We present a detailed characterization of the population of compact radio-continuum sources in W51 A using subarcsecond VLA and ALMA observations. We analyzed their 2-cm continuum, the recombination lines (RL's) H77$α$ and H30$α$, and the lines of $\rm H_{2}CO(3_{0,3}-2_{0,2})$, $\rm H_{2}CO(3_{2,1}-2_{2,0})$, and $\rm SO(6_{5}-5_{4})$. We derive diameters for 10/20 sources in the range $D \sim 10^{-3}$ to $\sim 10^{-2}$ pc, thus placing them in the regime of hypercompact HII regions (HC HII's). Their continuum-derived electron densities are in the range $n_{\rm e} \sim 10^4$ to $10^5$ cm$^{-3}$, lower than typically considered for HC HII's. We combined the RL measurements and independently derived $n_{\rm e}$, finding the same range of values but significant offsets for individual measurements between the two methods. We found that most of the sources in our sample are ionized by early B-type stars, and a comparison of $n_{\rm e}$ vs $D$ shows that they follow the inverse relation previously derived for ultracompact (UC) and compact HII's. When determined, the ionized-gas kinematics is always (7/7) indicative of outflow. Similarly, 5 and 3 out of the 8 HC HII's still embedded in a compact core show evidence for expansion and infall motions in the molecular gas, respectively. We hypothesize that there could be two different types of $hypercompact$ ($D< 0.05$ pc) HII regions: those that essentially are smaller, expanding UC HII's; and those that are also $hyperdense$ ($n_{\rm e} > 10^6$ cm$^{-3}$), probably associated with O-type stars in a specific stage of their formation or early life.
We have carried out the largest and most unbiased search for hypercompact (HC) HII regions. Our method combines four interferometric radio continuum surveys (THOR, CORNISH, MAGPIS and White2005) with far-infrared and sub-mm Galactic Plane surveys to identify embedded HII regions with positive spectral indices. 120 positive spectrum HII regions have been identified from a total sample of 534 positive spectral index radio sources. None of these HII regions, including the known HCHII regions recovered in our search, fulfills the canonical definition of an HCHII region at 5 GHz. We suggest that th…
We present 250~GHz continuum and H29alpha line data toward W49N:A2, a hypercompact HII region ionized by an O9 star. The data obtained with ALMA at a resolution of ~0"05 (600 au) confirmed the presence of an ionized ring with a radius of~700 au inclined by ~50degree (0degree for pole-on). It has a width of ~1000 au and is relatively flat with a scale height of less than several hundred au. The tilted ring, or the apparent ellipse, has a prominent velocity difference between its NW and SE ridges along the minor axis, suggesting that it is expanding in the equatorial plane at a velocity of 13.2 …
We present radio continuum observations of the high-mass young stellar object (HMYSO) G345.4938+01.4677 made using the Australia Telescope Compact Array (ATCA) at 5, 9, 17, and 19 GHz. These observations provide definite evidence that the outer and inner pairs of radio lobes consist of shock ionized material being excited by an underlying collimated and fast protostellar jet emanating from a hypercompact HII region. By comparing with images taken 6 yr earlier at 5 and 9 GHz using the same telescope, we assess the proper motions of the radio sources. The outer West and East lobes exhibit proper…
The formation of hypercompact (HC) HII regions is an important stage in massive star formation. Spectral line and continuum observations can explore its dynamic conditions. We present high angular resolution observations carried out with the Submillimeter Array (SMA) and the Very Large Array (VLA) toward the HC HII region G35.58-0.03. With the 1.3 mm SMA and 1.3 cm VLA, we detected a total of about 25 transitions of 8 different species and their isotopologues (CO, CH$_3$CN, SO$_2$, CH$_3$CCH, OCS, CS, H, and NH$_{3}$). G35.58-0.03 consists of an HC HII core with electron temperature $T_e^*\geq…
Context. The derived physical parameters for young HII regions are normally determined assuming the emission region to be optically thin. However, this assumption is unlikely to hold for young HII regions such as hyper-compact HII(HCHII) and ultra-compact HII(UCHII) regions and leads to the underestimation of their properties. This can be overcome by fitting the SEDs over a wide range of radio frequencies. Aims. The two primary goals of this study are (1) to determine the physical properties of young HII regions from radio SEDs in the search for potential HCHII regions, and (2) to use these ph…
The ngVLA will create a Galaxy-wide, volume-limited sample of HII regions; solve some long standing problems in the physics of HII regions; and provide an extinction-free star formation tracer in nearby galaxies.
Observations of millimeter wavelength radio recombination lines (mm-RRLs) are used to search for HII regions in an unbiased way that is complementary to many of the more traditional methods previously used. We carried out targeted mm-RRL observations (39$\leq$ principal quantum number ($n$) $\leq$65 and $Δn$ = 1, 2, 3 and 4, i.e. H$nα$, H$nβ$, H$nγ$ and H$nδ$) using the IRAM 30m and Mopra 22m telescopes. We observed 976 compact dust clumps selected from a catalog of $\sim$10,000 ATLASGAL sources. We detected H$nα$ mm-RRL emission toward 178 clumps; H$nβ$, H$nγ$ and H$nδ$ were also detected tow…
In this contribution we explore the new science that a Next Generation Very Large Array (ngVLA) would be able to perform on the topics of the youngest HII regions and (proto)stellar jets. Free-free continuum and radio recombination line (RRL) emission are often the only way of peering into the dense envelopes surrounding (proto)stars of all masses, and trace their initial feedback in the form of `radio jets', `hypercompact HII regions', or photoevaporating, partially-ionized flows. Properly disentangling free-free from dust emission is also mandatory in studies of protoplanetary and accretion …
HII regions are the ionized spheres surrounding high-mass stars. They are ideal targets for tracing Galactic structure because they are predominantly found in spiral arms and have high luminosities at infrared and radio wavelengths. In the Green Bank Telescope HII Region Discovery Survey (GBT HRDS) we found that >30% of first Galactic quadrant HII regions have multiple hydrogen radio recombination line (RRL) velocities, which makes determining their Galactic locations and physical properties impossible. Here we make additional GBT RRL observations to determine the discrete HII region velocity …
Physical characterization and modeling of candidate Hyper-Compact HII Regions
Universidad de Guanajuato（瓜纳华托大学）；Instituto de Cibernética, Matemática y Física（古巴数学物理与信息学研究所）；Universidad Nacional Autónoma de México（墨西哥国立自治大学）；Institute of Space Science & Technology（空间科学与技术研究所）；Institute of Space Sciences (ICE-CSIC)（空间科学研究所（巴塞罗那高等研究委员会））；Institute of Space Studies of Catalonia (IEEC)（加泰罗尼亚空间研究所）；Texas Southern University（德克萨斯南方大学）
We investigate the general stability of 1D spherically symmetric ionized Bondi accretion onto a massive object in the specific context of accretion onto a young stellar object. We first derive a new analytic expression for a steady state two temperature solution that predicts the existence of compact and hypercompact HII regions. We then show that this solution is only marginally stable if ionization is treated self-consistently. This leads to a recurring collapse of the HII region over time. We derive a semi-analytic model to explain this instability, and test it using spatially converged 1D …
The Southern HII Region Discovery Survey (SHRDS) is a survey of the third and fourth quadrants of the Galactic plane that will detect radio recombination line and continuum emission at cm-wavelengths from several hundred HII region candidates using the Australia Telescope Compact Array. The targets for this survey come from the WISE Catalog of Galactic HII Regions, and were identified based on mid-infrared and radio continuum emission. In this pilot project, two different configurations of the Compact Array Broad Band receiver and spectrometer system were used for short test observations. The …
Context: High-mass stars have a high degree of multiplicity and most likely form via disk accretion processes. The detailed physics of the binary and disk formation are still poorly constrained. Methods: Using the VLA in its most extended configuration at ~24GHz toward the prototypical high-mass star-forming region NGC7538IRS1 has allowed us to study the NH3 and thermal CH3OH emission and absorption as well as the cm continuum emission at an unprecedented spatial resolution of 0.06"x0.05", corresponding to a linear resolution of ~150AU at a distance of 2.7kpc. Results: A comparison of these ne…
HII regions made of gas ionized by radiations from young massive stars, are widely distributed in the Milky Way. They are tracers for star formation, and their distributions are correlated with the Galactic spiral structure. Radio recombination lines (RRLs) of hydrogen and other atoms allows for the precisest determination of physical parameters such as temperature and density. However, RRLs at around 1.4 GHz from HII regions are weak and their detections are difficult. As a result, only a limited number of detections have been obtained yet. The 19-beam receiver on board of the Five-hundred-me…
We have recently published observations of significant flux density variations at 1.3 cm in HII regions in the star forming regions Sgr B2 Main and North (De Pree et al. 2014). To further study these variations, we have made new 7 mm continuum and recombination line observations of Sgr B2 at the highest possible angular resolution of the Karl G. Jansky Very Large Array (VLA). We have observed Sgr B2 Main and North at 42.9 GHz and at 45.4 GHz in the BnA configuration (Main) and the A configuration (North). We compare these new data to archival VLA 7 mm continuum data of Sgr B2 Main observed in …
In this series of papers, we model the formation and evolution of the photoionized region and its observational signatures during massive star formation. Here we focus on the early break out of the photoionized region into the outflow cavity. Using results of 3-D magnetohydrodynamic-outflow simulations and protostellar evolution calculations, we perform post-processing radiative-transfer. The photoionized region first appears at a protostellar mass of 10Msun in our fiducial model, and is confined to within 10-100AU by the dense inner outflow, similar to some observed very small hypercompact HI…
Previous radio recombination line (RRL) observations of dust clumps identified in the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) have led to the detection of a large number of RRLs in the 3mm range. Here, we aim to study their excitation with shorter wavelength (sub)millimeter radio recombination line (submm-RRL) observations. We made observations of submm-RRLs with low principal quantum numbers ($n$ $\leq$ 30) using the APEX 12 m telescope, toward 104 HII regions associated with massive dust clumps from ATLASGAL. The observations covered the H25$α$, H28$α$, and H35$β$ transitio…
We have observed the compact HII region complex nearest to the dynamical center of the Galaxy, G-0.02-0.07, using ALMA in the H42a recombination line, CS J=2-1, H13CO+ J=1-0, and SiO v=0, J=2-1 emission lines, and 86 GHz continuum emission. The HII regions HII-A to HII-C in the cluster are clearly resolved into a shell-like feature with a bright-half and a dark-half in the recombination line and continuum emission. The absorption features in the molecular emission lines show that HII-A, B and C are located on the near side of the 50 km/s Molecular Cloud (50MC) but HII-D is located on the far s…
We investigate the surroundings of the hypercompact HII region M17 UC1 to probe the physical properties of the associated young stellar objects and the environment of massive star formation. Five of the seven point sources in this region show $L$-band excess emission. Geometric match is found between the H_2 emission and near-IR polarized light in the vicinity of IRS5A, and between the diffuse mid-IR emission and near-IR polarization north of UC1. The H_2 emission is typical for dense PDRs, which are FUV pumped initially and repopulated by collisional de-excitation. The spectral types of IRS5A…
The Lumina Project: Intergalactic Clumping and Recombination Sinks
The University of Texas at Dallas（德克萨斯大学达拉斯分校）；Institute for Basic Science（韩国基础科学研究所）；Massachusetts Institute of Technology（麻省理工学院）；York University（约克大学）；Max Planck Institute for Astrophysics（马克斯·普朗克天体物理学研究所）
We aim to investigate the chemistry of internal photon-dominated regions surrounding deeply embedded hypercompact and ultracompact HII regions. We search for specific tracers of this evolutionary stage of massive star formation that can be detected with current astronomical facilities. We modeled hot cores with embedded HC/UCHII regions, by coupling the astrochemical code Saptarsy to a radiative transfer framework obtaining the spatio-temporal evolution of abundances as well as time-dependent synthetic spectra. In these models where we focused on the internal PDR surrounding the HI region, the…
The dynamics of ionized gas around the W33 Main ultracompact HII region is studied using observations of hydrogen radio recombination lines and a detailed multiwavelength characterization of the massive star-forming region W33 Main is performed. We used the Giant Meterwave Radio Telescope (GMRT) to observe the H167$α$ recombination line at 1.4 GHz at an angular resolution of 10 arcsec, and Karl. G. Jansky Very Large Array (VLA) data acquired in the GLOSTAR survey to study the dynamics of ionized gas. We also observed the radio continuum at 1.4 GHz and 610 MHz with the GMRT and used GLOSTAR 4$-…
We observed radio recombination lines (RRLs) toward the W51 molecular cloud complex, one of the most active star forming regions in our Galaxy. The UV radiation from young massive stars ionizes gas surrounding them to produce HII regions. Observations of the W51 IRS1 HII region were made with the Arecibo 305 m telescope. Of the full 1-10 GHz database, we have analyzed the observations between 4.5 and 5 GHz here. The steps involved in the analysis were: a) bandpass calibration using on-source/off-source observations; b) flux density calibration; c) removing spectral baselines due to errors in b…
The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross-section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The mm/submm recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultra-compact HII regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra usi…
We present deep spectrophotometry of several Hii regions in the nearby low-mass spiral galaxies NGC 300 and M33. The data have been taken with UVES and OSIRIS spectrographs attached to the 8m VLT and 10.4m GTC telescopes, respectively. We have derived precise values of the physical conditions for each object making use of several emission line-intensity ratios. In particular, we have obtained direct determinations of the electron temperature in all the observed objects. We detect pure recombination lines (RLs) of Cii and Oii in several of the Hii regions, permitting to derive their C/H and C/O…
High-mass stars form in much richer environments than those associated with isolated low-mass stars, and once they reach a certain mass, produce ionised (HII) regions. The formation of these pockets of ionised gas are unique to the formation of high-mass stars (M $>8$ M$_\odot$), and present an excellent opportunity to study the final stages of accretion, which could include accretion through the HII region itself. This study of the dynamics of the gas on both sides of these ionisation boundaries in very young HII regions aims to quantify the relationship between the HII regions and their imme…
We presented a multi-wavelength study of AFGL 333-Ridge. The molecular line data reveals that the AFGL 333-Ridge has two independent velocity components at -50.5 km/s and -48.0 km/s. In the Position-Velocity diagram, the bridge feature connects with two parts that are spatially correlated but separated in velocity. These observational evidences support the scenario that the two velocity components have collided and merged into one molecular cloud. The majority of Class I YSOs are distributed within the collision region, suggesting that the cloud-cloud collision has induced the YSOs formation i…
We report Keck-NIRSPEC observations of the Brackett $α$ 4.05 $μ$m recombination line across the two candidate embedded super star clusters (SSCs) in NGC 1569. These SSCs power a bright HII region and have been previously detected as radio and mid-infrared sources. Supplemented with high resolution VLA mapping of the radio continuum along with IRTF-TEXES spectroscopy of the [SIV] 10.5 $μ$m line, the Brackett $α$ spectra data provide new insight into the dynamical state of gas ionized by these forming massive clusters. NIR sources detected in 2 $μ$m images from the Slit-viewing Camera are matche…
We detail tentative detections of low-frequency carbon radio recombination lines from within the Orion molecular cloud complex observed at 99-129 MHz. These tentative detections include one alpha transition and one beta transition over three locations and are located within the diffuse regions of dust observed in the infrared at 100umm, the Halpha emission detected in the optical, and the synchrotron radiation observed in the radio. With these observations, we are able to study the radiation mechanism transition from collisionally pumped to radiatively pumped within the HII regions of the Orio…
