Radio Recombination Line Observations Research Articles

4He abundances: Optical versus radio recombination line measurements

AbstractAccurate measurements of the 4He/H abundance ratio are important in constraining Big Bang nucleosynthesis, models of stellar and Galactic evolution, and H ii region physics. We discuss observations of radio recombination lines using the Green Bank Telescope toward a small sample of H ii regions and planetary nebulae. We report 4He/H abundance ratio differences as high as 15–20% between optical and ratio data that are difficult to reconcile. Using the H ii regions S206 and M17 we determine 4He production in the Galaxy to be dY/dZ = 1.71 ± 0.86.

Determination of the primordial helium abundance from radio recombination line observations

We describe a method for determining the primordial helium abundance (Y p) from radio recombination lines and present results of the first stage of our measurements. We analyze the observational data for hydrogen and helium radio recombination lines from six Galactic H II regions obtained at different times. We have found Y p = 25.2−25.5% with an error of ±0.9% from four sources.

THE ENORMOUS OUTER GALAXY H II REGION CTB 102

We present new radio recombination line observations of the previously unstudied H II region CTB 102. Line parameters are extracted and physical parameters describing the gas are calculated. We estimate the distance to CTB 102 to be 4.3 kpc. Through comparisons with H I and 1.42 GHz radio continuum data, we estimate the size of CTB 102 to be 100-130 pc, making it one of the largest H II regions known, comparable to the W4 complex. A stellar wind blown bubble model is presented as the best explanation for the observed morphology, size, and velocities.

RADIO RECOMBINATION LINES TOWARD THE GALACTIC CENTER LOBE

The Galactic Center lobe is a degree-tall shell seen in radio continuum images of the Galactic center (GC) region. If it is actually located in the GC region, formation models would require massive energy input (e.g., starburst or jet) to create it. At present, observations have not strongly constrained the location or physical conditions of the GC lobe. This paper describes the analysis of new and archival single-dish observations of radio recombination lines toward this enigmatic object. The observations find that the ionized gas has a morphology similar to the radio continuum emission, suggesting that they are associated. We study averages of several transitions from H106alpha to H191epsilon and find that the line ratios are most consistent with gas in local thermodynamic equilibrium. The radio recombination line widths are remarkably narrow, constraining the typical electron temperature to be less than about 4000 K. These observations also find evidence of pressure broadening in the higher electronic states, implying a gas density of n_e=910^{+310}_{-450} cm^{-3}. The electron temperature, gas pressure, and morphology are all consistent with the idea that the GC lobe is located in the GC region. If so, the ionized gas appears to form a shell surrounding the central 100 parsecs of the galaxy with a mass of roughly 10^5 Msun, similar to ionized outflows seen in dwarf starbursts.

Internal Dynamics of the Hypercompact HiiRegion G28.20−0.04N

High-resolution (0.15'') Very Large Array observations of 7 mm continuum and H53α line emission toward the hypercompact H II region G28.20-0.04N reveal the presence of large-scale ordered motions. We find a velocity gradient of 103 km s−1 pc−1 along the minor axis of the continuum source. Lower resolution (1.0''-2.3'') radio recombination line observations indicate a systematic increase of line width from H30α to H92α. Under the assumption that the H30α line does not suffer significant pressure broadening, we have deconvolved the contributions of statistical broadening (thermal, turbulent, and pressure) from large-scale motions. The pressure broadening of the H53α, H76α, and H92α lines implies an electron density of 7 × 106, 9 × 105, and 3 × 105 cm−3, respectively.

GMRT and VLA observations at 49 cm and 20 cm of the HII region near l = 24.8°, b = 0.1°

We report multi-frequency radio continuum and hydrogen radio recombination line observations of HII regions near l = 24.8°, b = 0.1° using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n = 172), 0.61 GHz (n = 220) and the Very Large Array (VLA) at 1.42 GHz (n = 166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l = 24.83° and b = 0.1° is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ∼550 M⊙ with a linear size of 7 pc and an rms electron density of ∼110 cm−3 at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5.We also report detection of hydrogen recombination lines from the HII region at l = 24.83° and b = 0.1° at all observed frequencies near V lsr = 100 km s−1. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.

On the Distance and Structure of W80

The continuum radiation of the Galactic region located at l = 85°, b = -0.5° is well defined by its 11 cm emission. The region, extended 3° in diameter, is cataloged as W80, and its optical images show the North America and Pelican Nebulae complexes. In this paper we present new distance measurements and physical parameters obtained from radio recombination line observations at a frequency near 1.4 GHz. Four structures are identified in the region under study: a near structure is located at a distance of about 0.7 kpc, another structure lies on the east side of W80 at a distance of 1.7 kpc, there is a concentration of ionized gas on the west side at a distance of 2.7 kpc, and a fourth structure is found further away at 3.3 kpc, which does not seem to belong to the W80 complex. The results are presented using the (X, Y, Z)-coordinates of the Galactic plane. The model used assumes a constant electron temperature Te = 6800 K and also assumes homogeneity. Our results seem to indicate that the ionized hydrogen of W80 spreads along the line of sight instead of clumping at one distance. We obtain values of the electron density and the ionization parameter between ne = 5 and 16 cm-3 and U = 51 and 110 cm-2 pc, respectively.

The Electron Temperature Gradient in the Galactic Disk

We derive the electron temperature gradient in the Galactic disk using a sample of HII regions that spans Galactocentric distances 0--17 kpc. The electron temperature was calculated using high precision radio recombination line and continuum observations for more than 100 HII regions. Nebular Galactocentric distances were calculated in a consistent manner using the radial velocities measured by our radio recombination line survey. The large number of nebulae widely distributed over the Galactic disk together with the uniformity of our data provide a secure estimate of the present electron temperature gradient in the Milky Way. Because metals are the main coolants in the photoionized gas, the electron temperature along the Galactic disk should be directly related to the distribution of heavy elements in the Milky Way. Our best estimate of the electron temperature gradient is derived from a sample of 76 sources for which we have the highest quality data. The present gradient in electron temperature has a minimum at the Galactic Center and rises at a rate of 287 +/- 46 K/kpc. There are no significant variations in the value of the gradient as a function of Galactocentric radius or azimuth. The scatter we find in the HII region electron temperatures at a given Galactocentric radius is not due to observational error, but rather to intrinsic fluctuations in these temperatures which are almost certainly due to fluctuations in the nebular heavy element abundances. Comparing the HII region gradient with the much steeper gradient found for planetary nebulae suggests that the electron temperature gradient evolves with time, becoming flatter as a consequence of the chemical evolution of the Milky Way's disk.

Radio Recombination Lines in Galactic H ii Regions

We report radio recombination line (RRL) and continuum observations of a sample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m) radio telescope in Green Bank, West Virginia. We believe this to be the most sensitive RRL survey ever made for a sample this large. Most of our source integration times range between 6 and 90 hr, yielding typical rms noise levels of ~1.0-3.5 mK. Our data result from two different experiments performed, calibrated, and analyzed in similar ways. A C II survey was made at the 3.5 cm wavelength to obtain accurate measurements of carbon radio recombination lines. When combined with atomic (C I) and molecular (CO) data, these measurements will constrain the composition, structure, kinematics, and physical properties of the photodissociation regions that lie on the edges of H II regions. A second survey was made at the 3.5 cm wavelength to determine the abundance of 3He in the interstellar medium of the Milky Way. Together with measurements of the 3He+ hyperfine line, we get high-precision RRL parameters for H, 4He, and C. Here we discuss significant improvements in these data with both longer integrations and newly observed sources.

Radio and infrared recombination studies of the southern massive star forming region G333.6-0.2

We present high spatial resolution radio and near-infrared hydrogen recombination line observations of the southern massive star-forming region G333.6 0.2. The 3.4-cm continuum peak is found slightly offset from the infrared source. The H90α spectra show for the first time a double peak profile at some positions. The complex velocity s tructure may be accounted for by champagne outflows, which may also explain the offset betw een the radio and infrared sources. The 2.17-µm Brγ image and H90α map are combined to construct an extinction map which shows a trend probably set by the blister nature of the H II region. The total number of Lyman continuum photons in the central 50-arcsec is estimated to be equivalent to that emitted by up to 19 O7V stars.

The Chemical Evolution of Helium

We report on measurements of the 4He abundance toward the outer Galaxy H II region S206 with the NRAO Green Bank Telescope. Observations of hydrogen and helium radio recombination lines between 8 and 10 GHz were made toward the peak radio continuum position in S206. We derive 4He/H = 0.08459 ± 0.00088 (random) ± 0.0010 (known systematic), 20% lower than optical recombination line results. It is difficult to reconcile the large discrepancy between the optical and radio values even when accounting for temperature, density, and ionization structure or for optical extinction by dust. Using only M17 and S206 we determine ΔY/ΔZ = 1.41 ± 0.62 in the Galaxy, consistent with standard chemical evolution models. High helium abundances in the old stellar population of elliptical galaxies can help explain the increase in emission with shorter wavelength between 2000 and 1200 A, called the UV upturn or UVX. Our lower values of ΔY/ΔZ are consistent with a normal helium abundance at higher metallicity and suggest that other factors, such as a variable red giant branch mass loss with metallicity, may be important. When combined with 4He abundances in metal-poor galaxy H II regions, Magellanic Cloud H II regions, and M17 that have been determined from optical recombination lines, including the effects of temperature fluctuations, our radio 4He/H abundance ratio for S206 is consistent with a helium evolution of ΔY/ΔZ = 1.6. A linear extrapolation to zero metallicity predicts a 4He/H primordial abundance ratio about 5% lower than that given by the Wilkinson Microwave Anisotropy Probe and standard big bang nucleosynthesis. The measured 4He abundances may be systematically underestimated by a few percent if clumping exists in these H II regions.

High-mass star formation within the bright-rimmed cloud SFO 79

We report Radio Recombination Line (RRL) and continuum observations toward the IRAS point source 16362−4845, embedded within the Bright-Rimmed Cloud (BRC) SFO 79, a small molecular cloud lying at the edge of the HII region RCW 108. High resolution observations of the H92α hydrogen recombination line and of the continuum emission (3.6 and 6 cm) confirm the presence of a resolved Ultra Compact (UC) HII region embedded within the molecular cloud. The integrated radio fluxes suggest the source of the ionisation to be an O9 Zero Age Main Sequence (ZAMS) star. Millimetre observations of 12 CO, 13 CO and C 18 O( J = 1-0) molecular lines reveal the presence of a molecular condensation offset ∼30 �� to the north of the IRAS position on the boundary of the UC HII region. Analysis of 2MASS data has led to the identification of a small IR cluster of Young Stellar Objects (YSOs) that are positionally coincident with the UC HII region, lying to the south east of the peak of the radio emission. Moreover, the UC HII region appears to be extended in the direction of the IR cluster, which suggests that the radio emission and the IR cluster are in some way related to each other. MSX 8.3 µm and 21.3 µ mi mages have been used to trace the large scale structure of the BRC, revealing the presence of a Photo Dominated Region (PDR) and three embedded thermal sources within the molecular cloud. The PDR has a plane parallel morphology which correlates extremely well with the morphology of the ionised gas traced by the optical emission. The three thermal sources (labelled A, B, C) all lie at a similar projected distance from the interface between the HII region and the molecular gas of the cloud. Thermal sources A and C are positionally coincident with the IRAS point sources 16362-4845 and 16362-4841 respectively, both of which have IRAS colours consistent with the presence of UC HII regions. Given that UC HII regions are relatively short lived (∼10 5 yrs) it is reasonable to suggest that these two UC HII regions are of a similar age. The alignment of the three thermal sources along a line parallel to the bright rim suggests that they could have been triggered by the propagation of a plane parallel shock through the cloud.

PRIMORDIAL HELIUM ABUNDANCE BY RRL: NEW RESULT

NewresultsofthePrimordial Helium abundance (Yp) measurement by radio recombination lines (RRL) observations from galactic HII regions are presented. The RRL observations were carried out with two Parabolas: RT32(‚ =13.5 mm and 3.5 cm, Medicina, Italy) and RT22 (‚ = 8 and 13.5 mm, Pushchino, Russia). The current value was obtained on the base of flve source observations with the accurate correction for both the ionization structureanddustin∞uence. Atpresent,theobtained Yp=25:74(§1:0)%valueseemstobehigherthanthat suggested by optical data, allowing the presence of unknown light particles, and is in agreement with the result from cosmic microwave background asymmetry.

The electron temperature and density in interstellar medium by using carbon radio recombination line observations

The intensities of carbon radio recombination lines (RRL’s) are definedallowing for the effect of dielectronic-like recombination. The rate ofdielectronic-like recombination is calculated as functions of line number,electron density and temperature accurate to 0.05. Following from the balanceequation solutions for populations, the RRL intensities are analytically foundby the method of successive approximations to an accuracy of 0.15. Theobservations of carbon RRL’s are analyzed toward Cassiopeia A. The averageelectron temperature, density, expanded CII region lengths and inaccuraciesare found with the experimental values of RRL widths and intensities.

Identification of the ionizing source of NGC 2024

We propose the late-O, early-B star IRS2b as the ionizing source of the Flame Nebula (NGC 2024). It has been clear that such a hot, massive star must be present in this heavily obscured region, and now it has been identified. New near-infrared photometry shows that IRS2b is the most luminous and hottest star in the young star cluster embedded in the center of NGC 2024. The near-infrared observations (5' x 5') cover ~90 % of the HII region detected in radio continuum radiation, making the probability very low that the ionizing star is not present in the field. A K-band spectrum of IRS2b obtained with ISAAC on the Very Large Telescope indicates that the spectral type of IRS2b is in the range O8V - B2V. Additional arguments indicate that its spectral type is likely closer to O8 than to B2. The corresponding amount of ionizing radiation is consistent with published radio continuum and recombination line observations.

Radio Recombination Lines as a Tool for Study Dynamics of Planetary Nebulae

An observational program for study dynamics of planetary nebulae with the help of RRL is discussed. The expansion velocities may be found from observations of radio recombination lines in different wavelength. The method is free of the microturbulence problem inherent in standard spectroscopic methods. Set of theoretical models was computed for NGC 7027. Observations of RRL with n > 150 may allow making choice between models with high and low microturbulence.

On the ionizing source of the Flame Nebula (NGC 2024)

We have identified IRS 2b as the ionizing source of the Flame Nebula (NGC 2024). For several decades it has been clear that such a hot, massive star must be present in this heavily obscured region, and now it has been found. New J- and K-band photometry and a VLT-isaac K-band spectrum show that IRS 2b is an 08 - 09.7 V star, consistent with published radio continuum and recombination line observations. IRS 2b seems to be slightly evolved from the zero-age main sequence.

Rosette Nebula: RRL observations of 92 α transition and the modelling

Observations of H92 α radio recombination line (RRL) and continuum emission were made towards three Celnik positions of the Rosette Nebula. The electron temperature T e(LTE) was derived for each position giving an average value of 7980±580 K. A non-LTE modelling was made which could describe the dependence of line contrast ( T L/ T C), T e(LTE) and line width on quantum number ( n). The modelling together with a review of available data show that the Rosette Nebula should contain clumps with densities up to ∼10 3 cm −3. The unexpected V LSR measured at one of the three positions could be interpreted as a possible jet, or some dense compact part (clump) of the HII region, moving toward the observer.

Maser effects in the recombination lines of Eta Carinae

Eta Car is a variable star surrounded by a cloud of gas and dust that absorbs a large part of its optical emission and that showed, in the past two hundred years, several events of mass ejection. It presents a 5.52 year period in the high excitation Heλ 10830 line intensity, indicating periodic changes in the ionizing radiation. Changes in the central velocity of optical lines, with the same periodicity, suggests the presence of a companion star, in a binary orbit. At radio frequencies, the region shows an optically thick bremsstrahlung spectrum, extending to millimeter and submillimeter wavelengths, and strong recombination lines at low quantum numbers showing maser effects. The flux densities, both from the continuum and recombination lines show a periodic behavior similar to that observed at optical wavelengths. We present observations of radio continuum and recombination lines at different millimeter wavelengths for several epochs. Based on these observations we modeled the emitting region as a thin disk and determined its temperature, electron density and velocity distributions using the radiation transfer equations in NLTE.

Preliminary Results of Galactic Radio Recombination Line Observations using the GMRT

A pilot project to observe recombination lines of hydrogen and carbon from Galactic HII regions near 20cm (n = 168) and 49cm (n = 220) using the 30-station correlator at the Giant Metre-wave Radio Telescope was undertaken. The preliminary results from observations of the HII regions W3 and S106 are presented here.