Introducing the POETS survey

Outflow activity is a proxy for ongoing star formation. A number of outflow studies have established a correlation between the bolometric luminosity (Lbol) of a star-forming region and the integrated (molecular) outflow properties. These properties include the mechanical force and momentum released into the ambient gas, which, at parsec scales, quantify the overall contribution of an ensemble of outflows. Indeed, parsec scale surveys sum up the outflow activity from diverse young stars, both Sun-like and more massive, because the latter preferentially form in associations of a few tens to a few thousands. Nevertheless, the above correlation holds over six orders of magnitude of Lbol  and is interpreted as evidence for a single outflow mechanism, which scales with the stellar luminosity, and the motion of the outflows being momentum driven. However, on scales of a few 1000 au, representative of individual young stars, there are poor statistics on outflow properties for stellar luminosities exceeding thousands Solar luminosities (1000 L⊙). In turn, large-scale measurements performed on molecular gas have left out fundamental questions, such as how efficiently primary outflows converts energy and momentum into large-scale motions and eventually cloud turbulence — "primary outflows" are those whose material was directly (gravitationally) bound to the driving star.

For the purpose of quantifying the morphology and dynamical properties of the primary outflow emission in the vicinity of luminous young stars, we have started the Protostellar Outflows at the EarliesT Stages (POETS) survey.  The target sample of about 40 sources has been selected with the idea of combining the kinematic information of outflowing gas, inferred from the H2O maser emission, with the information of ejected mass, inferred from the H II, free–free continuum emission. The starting dataset was made up of:

1.  phase-referencing, multi-epoch, H2O maser observations conducted with the VLBA antennas in USA at 10 mJy sensitivities; 

2.  a radio continuum survey at 3 frequency bands (6, 15, and 22 GHz) conducted with the VLA antennas in New Mexico (in the A and B configurations) at 5-10 μJy sensitivities.

To interpret the POETS observations one has to keep in mind the following:

• On the one hand, H2O maser emission traces shocked gas propagating in dense regions (densities greater than a million particles per cubic centimeter) at velocities between 10 and 200 km/s: these properties make H2O masers signposts of proto-stellar outflows within a few 1000 au from their driving source. Maser spots, namely, cloudlets of the order of a few au in size, are ideal test particles to measure the local three-dimensional motion of gas shocked where stellar winds and jets impact ambient gas.

• On the other hand, thermal (bremsstrahlung) continuum emission, with flux densities lower than a few mJy, and spectral index values (α) at centimeter wavelengths between −0.1 and below 2, trace the ionized gas component of stellar winds and jets. Ionization is caused by shocks around young stars with spectral types later than B, which emit negligible Lyman flux. In these sources, the centimeter continuum luminosity scales with the stellar luminosity as a power law of approximately index 0.6. By extrapolating this law to young stars of spectral types B and earlier, for instance, a continuum flux of approximately 1 mJy at 8 GHz is expected for a B1 zero-age main sequence (ZAMS) star at a distance of 1 kpc from the Sun. For comparison, a homogeneous, optically thin H II region, that is excited by the Lyman photons of a B1 ZAMS star, emits a continuum flux more than two orders of magnitude higher (≈ 0.2 Jy) at the same frequency and distance. It follows that, around young massive stars, ionized stellar winds and jets can be only detected prior to the ultra-compact (UC) H II region phase, the latter implying typical lifetimes older than ten thousand years.

• While the flux density (S) of radio stellar winds and jets typically increases with frequency, Sν ∝ ν ^(α), and shows a partially opaque spectral index (0 < α < 2) where the emission is confined at a few 1000 au of the star, this phenomenology can change moving at larger distances. In the former case, radio spectra are interpreted as thermal free-free emission from ionized particles that are accelerated within their own electric field, but, along the axis of a few radio jets, the spectrum is occasionally inverted at the loci of bright knots of ionized gas, showing spectral index values much lower than the optically thin limit (−0.1). In this latter case, radio spectra are interpreted as evidence for (non-thermal) synchrotron emission from strong jet shocks against the ambient medium, where ionized particles would be sped up to relativistic velocities via diffusive shock acceleration. 

More details in, Sanna et al. 2018, A&A, 619, 107  (and references therein)

                                                        Featured examples from POETS:

                                                                                                                                A case study of radio continuum emission tracing a proto-stellar jet

                                                                                                                     A case study of H2O maser emission tracing a proto-stellar jet