at 10 GHz over a field-of-view of 6′ and with a spatial resolution ofθmaj×θmin~ 0.′′4 × 0.′′2. We achieved a 5μJy beam−1rms noise level at the center of our mosaic field of view. Among the 18 sources we detected, 16 were YSOs (three Class 0, five Class I, six Class II, and two Class III) and two were extragalactic candidates. We find that thermal dust emission generally contributed less than 30% of the emission at 10 GHz. The radio emission is dominated by other types of emission, such as gyro-synchrotron radiation from active magnetospheres, free–free emission from thermal jets, free–free emission from the outflowing photoevaporated disk material, and synchrotron emission from accelerated cosmic-rays in jet or protostellar surface shocks. These different types of emission could not be clearly disentangled. Our non-detections for Class II/III disks suggest that extreme UV-driven photoevaporation is insufficient to explain disk dispersal, assuming that the contribution of UV photoevaporating stellar winds to radio flux does not evolve over time. The sensitivity of our data cannot exclude photoevaporation due to the role of X-ray photons as an efficient mechanism for disk dispersal. Deeper surveys using the Square Kilometre Array (SKA) will have the capacity to provide significant constraints to disk photoevaporation.
Highlights
The first step towards forming the building blocks of planets occurs via grain growth in disks composed of dust and gas surrounding young stars (e.g., Testi et al 2014; Johansen et al 2014)
Nature of the emission at 10 GHz we evaluate how much of the flux measured towards the young stellar objects (YSOs) in our 10 GHz Very Large Array (VLA) observations is due to: (i) thermal emission from dust, and (ii) other mechanisms such as free–free emission from ionized radio jets or photoevaporative winds, gyro-synchrotron emission from active magnetospheres, and synchrotron emission produced through the acceleration of cosmic-rays by jet or protostellar surface shocks (e.g., Macías et al 2016; Gibb 1999; Forbrich et al 2007; Padovani et al 2016; Padovani & Galli 2018)
We carried out very sensitive continuum observations of the Ophiuchus A (Oph A) star-forming region at 10 GHz with the VLA (1σ = 5 μJy beam−1 at the center of the field of view)
Summary
The first step towards forming the building blocks of planets occurs via grain growth in disks composed of dust and gas surrounding young stars (e.g., Testi et al 2014; Johansen et al 2014). The detection of transition disks where dust has been cleared within the inner regions (e.g., Strom et al 1989; Pascucci et al 2016; van der Marel et al 2018; Ansdell et al 2018) has favored the development of theoretical models where disk dispersal occurs from the inside out (e.g., photoevaporation, grain growth, giant planet formation). All five epochs of observation (see Table 1) were carried out in the most extended, A array configuration, which provides a projected baseline range from 310 to 34 300 m. They are separated by 2.6 , while the primary beam FWHM is 4.2. The flux calibration uncertainty is expected to be about 5%
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