Radio Recombination Line Observations Research Articles

Multiline observations of hydrogen, helium, and carbon radio-recombination lines toward Orion A: A detailed dynamical study and direct determination of physical conditions

We present a study of hydrogen, helium, and carbon millimeter-wave radio-recombination lines (RRLs) toward ten representative positions throughout the Orion Nebula complex, using the Yebes 40m telescope in the Q band (31.3 GHz to 50.6 GHz) at an angular resolution of about $45 (sim 0.09\,pc). The observed positions include the Orion Nebula (M42) with the Orion Molecular Core 1, M43, and the Orion Molecular Core 3 bordering on NGC 1973, 1975, and 1977. While hydrogen and helium RRLs arise in the ionized gas surrounding the massive stars in the Orion Nebula complex, carbon RRLs stem from the neutral gas of the adjacent photo-dissociation regions (PDRs). The high velocity resolution ($0.3\ $) enables us to discern the detailed dynamics of the RRL emitting neutral and ionized gas. We compare the carbon RRLs with SOFIA/upGREAT observations of the ii m $ line and IRAM 30m observations of the 13CO (J=2-1) line (the complete map is presented here for the first time). We observe small differences in peak velocities between the different tracers, which cannot always be attributed to geometry but potentially to shear motions. Using the far-infrared ii and ii intensities with the carbon RRL intensities, we can infer physical conditions (electron temperature $T_ e $ and electron density $n_ e $, converted to hydrogen nuclei density $n_ H $ by dividing by the carbon gas-phase abundance $ A C $) in the PDR gas using nonlocal thermal equilibrium excitation models. For positions in OMC1, we infer $n_ e cm^ $ and $T_ e K $. On the border between OMC1 and M43, we observe two gas components with $n_ e cm^ $ and $n_ e cm^ $, and $T_ e K $ and $T_ e K $. In M43, we infer $n_ e cm^ $ and $T_ e K $. The Extended Orion Nebula southeast of OMC1 is characterized by $n_ e cm^ $ and $T_ e K $, while OMC3 has $n_ e cm^ $ and $T_ e K $. Our observations are sensitive enough to detect faint lines toward two positions in OMC1, in the BN/KL PDR and the PDR close to the Trapezium stars, that may be attributed to RRLs of C$^+$ or O$^+$. In general, the RRL line widths of both the ionized and neutral gas, as well as the ii and 13CO line widths, are broader than thermal, indicating significant turbulence in the interstellar medium, which transitions from super-Alfv\'enic and subsonic in the ionized gas to sub-Alfv\'enic and supersonic in the molecular gas. At the scales probed by our observations, the turbulent pressure dominates the pressure balance in the neutral and molecular gas, while in the ionized gas the turbulent pressure is much smaller than the thermal pressure.

On the Question of the Primordial Helium Abundance by the RRL Observations in Orion A

Radio recombination lines comprise a powerful tool for studying the interstellar medium. One of the important tasks is to measure the primordial abundance of helium formed during the primordial nucleosynthesis of the Universe, which, in turn, allows us to verify the conclusions of the Standard Cosmological Model. The Orion A nebula is an interesting object for studying this problem. Previously, we found that, in this HII region, the region of ionized helium is smaller than the region of ionized hydrogen. Therefore, the actual helium abundance, n(He)/n(H), is not less than the maximum measured value of y+ = n(He+)/n(H+). This makes it possible to obtain restrictions on the primordial helium abundance. This article presents new observations of radio recombination lines in Orion A at a wavelength of 13 mm. It was found that the maximum value of y+ is in the range 10.03–11.55%. Therefore, it can be expected that the primordial helium abundance (Yp, He/H mass ratio) can be not less than ≈24.93–29.40%, which allows deviations from the conclusions of the Standard Model; for instance, it allows the presence of unknown light particles during primordial nucleosynthesis. Observations of doubly ionized helium in Orion A and the planetary nebula NGC 7027 were also made. It was found that the contribution of doubly ionized helium, y++ = n(He++)/n(H+), is 7 × 10–4 in Orion A and 2.7(±1.3)% for NGC 7027. Estimates of the electron temperature are also made. In particular, for NGC7027, it was found that Тe ≈ 11 900–12 300 K, which is higher than for the HII regions.

Discovery of Hydrogen Radio Recombination Lines at z = 0.89 toward PKS 1830-211

We report the detection of stimulated hydrogen radio recombination line (RRL) emission from ionized gas in a z = 0.89 galaxy using 580–1670 MHz observations from the MeerKAT Absorption Line Survey. The RRL emission originates in a galaxy that intercepts and strongly lenses the radio blazar PKS 1830−211 (z = 2.5). This is the second detection of RRLs outside of the local Universe and the first clearly associated with hydrogen. We detect effective H144α (and H163α) transitions at observed frequencies of 1156 (798) MHz by stacking 17 (27) RRLs with 21σ (14σ) significance. The RRL emission contains two main velocity components and is coincident in velocity with H i 21 cm and OH 18 cm absorption. We use the RRL spectral line energy distribution and a Bayesian analysis to constrain the density (n e ) and the volume-averaged path length (ℓ) of the ionized gas. We determine cm−3 and pc toward the northeast (NE) lensed image, likely tracing the diffuse thermal phase of the ionized ISM in a thin disk. Toward the southwest (SW) lensed image, we determine cm−3 and pc, tracing gas that is more reminiscent of H scii regions. We estimate a star formation (surface density) rate of ΣSFR ∼ 0.6 M ⊙ yr−1 kpc−2 or SFR ∼ 50 M ⊙ yr−1, consistent with a star-forming main-sequence galaxy of M ⋆ ∼ 1011 M ⊙. The discovery presented here opens up the possibility of studying ionized gas at high redshifts using RRL observations from current and future (e.g., SKA and ngVLA) radio facilities.

Observations of decameter carbon radio recombination lines in several galactic directions. Part 1. Experimental study

Subject and Purpose. Since decameter carbon radio recombination lines (RRLs) were detected for the first time more than forty years ago, they have significantly extended our knowledge of the physics, kinematics and chemistry of the cold rarefied interstellar medium (ISM). A large number of these lines have been observed towards various Galactic radio sources. The present paper describes our studies of decameter carbon RRLs in such Galactic directions as the sight-lines to the S140 emission nebula and to the large volume of cold neutral hydrogen known as the GSH 139-03-69 super shell. Methods and Methodology. Observations within a 1-MHz frequency band centered at 26 MHz were performed using the UTR-2 radio telescope and a multi-channel digital correlator. The UTR-2 is still the world largest and the most sensitive low-frequency radio telescope. Results.We report the detection of decameter carbon RRL series C627α – C637α from the medium lying towards the S140 nebula. The extents of RRL forming regions have been estimated. It is suggested that RRLs in the S140 direction are formed in the local ISM lying along the line of sight. The RRL-forming region is probably associated with omnipresent diffuse neutral HI gas in the Galactic plane rather than with S140 nebula itself. Toward the GSH 139-03-69 super shell, decameter RRLs have been detected as well. Likewise, they apparently originate from the local medium lying along the sight-line. Yet, the spectrum contains a RRL component corresponding to the absorption of the cold gas of the GSH 139-03-69 itself in the ISM. Conclusions. The obtained results indicate great possibilities of decameter carbon RRLs not only for cold ISM probing but also for making a good auxiliary tool for studying large complexes of extremely cold hydrogen HI in the Galaxy.

Non-detection of broad hydrogen radio recombination lines in the Circinus galaxy

ABSTRACT The line widths of broad-line regions (BLRs) of active galactic nuclei (AGNs) are key parameters for understanding central supermassive black holes. However, owing to obscuration from dusty tori, optical recombination lines from BLRs in type II AGNs cannot be directly detected. Radio recombination lines (RRLs), with low extinction, could be ideal tracers to probe the emission from BLRs in type II AGNs. We performed RRL observations for H35α and H36α towards the centre of the Circinus galaxy with Atacama Large Millimeter/submillimeter Array (ALMA). The narrow components of H35α and H36α, which are thought to be mainly from star-forming regions around the nuclear region, are detected. However, only upper limits are obtained for broad H35α and H36α. Because Circinus is one of the nearest AGNs, the non-detection of broad RRLs in Circinus in this band tells us that it is hopeless to try to detect broad RRL emission in local AGNs with current facilities. Submillimetre RRLs, with flux densities that are dozens of times higher than those at the millimetre level, could be the tools to directly detect BLRs in type II AGNs with ALMA, once its backend frequency coverage has been upgraded to several times better than its current capabilities.

A multiwavelength study of the W33 Main ultracompact HII region

Aims. 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. Methods. We used the Giant Meterwave Radio Telescope (GMRT) to observe the H167α recombination line at 1.4 GHz at an angular resolution of 10″, and Karl. G. Jansky Very Large Array (VLA) data acquired in the GLOSTAR survey that stacks six recombination lines from 4–8 GHz at 25″ resolution 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 4-8 GHz continuum data to characterize the nature of the radio emission. In addition, archival data from submillimeter to near-infrared wavelengths were used to study the dust emission and identify young stellar objects in the W33 Main star-forming region. Results. The radio recombination lines were detected at good signal to noise in the GLOSTAR data, while the H167α radio recombination line was marginally detected with the GMRT. The spectral index of radio emission in the region determined from GMRT and GLOSTAR shows the emission to be thermal in the entire region. Along with W33 Main, an arc-shaped diffuse continuum source, G12.81–0.22, was detected with the GMRT data. The GLOSTAR recombination line data reveal a velocity gradient across W33 Main and G12.81–0.22. The electron temperature is found to be 6343 K and 4843 K in W33 Main and G12.81–0.22, respectively. The physical properties of the W33 Main molecular clump were derived by modeling the dust emission using data from the ATLASGAL and Hi-GAL surveys and they are consistent with the region being a relatively evolved site of massive star formation. The gas dynamics and physical properties of G12.81–0.22 are consistent with the HII region being in an evolved phase and its expansion on account of the pressure difference is slowing down.

Radio recombination line observations at 1.0 – 1.5GHz with FAST

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 allow for the most precise determination of physical parameters such as temperature and density. However, RRLs at around 1.4 GHz from H II 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-meter Aperture Spherical radio Telescope (FAST) can simultaneously cover 23 RRLs for Hnα, Henα, and Cnα (n = 164 – 186), respectively. This, combined with its unparalleled collecting area, makes FAST the most powerful telescope to detect weak RRLs. In this pilot survey, we use FAST to observe nine H II regions at L band. We allocate 20 minutes pointing time for each source to achieve a sensitivity of around 9 mK in a velocity resolution of 2.0 kms. In total, 21 RRLs for Hnα and Cnα at 1.0 – 1.5 GHz have been simultaneously detected with strong emission signals. Overall, the detection rates for the H167α and C167α RRLs are 100%, while that for the He167α RRL is 33.3%. Using hydrogen and helium RRLs, we measure the electron density, electron temperature, and pressure for three H II regions. This pilot survey demonstrates the capability of FAST in RRL measurements, and a statistically meaningful sample with RRL detection, through which knowledge about Galactic spiral structure and evolution can be obtained, is expected in the future.

The dense warm ionized medium in the inner Galaxy

Context. Ionized interstellar gas is an important component of the interstellar medium and its lifecycle. The recent evidence for a widely distributed highly ionized warm interstellar gas with a density intermediate between the warm ionized medium (WIM) and compact H II regions suggests that there is a major gap in our understanding of the interstellar gas. Aims. Our goal is to investigate the properties of the dense WIM in the Milky Way using spectrally resolved SOFIA GREAT [N II] 205 μm fine-structure lines and Green Bank Telescope hydrogen radio recombination lines (RRL) data, supplemented by spectrally unresolved Herschel PACS [N II] 122μm data, and spectrally resolved 12CO. Methods. We observed eight lines of sight (LOS) in the 20° < l < 30° region in the Galactic plane. We analyzed spectrally resolved lines of [N II] at 205 μm and RRL observations, along with the spectrally unresolved Herschel PACS 122 μm emission, using excitation and radiative transfer models to determine the physical parameters of the dense WIM. We derived the kinetic temperature, as well as the thermal and turbulent velocity dispersions from the [N II] and RRL linewidths. Results. The regions with [N II] 205 μm emission are characterized by electron densities, n(e) ~ 10−35 cm−3, temperatures range from 3400 to 8500 K, and nitrogen column densities N(N+) ~ 7 × 1016 to 3 × 1017 cm−2. The ionized hydrogen column densities range from 6 × 1020 to 1.7 × 1021 cm−2 and the fractional nitrogen ion abundance x(N+) ~ 1.1 × 10−4 to 3.0 × 10−4, implying an enhanced nitrogen abundance at a distance ~4.3 kpc from the Galactic Center. The [N II] 205 μm emission lines coincide with CO emission, although often with an offset in velocity, which suggests that the dense warm ionized gas is located in, or near, star-forming regions, which themselves are associated with molecular gas. Conclusions. These dense ionized regions are found to contribute ≳50% of the observed [C II] intensity along these LOS. The kinetic temperatures we derive are too low to explain the presence of N+ resulting from electron collisional ionization and/or proton charge transfer of atomic nitrogen. Rather, these regions most likely are ionized by extreme ultraviolet (EUV) radiation from nearby star-forming regions or as a result of EUV leakage through a clumpy and porous interstellar medium.

Time-variable Radio Recombination Line Emission in W49A

Abstract We present new Jansky Very Large Array (VLA) images of the central region of the W49A star-forming region at 3.6 cm and at 7 mm at resolutions of 015 (1650 au) and 004 (440 au), respectively. The 3.6 cm data reveal new morphological detail in the ultracompact H ii region population, as well as several previously unknown and unresolved sources. In particular, source A shows elongated, edge-brightened bipolar lobes, indicative of a collimated outflow, and source E is resolved into three spherical components. We also present VLA observations of radio recombination lines at 3.6 cm and 7 mm, and IRAM Northern Extended Millimeter Array (NOEMA) observations at 1.2 mm. Three of the smallest ultracompact H ii regions (sources A, B2, and G2) all show broad kinematic linewidths, with ΔV FWHM ≳ 40 km s−1. A multi-line analysis indicates that broad linewidths remain after correcting for pressure broadening effects, suggesting the presence of supersonic flows. Substantial changes in linewidth over the 21 yr time baseline at both 3.6 cm and 7 mm are found for source G2. At 3.6 cm, the linewidth of G2 changed from 31.7 ± 1.8 km s−1 to 55.6 ± 2.7 km s−1, an increase of +23.9 ± 3.4 km s−1. The G2 source was previously reported to have shown a 3.6 cm continuum flux density decrease of 40% between 1994 and 2015. This source sits near the center of a very young bipolar outflow whose variability may have produced these changes.

Unusual Galactic H ii Regions at the Intersection of the Central Molecular Zone and the Far Dust Lane

Abstract Sgr E is a massive star formation complex found toward the Galactic center that consists of numerous discrete, compact H ii regions. It is located at the intersection between the central molecular zone (CMZ) and the far dust lane of the Galactic bar, similar to “hot spots” seen in external galaxies. Compared with other Galactic star formation complexes, the Sgr E complex is unusual because its H ii regions all have similar radio luminosities and angular extents, and they are deficient in ∼10 μm emission from their photodissociation regions (PDRs). Our Green Bank Telescope radio recombination line observations increase the known membership of Sgr E to 19 H ii regions. There are 43 additional H ii region candidates in the direction of Sgr E, 26 of which are detected for the first time here using MeerKAT 1.28 data. Therefore, the true H ii region population of Sgr E may number >60. Using APEX SEDIGISM 13CO data we discover a 3.0 × 105 molecular cloud associated with Sgr E, but find few molecular or far-infrared concentrations at the locations of the Sgr E H ii regions. Comparison with simulations and an analysis of its radio continuum properties indicate that Sgr E formed upstream in the far dust lane of the Galactic bar a few million years ago and will overshoot the CMZ, crashing into the near dust lane. We propose that the unusual infrared properties of the Sgr E H ii regions are caused by their orbits about the Galactic center, which have possibly stripped their PDRs.

The GBT Diffuse Ionized Gas Survey: Tracing the Diffuse Ionized Gas around the Giant Hii Region W43

Abstract The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) is a fully sampled radio recombination line (RRL) survey of the inner Galaxy at C-band (4–8 GHz). We average together ∼15 Hnα RRLs within the receiver bandpass to improve the spectral signal-to-noise ratio. The average beam size for the RRL observations at these frequencies is ∼2′. We grid these data to have spatial and velocity spacings of 30″ and 0.5 , respectively. Here we discuss the first RRL data from GDIGS: a 6 deg2 area surrounding the Galactic H II region complex W43. We attempt to create a map devoid of emission from discrete H II regions and detect RRL emission from the diffuse ionized gas (DIG) across nearly the entire mapped area. We estimate the intensity of the DIG emission by a simple empirical model, taking only the H II region locations, angular sizes, and RRL intensities into account. The DIG emission is predominantly found at two distinct velocities: ∼40 and ∼100 . While the 100 component is associated with W43 at a distance of ∼6 kpc, the origin of the 40 component is less clear. Since the distribution of the 40 emission cannot be adequately explained by ionizing sources at the same velocity, we hypothesize that the plasma at the two velocity components is interacting, placing the 40 DIG at a similar distance as the 100 emission. We find a correlation between dust temperature and integrated RRL intensity, suggesting that the same radiation field that heats the dust also maintains the ionization of the DIG.

Radio recombination line observations towards Spitzer infrared bubbles with the TianMa radio telescope

ABSTRACT Many of the Spitzer infrared bubbles identified by the Milky Way Project (MWP) are suggested to be $\rm{H \small {II}} $ regions in nature. More than 70 per cent of the ∼5000 known bubbles do not have radio recombination line (RRL) observations, hence have not been confirmed as $\rm{H \small {II}} $ regions. A systematic RRL survey should be helpful to identify the nature of the bubbles. With the Shanghai TianMa 65-m radio telescope, we searched for RRLs towards 216 selected Spitzer bubbles by simultaneously observing 19 RRLs in the C band (4–8 GHz). RRLs are detected in the directions of 75 of the 216 targets. 31 of the 75 RRL sources are classified as new detections, which are possibly from new $\rm{H \small {II}} $ regions or diffuse warm ionized medium; 36 of them are probably from the outskirts of nearby bright $\rm{H \small {II}} $ regions, rather than bubble-encircled ionized gas; and the detected RRLs towards 8 bubbles are identified from known $\rm{H \small {II}} $ regions. For 58 of the 75 RRL sources, we obtained their distances after resolving the kinematic distance ambiguity by combining the results of the H2CO absorption method, the $\rm{H \small {I}} $ emission/absorption method, and the $\rm{H \small {I}} $ self-absorption method. The low detection rate of new $\rm{H \small {II}} $ regions implies that a number of MWP bubbles in the DR1 catalogue are too faint if they are $\rm{H \small {II}} $ regions.

The first detection of radio recombination lines at cosmological distances

Context. Recombination lines involving high principal quantum numbers (n ∼ 50 − 1000) populate the radio spectrum in large numbers. Low-frequency (< 1 GHz) observations of radio recombination lines (RRLs) primarily from carbon and hydrogen offer a new, if not unique, way to probe cold, largely atomic gas and warm, ionised gas in other galaxies. Furthermore, RRLs can be used to determine the physical state of the emitting regions, such as temperature and density. These properties make RRLs, potentially, a powerful tool of extragalactic interstellar medium (ISM) physics. At low radio frequencies, it is conceivable to detect RRLs out to cosmological distances when illuminated by a strong radio continuum. However, they are extremely faint (τpeak ∼ 10−3 − 10−4) and have so far eluded detection outside of the local universe. Aims. With observations of the radio quasar 3C 190 (z = 1.1946), we aim to demonstrate that the ISM can be explored out to great distances through low-frequency RRLs. Methods. 3C 190 was observed with the LOw Frequency ARray (LOFAR) and processed using newly developed techniques for spectral analysis. Results. We report the detection of RRLs in the frequency range 112 MHz–163 MHz in the spectrum of 3C 190. Stacking 13 α-transitions with principal quantum numbers n = 266 − 301, a peak 6σ feature of optical depth τpeak = (1.0 ± 0.2)×10−3 and FWHM = 31.2 ± 8.3 km s−1 was found at z = 1.124. This corresponds to a velocity offset of −9965 km s−1 with respect to the systemic redshift of 3C 190. Conclusions. We consider three interpretations of the origin of the RRL emission: an intervening dwarf-like galaxy, an active galactic nucleus (AGN) driven outflow, and the inter-galactic medium. We argue that the recombination lines most likely originate in a dwarf-like galaxy (M ∼ 109 M⊙) along the line of sight, although we cannot rule out an AGN-driven outflow. We do find the RRLs to be inconsistent with an inter-galactic medium origin. With this detection, we have opened up a new way to study the physical properties of cool, diffuse gas out to cosmological distances.

New detections of (sub)millimeter hydrogen radio recombination lines towards high-mass star-forming clumps

Aims. 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 3 mm range. Here, we aim to study their excitation with shorter wavelength (sub)millimeter radio recombination line (submm-RRL) observations. Methods. We made observations of submm-RRLs with low principal quantum numbers (n ≤ 30) using the APEX 12 m telescope, toward 104 H II regions associated with massive dust clumps from ATLASGAL. The observations covered the H25α, H28α, and H35β transitions. Toward a small subsample the H26α, H27α, H29α, and H30α lines were observed to avoid contamination by molecular lines at adjacent frequencies. Results. We have detected submm-RRLs (signal-to-noise (S∕N)≥ 3 σ) from compact H II regions embedded within 93 clumps. The submm-RRLs are approximately a factor of two brighter than the mm-RRLs and consistent with optically thin emission in local thermodynamic equilibrium (LTE). The average ratio (0.31) of the measured H35β/H28α fluxes is close to the LTE value of 0.28. No indication of RRL maser emission has been found. The Lyman photon flux, bolometric, and submm-RRL luminosities toward the submm-RRL detected sources present significant correlations. The trends of dust temperature and the ratio of bolometric luminosity to clump mass, Lbol ∕Mclump, indicate that the H II regions are related to the most massive and luminous clumps. By estimating the production rate of ionizing photons, Q, from the submm-RRL flux, we find that the Q(H28α) measurements provide estimates of the Lyman continuum photon flux consistent with those determined from 5 GHz radio continuum emission. Six RRL sources show line profiles that are a combination of a narrow and a broad Gaussian feature. The broad features are likely associated with high-velocity ionized flows. Conclusions. We have detected submm-RRLs toward 93 ATLASGAL clumps. Six RRL sources have high-velocity RRL components likely driven by high-velocity ionized flows. Their observed properties are consistent with thermal emission that correlates well with the Lyman continuum flux of the H II regions. The sample of H II regions with mm/submm-RRL detections probes, in our Galaxy, luminous clumps (Lbol > 104 L⊙) with high Lbol∕Mclump. We also provide suitable candidates for further studies of the morphology and kinematics of embedded, compact H II regions with the Atacama Large Millimeter/submillimeter Array (ALMA).

The Cosmic-Ray Ionization Rate in the Galactic Disk, as Determined from Observations of Molecular Ions

Abstract We have obtained estimates for the cosmic-ray ionization rate (CRIR) in the Galactic disk, using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously published measurements of the abundance of four molecular ions: ArH+, OH+, , and . For diffuse atomic clouds at Galactocentric distances in the range , observations of ArH+, OH+, and imply a mean primary CRIR of per hydrogen atom, where . Within diffuse molecular clouds observed toward stars in the solar neighborhood, measurements of and H2 imply a primary CRIR of per H atom, corresponding to a total ionization rate per H2 molecule of in good accord with previous estimates. These estimates are also in good agreement with a rederivation, presented here, of the CRIR implied by recent observations of carbon and hydrogen radio recombination lines along the sight line to Cas A. Here, our best-fit estimate for the primary CRIR is per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, , with a best-fit dependence for .

Improved Constraints on the Disk around MWC 349A from the 23 m LBTI

Abstract We present new spatially resolved observations of MWC 349A from the Large Binocular Telescope Interferometer (LBTI), a 23 m baseline interferometer made up of two, co-mounted 8 m telescopes. MWC 349A is a B[e] star with an unknown evolutionary state. Proposed scenarios range from a young stellar object, to a B[e] supergiant, to a tight binary system. Radio continuum and recombination line observations of this source revealed a sub-arcsecond bipolar outflow surrounding an ∼100 mas circumstellar disk. Follow-up infrared studies detected the disk, and suggested that it may have skew and an inner clearing. Our new infrared interferometric observations, which have more than twice the resolution of previously published data sets, support the presence of both skew and a compact infrared excess. They rule out inner clearings with radii greater than ∼14 mas. We show the improvements in disk parameter constraints provided by LBTI, and discuss the inferred disk parameters in the context of the posited evolutionary states for MWC 349A.

ATLASGAL-selected massive clumps in the inner Galaxy

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 $\Delta n$ = 1, 2, 3 and 4, i.e. H$n\alpha$, H$n\beta$, H$n\gamma$ and H$n\delta$) using the IRAM 30m and Mopra 22m telescopes. We observed 976 compact dust clumps selected from a catalog of $\sim$10,000 ATLASGAL sources. We detected H$n\alpha$ mm-RRL emission toward 178 clumps; H$n\beta$, H$n\gamma$ and H$n\delta$ were also detected toward 65, 23, and 22 clumps, respectively. This is the largest sample of mm-RRLs detections published to date. Comparing the positions of these clumps with radio continuum surveys we identified compact radio counterparts for 134 clumps. The nature of the other 44 detections is unclear, but 8 detections are thought to be potentially new HII regions. Broad linewidths are seen toward nine clumps (linewidth > 40 km s$^{-1}$) revealing significant turbulent motions within the ionized gas. We find that the linewidth of the H$^{13}$CO$^{+}$ (1$-$0) emission is significantly wider than those without mm-RRL detection. We also find a correlation between the integrated fluxes of the mm-RRLs and the 6 cm continuum flux densities of their radio counterparts. We find that the mm-RRL emission is associated with HII regions with $n_{e}<10^{5}$ cm$^{-3}$ and HII region diameter >0.03 pc. We detected mm-RRLs toward 178 clumps and identified eight new HII region candidates. The broad mm-RRL from nine clumps may indicate that they arise in very young hyper-compact HII regions. The mm-RRLs trace the radio continuum sources detected by high-resolution observations and their line parameters show associations with the embedded radio sources and their parental molecular clumps.

Ionized gas clouds near the Sagittarius Arm tangent

Radio recombination lines (RRLs) are the best tracers of ionized gas. Simultaneous observations of multi-transitions of RRLs can significantly improve survey sensitivity. We conducted pilot RRL observations near the Sagittarius Arm tangent by using the 65-m Shanghai Tian Ma Radio Telescope (TMRT) equipped with broadband feeds and a digital backend. Six hydrogen RRLs (H96 H101α) at C band (6289 MHz–7319 MHz) were observed simultaneously toward a sky area of 2° × 1.2° by using on-the-fly mapping mode. These transitions were then stacked together for detection of ionized gas. Star forming complexes G48.6+0.1 and G49.5−0.3 were detected in the integrated intensity map. We found agreements between our measured centroid velocities and previous results for the 21 known HII regions in the mapped area. For more than 80 cataloged HII region candidates without previous RRL measurements, we obtained new RRL spectra at 30 targeted positions. In addition, we detected 25 new discrete RRL sources with spectral S/N σ, and they were not listed in the catalogs of previously known HII regions. The distances for 44 out of these 55 new RRL sources were estimated.

Low-frequency Carbon Radio Recombination Lines. I. Calculations of Departure Coefficients

Abstract In the first paper of this series, we study the level population problem of recombining carbon ions. We focus our study on high quantum numbers, anticipating observations of carbon radio recombination lines to be carried out by the Low Frequency Array. We solve the level population equation including angular momentum levels with updated collision rates up to high principal quantum numbers. We derive departure coefficients by solving the level population equation in the hydrogenic approximation and including low-temperature dielectronic capture effects. Our results in the hydrogenic approximation agree well with those of previous works. When comparing our results including dielectronic capture, we find differences that we ascribe to updates in the atomic physics (e.g., collision rates) and to the approximate solution method of the statistical equilibrium equations adopted in previous studies. A comparison with observations is discussed in an accompanying article, as radiative transfer effects need to be considered.

LOFAR observations of decameter carbon radio recombination lines towards Cassiopeia A

We present a study of carbon radio recombination lines towards Cassiopeia A using LOFAR observations in the frequency range 10-33 MHz. Individual carbon $\alpha$ lines are detected in absorption against the continuum at frequencies as low as 16 MHz. Stacking several C$\alpha$ lines we obtain detections in the 11-16 MHz range. These are the highest signal-to-noise measurements at these frequencies. The peak optical depth of the C$\alpha$ lines changes considerably over the 11-33 MHz range with the peak optical depth decreasing from 4$\times10^{-3}$ at 33 MHz to 2$\times10^{-3}$ at 11 MHz, while the line width increases from 20 km s$^{-1}$ to 150 km s$^{-1}$. The combined change in peak optical depth and line width results in a roughly constant integrated optical depth. We interpret this as carbon atoms close to local thermodynamic equilibrium. In this work we focus on how the 11-33 MHz carbon radio recombination lines can be used to determine the gas physical conditions. We find that the ratio of the carbon radio recombination lines to that of the 158 $\mu$m [CII] fine-structure line is a good thermometer, while the ratio between low frequency carbon radio recombination lines provides a good barometer. By combining the temperature and pressure constraints with those derived from the line width we are able to constrain the gas properties (electron temperature and density) and radiation field intensity. Given the 1$\sigma$ uncertainties in our measurements these are; $T_{e}\approx68$-$98$ K, $n_{e}\approx0.02$-$0.035$ cm$^{-3}$ and $T_{r,100}\approx1500$-$1650$ K. Despite challenging RFI and ionospheric conditions, our work demonstrates that observations of carbon radio recombination lines in the 10-33 MHz range can provide insight into the gas conditions.