Deciphering the Ionized Gas Content in the Massive Star-Forming Complex G75.78+0.34

Abstract

We present sub-arcsecond observations toward the massive star forming region G75.78+0.34. We used the Very Large Array to study the centimeter continuum and H 2 O and CH 3 OH maser emission, and the Owens Valley Radio Observatory and Submillimeter Array to study the millimeter continuum and recombination lines (H40α and H30α). We found radio continuum emission at all wavelengths, coming from three components:(1) a cometary ultracompact (UC) H II region with an electron density ∼3.7 × 10 4 cm −3 , excited by a B0 type star, and with no associated dust emission; (2) an almost unresolved UCH II region (EAST), located ∼6 ′′ to the east of the cometary UCH II region, with an electron density ∼1.3 × 10 5 cm −3 , and associated with a compact dust clump detected at millimeter and mid-infrared wavelengths; and (3) a compact source (CORE), located ∼2 ′′ to the southwest of the cometary arc, with a flux density increasing with frequency, and embedded in a dust condensation of 30 M ⊙ . The CORE source is resolved into two compact and unresolved sources which can be well-fit by two homogeneous hypercompact H II regions each one photo-ionized by a B0.5 ZAMS star, or by free-free radiation from shock-ionized gas resulting from the interaction of a jet/outflow system with the surrounding environment. The spatial distribution and kinematics of water masers close to the CORE-N and S sources, together with excess emission at 4.5 µm and the detected dust emission, suggest that the CORE source is a massive protostar driving a jet/outflow. Subject headings: stars: formation -ISM: individual objects (G75.78+0.34) -ISM: HII regions -ISM:dust -radio continuum: ISM

Outflow structure within 1000 au of high-mass YSOs

“…7). The non detection in the C-band is consistent with the upper limit obtained by Sánchez-Monge et al (2013) at the same frequency, while the differences in the Ku-band fluxes might be explained by the different sampling and resolution of the observations: Sánchez-Monge et al (2013) used the VLA in the B and C configurations, sensitive to relatively extended emission and measuring a flux of 2 mJy, whereas in our observations with the A-Array, we filter out a fraction of this emission and detect only about 1 mJy. In the direction of the SW compact source, we observe a tight water maser cluster with a rather scattered pattern of proper motions.…”

“…Following the analysis by Sánchez-Monge et al (2013), the properties of the radio emission from the two compact sources are consistent with both homogeneous, hypercompact Hii regions and free-free radiation from a jet. Therefore, with the present data, it is difficult to decide if two distinct YSOs are driving two independent outflows collimated at different PAs or if the radio continuum and the water masers trace, respectively, ionized gas along the axis and an extended, wide-angle bow shock, generated by the same outflow.…”

“…7)) suggest the presence in this region of an outflow collimated along NE-SW, Sánchez-Monge et al (2013) interpret the water maser pattern in terms of a wide-angle bow shock produced by the interaction of the jet/outflow with dense clump material. However, we note that the two nearby radio sources could be ionized by two distinct, high-mass YSOs, each emitting a massive, collimated outflow.…”

Weak and Compact Radio Emission in Early High-Mass Star-Forming Regions. I. Vla Observations

“…Thermal free-free emission at centimeter wavelengths in high-mass star-forming regions can have distinct origins, such as H II regions, photoionized internally or externally, ionized accreting flows (Keto 2002(Keto , 2003(Keto , 2007, stellar winds (Carrasco-González et al 2015), or shocks produced by the collision of thermal jets with surrounding material (see (Rodríguez et al 2012) and Sánchez-Monge et al (2013b) for an extensive list of different ionization processes in highmass young stellar objects). The non-thermal components could be protostars with an active magnetosphere (Deller et al 2013) or high-mass binary stars producing synchrotron radiation in the region where their winds collide (Rodríguez et al 2012).…”

A study on subarcsecond scales of the ammonia and continuum emission toward the G16.59−0.05 high-mass star-forming region

“…The origin of such emission has been interpreted in terms of photo-ionized spherical (Panagia & Felli 1975) and A&A 558, A145 (2013) collimated (Reynolds 1986) stellar winds, shock-induced radiation (Ghavamian & Hartigan 1998), and, more recently, trapped Hii regions (Keto 2003). See also Rodríguez et al (2012) and Sánchez-Monge et al (2013b) and references therein, for a list of different mechanisms proposed to explain the observation of thermal centimeter continuum emission in star-forming regions.…”