Maser Lines Research Articles

The λ0 = 1.35 cm H2O maser line: The hyperfine structure and profile asymmetry

The spectra of several H2O maser sources exhibit single λ0 = 1.35 cm maser lines with narrow asymmetric profiles. We consider the hyperfine structure of the line that corresponds to the transition between the rotational 616 → 523 levels of ortho-H2O molecules to account for the line asymmetry. Our numerical simulations of the maser line profile agree well with the observations if the hyperfine structure is taken into account.

VLA H53α Radio Recombination Line Observations of the Ultraluminous Infrared Galaxy Arp 220

We present high angular resolution (07) observations made with the Very Large Array (VLA) of the radio recombination line (RRL) H53α and radio continuum emission at 43 GHz from the ultraluminous infrared galaxy (ULIRG) Arp 220. The 43 GHz continuum emission shows a compact structure (~2'') with two peaks separated by ~1'', the east (E) and west (W) components, that correspond to each galactic nucleus of the merger. The spectral indices for both the E and W components, using radio continuum images at 8.3 and 43 GHz are typical of synchrotron emission (α ~ -1.0). Our 43 GHz continuum and H53α line observations confirm the flux densities predicted by the models proposed by Anantharamaiah et al. (2000). This agreement with the models implies the presence of high-density (~105 cm-3) compact H II regions (~0.1 pc) in Arp 220. The integrated H53α line emission is stronger toward the nonthermal radio continuum peaks, which are also coincident with the peaks of molecular emission of the H2CO. The coincidence between the integrated H53α and the H2CO maser line emission suggests that the recent star-forming regions, traced by the high-density gas, are located mainly in regions that are close to the two radio continuum peaks. A velocity gradient of ~0.30 km s-1 pc-1 in the H53α RRL is observed toward the E component, and a second velocity gradient of ~0.15 km s-1 pc-1 is detected toward the W component. The orientations of these velocity gradients are in agreement with previous CO, H I, and OH observations. The kinematics of the high-density ionized gas traced by the H53α line are consistent with two counterrotating disks as suggested by the CO and H I observations.

The OH Maser Line Receiving System for the Urumqi 25m Radio Telescope

A maser spectral line system is newly implemented on the Urumqi 25m Radio Telescope. The system consists mainly of a cooling receiver and a 4096 channels digital correlation spectrometer. The frequency resolution of the spectrometer at the maximum signal bandwidth of 80 MHz is 19.5 kHz. After careful calibrations observation at the 1665 MHz OH maser emission was made towards a number of sources, including W49N and W75N. The observed results demonstrate that the digital correlation spectrometer is suitable for astronomical spectral line observations.

Water masers in the massive protostar IRAS 20126+4104: ejection and deceleration

Received date; accepted date Abstract. We report on the first multi-epoch, phase referenced VLBI observations of the H2O maser emission in a high-mass protostar associated with a disk-jet system. The source under study, IRAS20126+4104, has been extensively investigated in a large variety of tracers, including H2O maser VLBA data acquired by us three years before the present observations. The new findings fully confirm the interpretation proposed in our previous study, namely that the maser spots are expanding from a common origin coincident with the protostar. We also demonstrate that the observed 3-D velocities of the maser spots can be fitted with a model assuming that the spots are moving along the surface of a conical jet, with speed increasing for increasing distance from the cone vertex. We also present the results of single-dish monitoring of the H2O maser spectra in IRAS20126+4104. These reveal that the peak velocity of some maser lines decreases linearly with time. We speculate that such a deceleration could be due to braking of the shocks from which the maser emission originates, due to mass loading at the shock front or dissipation of the shock energy.

Influence of a mesoscopic bath on quantum coherence

For a quantum double well system interacting with a mesoscopic bath, it is shown that a ``bath'' consisting of a single particle is sufficient to substantially reduce tunneling between the two wells. This is demonstrated by considering an ammonia molecule in the center of a ring; in addition to halving the maser line frequency, there is an increase in intensity by four orders of magnitude. The tunneling varies nonmonotonically with the number $N$ of electrons in the ring, reflecting the changing electronic correlations. Although the tunneling is reduced for small $N$, it turns around and grows to its free value for large $N$. This is shown to not violate Anderson's orthogonality theorem. Experimental implementations are discussed.

Probing the Magnetic Field at Subparsec Radii in the Accretion Disk of NGC 4258

We present an analysis of polarimetric observations at 22 GHz of the water vapor masers in NGC 4258 obtained with the VLA and the GBT. We do not detect any circular polarization in the spectrum indicative of Zeeman-induced splitting of the maser lines of water, a non-paramagnetic molecule. We have improved the 1 σ upper limit estimate on the toroidal component of the magnetic field in the circumnuclear disk of NGC 4258 at a radius of 0.2 pc from 300 to 90 mG. We have developed a new method for the analysis of spectra with blended features and derive a 1 σ upper limit of 30 mG on the radial component of the magnetic field at a radius of 0.14 pc. Assuming thermal and magnetic pressure balance, we estimate an upper limit on the mass accretion rate of ~10-3.7 M☉ yr-1 for a total magnetic field of less than 130 mG. We discuss the ramifications of our results on current maser models proposed to explain the observed maser emission structure and the consequences for current accretion theories. We find from our magnetic field limits that the thin-disk model and the jet-disk model are better candidates for accounting for the extremely low luminosity nature of NGC 4258 than models that include advection-dominated accretion flows.

Astrophysical water masers: Line profiles analysis

The changes in the spectral line profile of the 22GHz H2O maser are calculated as a function of emerging maser flux. We address not only the narrowing and re-broadening of the maser lines, but also the deviations of Gaussian symmetry as a result of the various hyperfine components of the 22 GHz maser line. Non-LTE models of the H2O maser transition, including hyperfine structure and cross-relaxation, are compared with high spectral resolution Very Long Baseline Interferometry (VLBI) observations of the H2O masers in the circumstellar envelopes (CSEs) of a sample of late-type stars. This yields estimates on the thermal width in the maser region as well as the emerging maser flux and thus the level of saturation. We also discuss the effect of a velocity gradient along the maser path on the line widths and shapes of the line profile and the effect of the geometry of the maser region. We find that the typical velocity shift along the maser path is of the order of 1.0 km/s. The effect of this shift on the shape of the maser spectrum is difficult to distinguish from the effect of the hyperfine components.

Masers as signposts of high-mass protostars

The 22 GHz H2O maser line was observed towards 79 candidate high-mass protostellar objects from a flux-limited sample of 6.7 GHz methanol sources. The emission was detected in 41 sources, towards 28 of these for the first time. The detection rate of 52% was similar to rates reported for other samples of high-mass protostars selected mainly with far-infrared (FIR) colour criteria. The median value of H2O maser luminosity of 10 −5.5 Lis equal to that of the CH3OH maser luminosity, whereas the median OH maser luminosity was found to be ∼1.5 orders of magnitude lower. Comparison of the velocity ranges showing maser emission implies that for the majority of sources the H2 Oa nd CH 3OH maser lines originate from different regions. The percentage of sources with emission in two or three of the maser species, their association with radio contin- uum and IR emission and the maser and IR luminosities are consistent with the view that evolutionary phases with H2 Oa nd CH3OH masers largely overlap and precede the OH maser phase, while at a later stage OH and CH3OH masers may coexist. Strong correlations of OH and CH3OH maser luminosities with IR luminosity and only a marginal correlation of H2 Oa nd IR luminosity confirm current pumping schemes of all three maser lines.

Very Large Array observations of broad 6-cm excited-state OH lines in W49A

Using the Very Large Array (VLA), we observed all three of the 6-cm lines of the 2 Π 1/2 , J = 1/2 state of OH with sub-arcsecond resolution (∼0.4 arcsec) in W49A. While the spatial distribution and the range in velocities of the 6-cm lines are similar to those of the ground-state (18-cm) OH lines, a large fraction of the total emission in all three 6-cm lines has large linewidths (∼5-10 km s -1 ) and is spatially extended, very unlike typical ground-state OH masers, which typically are point-like at VLA resolutions and have linewidths ≤1 km s -1 . We find brightness temperatures of 5900, 4700 and ≥730 K for the 4660, 4750 and 4765-MHz lines, respectively. We conclude that these are indeed maser lines. However, the gains are ∼0.3, again very unlike the 18-cm lines, which have gains ≥10 4 . We compare the excited-state OH emission with that from other molecules observed with comparable angular resolution to estimate physical conditions in the regions emitting the peculiar, low-gain maser lines. We also comment on the relationship with the 18-cm masers.

First VLBI mapping of circumstellar $\mathsf{^{29}}$SiO maser emission

We report the first VLBI map of the $v=0$ $J=$ 1–0 maser line of 29 SiO in the long-period variable star IRC +10011. We have found that this maser emission is composed of multiple spots distributed in an incomplete ring, suggesting that this maser is also amplified tangentially, as already proposed in other SiO circumstellar masers. We present also VLBI maps for the 7 mm 28 SiO $v=1$ and 2 $J=$ 1–0 and the 3 mm $v=1$ $J=$ 2–1 lines. The 29 SiO masing region appears to be located in a layer in between the 28 SiO $v=1$ $J=$ 1–0 and 28 SiO $v=1$ $J=$ 2–1 lines. In addition, we confirm that the 86 GHz maser $v=1$ $J=$ 2–1 forms in an outer region of the circumstellar envelope compared to the other 28 SiO masers studied. Finally, we discuss the possible implications of the observational results on the SiO maser pumping theory.

Emergence of a Narrow H2O Maser Feature in NGC 1052

We report the emergence of a narrow H2O maser feature with an FWHM of 21 km s-1 in the LINER NGC 1052, which has been known to show only a broad (FWHM > 100 km s-1) maser line profile with relatively bright continuum radio emission. The new narrow maser feature with a peak flux density of 47 mJy at VLSR = 1787 km s-1 is redshifted by 328 km s-1 with respect to the systemic velocity. Broad features with peak velocities of 1510 and 1704 km s-1, more redward than ever observed before, are also detected. The profile of the new narrow feature possibly shows brightening by 16% ± 9% and narrowing by 30% ± 12% between 2003 May 30 and June 2. During the same time, the continuum flux density has increased by 21%. Synchronous variation of maser and continuum flux densities on a timescale of days resembles that in Mrk 348, which is also a broad megamaser source with a bright radio continuum. Continuum and maser brightening and narrowing indicate that an increase of the background seed photon and an increase of maser gain have occurred simultaneously. A jet component running behind a mixture of ionized regions and X-ray dissociation regions at a subrelativistic velocity can produce such short-time variation. Another explanation is an interaction between the jet and molecular clouds.

Methanol Proplyds and a Double Protoplanetary System in the Constellation Norma

We have carried out detailed studies of a star-forming region containing two maser sources in the constellation Norma. The sources display a complex spectral distribution of the maser lines and spatial distribution of the maser condensations. The maser condensations may have formed around objects that are hidden by dense molecular cocoons; the velocities of the maser features may represent Keplerian orbital motions. The cocoons, which radiate in thermal methanol and CS lines, correspond to the centers of mass in the maser sources + dense molecular core systems. The velocities of the CS lines or thermal methanol lines can be used to identify the locations of the centers of mass of these systems. If the maser radiation is generated in the atmospheres of protoplanets, the Norma radio source may correspond to two protoplanetary disks, each with a protostar and protoplanetary system. In this case, the masses of the protostars are approximately 13 Mand 38 M� . c � 2005Pleiades Publishing, Inc. Radio-spectroscopic observations of star-forming regions containing dense clumps of gas and dust with various molecular compounds are very promising for studies of the physical states of prestellar material. Radio lines emitted by such clumps can be related to various temporal, dynamical, and thermal evolu- tionary phases of the interstellar medium, and the parameters obtained from the observed emission of various molecules can be used to reconstruct the physical conditions in star-forming regions. Some molecules in prestellar condensates are subject to intense irradiation from neighboring, recently formed, young stars or to dynamical compression by flows of matter from neighboring stars, both of which can act to redistribute the level populations. This can result in nonequilibrium (maser) radiation by these molecules, which are virtually always found in associ- ation with protostellar objects. The spatial structures of the maser condensations shows that they are often laid out in chains or arcs, sometimes displaying char- acteristics consistent with Keplerian disks containing protoplanetary systems. Such disks are called proto- planetary disks, or proplyds (1).

Accurate rest frequencies of methanol maser and dark cloud lines

We report accurate laboratory measurements of selected methanol transition frequencies between 0.834 and 230 GHz in order to facilitate astronomical velocity analyses. New data have been obtained between 10 and 27 GHz and between 60 and 119 GHz. Emphasis has been put on known or potential interstellar maser lines as well as on transitions suitable for the investigation of cold dark clouds. Because of the narrow line widths (<0.5 kms-1) of maser lines and lines detected in dark molecular clouds, accurate frequencies are needed for comparison of the velocities of different methanol lines with each other as well as with lines from other species. In particular, frequencies for a comprehensive set of transitions are given which, because of their low level energies (< 20 cm-1 or 30 K) are potentially detectable in cold clouds. Global Hamiltonian fits generally do not yet yield the required accuracy. Additionally, we report transition frequencies for other lines that may be used to test and to improve existing Hamiltonian models.

H 2 O Masers in W49 North and Sagittarius B2

Using the Very Large Array (VLA) of the National Radio Astronomy Observatory in the A and B configurations, we have obtained simultaneous high-resolution observations of both the 22 GHz H2O maser lines as well as the 22 GHz continuum for the H II regions W49N and Sagittarius B2. The angular resolution of both observations is ~ 01, which at the distance of W49N (11.4 kpc) and Sgr B2 (8.5 kpc) corresponds to a physical size of less than 1000 AU in both sources. The velocity coverage for W49N is ±435 km s-1; positions for 316 H2O maser components were obtained. The velocity coverage for Sgr B2 is -40 to +120 km s-1; positions for 68 maser components were determined in Sgr B2 Main, 79 in Sgr B2 North, 14 in Sgr B2 Mid-North, and 17 in Sgr B2 South, for a total of 178 H2O maser positions in Sgr B2.

Searching for high-redshift centimeter-wave continuum, line and maser emission using the Square Kilometer Array

We discuss the detection of redshifted line and continuum emission at radio wavelengths using a Square Kilometer Array (SKA), specifically from low-excitation rotational molecular line transitions of CO and HCN (molecular lines), the recombination radiation from atomic transitions in almost-ionized hydrogen (radio recombination lines; RRLs), OH and H 2O maser lines, as well as from synchrotron and free–free continuum radiation and HI 21-cm line radiation. The detection of radio lines with the SKA offers the prospect to determine the redshifts and thus exact luminosities for some of the most distant and optically faint star-forming galaxies and active galactic nuclei, even those galaxies that are either deeply enshrouded in interstellar dust or shining prior to the end of reionization. Moreover, it provides an opportunity to study the astrophysical conditions and resolved morphologies of the most active regions in galaxies during the most active phase of star formation at redshift z ∼ 2. A sufficiently powerful and adaptable SKA correlator will enable wide-field three-dimensional redshift surveys at chosen specific high redshifts, and will allow new probes of the evolution of large-scale structure (LSS) in the distribution of galaxies. The detection of molecular line radiation favours pushing the operating frequencies of SKA up to at least 26 GHz, and ideally to 40 GHz, while very high redshift maser emissions requires access to about 100 MHz. To search for LSS the widest possible instantaneous field of view would be advantageous.

SiO Maser Survey of IRAS Sources in the Inner Galactic Disk

Abstract We surveyed 401 color-selected IRAS sources in the galactic disk in the SiO $J = 1 \hbox{--} 0, v = 1$ and 2 maser lines at 43 GHz, resulting in 254 (239 new) detections. The observed sources lie mostly in a strip of the inner galactic disk with boundaries $-10^{\circ} &amp;lt; l &amp;lt; 40^{\circ}$ and $\vert b\vert &amp;lt; 3^{\circ}$. This survey provides radial velocities of inner-disk stars for which optical measurements cannot be made due to interstellar extinction. The SiO $l \hbox{--} v$ diagram in the area $-10^{\circ} &amp;lt; l &amp;lt; 40^{\circ}$ exhibits fewer objects coincident with the molecular ring feature than does the OH 1612 MHz source $l \hbox{--} v$ map, indicating a slight difference of stellar type between SiO and OH emitting stars. After identifying all of the SiO detected sources in the 2MASS near-infrared catalog, we computed their luminosity distances based on the infrared fluxes. We then mapped these objects onto the first quadrant of the galactic plane. Combining the distances with the SiO radial velocities, we obtained a pattern speed for SiO maser sources, $\Omega_{\mathrm{P}} = 21 (\pm 13) \,\mathrm{km} \,\mathrm{s}^{-1} \,\mathrm{kpc}^{-1}$, between the distances 1 and 5.5 kpc, without the use of any dynamical models. The increase of the pattern speed toward the galactic center (up to $60 \,\mathrm{km} \,\mathrm{s}^{-1} \,\mathrm{kpc}^{-1}$ between the distances, 5.5 and 7 kpc) suggests the presence of two pattern speeds in the Galaxy.

A statistical analysis of methanol maser groups

An analysis of the flux densities of the 51-60A+ (6.7 GHz) and 20-3−1E (12.2 GHz) class II methanol maser lines in a large and homogeneous sample of maser sources has been carried out. For convenience, the maser lines were divided into three groups: group I contains spectral features for the lines most prominent in the 51-60A+ (6.7 GHz) transition, group II contains spectral features for the lines strongest in the 20-3−1E (12.2 GHz) transition, group III contains spectral features for which the velocities of the emission maxima of the two lines coincide. The same dependence was found for group II and group III: log S6.7=(0.79±0.05)×log S12.2+(0.79±0.05). The spectral features in group I do not obey this relation, and deviations from a linear dependence are considerably greater. It is suggested that methanol class II masers be divided into a subclass IIa, which has special conditions favoring 6.7 GHz masers, and a subclass IIb, which is comprised of the 12.2 GHz masers and those 6.7 GHz masers that necessarily accompany them under the same conditions.

Multiple Outflows in the LkHα 234 Region

We report results of radio continuum (1.3 and 3.6 cm) and H2O maser line observations, carried out with the Very Large Array (VLA) in its A configuration, toward the star-forming region LkHα 234. We detected five radio continuum sources (VLA 1, VLA 2, VLA 3A, VLA 3B, and LkHα 234) in a region of 5'' (5000 AU), of which three were previously unknown (VLA 1, VLA 2, and VLA 3B). VLA 3A and VLA 3B seem to form a close (220 AU) binary system. Their elongated morphologies and positive spectral indices suggest that both VLA 3A and VLA 3B could be thermal radio jets. In addition, we detected three clusters of water masers, which are spatially associated with VLA 1, VLA 2, and VLA 3B. Based on the analysis of the distribution of the water masers and the characteristics of the continuum emission, we favor the new radio continuum source VLA 2 as the exciting source of the large-scale CO/[S II] outflow observed in the region. Moreover, we find that the multiple outflows observed in the region share a similar orientation. Finally, our data confirm that there is no evidence indicating that the Herbig Be star LkHα 234 is driving any of the outflows in the region.

$\boldmath\mathsf{^{28}}$SiOv = 1 andv = 2,J = 1–0 maser variability in evolved stars. Eleven years of short spaced monitoring

This paper presents and discusses the final data set of a long-term and short-spaced monitoring of 21 SiO maser sources, mostly evolved stars, carried out in two SiO maser lines at 43 GHz with the Observatorio Astronomico Nacional 13.7 m telescope at the Centro Astronomico de Yebes (Guadalajara, Spain). In most objects, more than 80 spectra per transition over a period of 11 years have been recorded. The new data presented here, previously unpublished, represent nearly 50% of the total SiO data collected in the project. In addition, the availability of optical light curves from the AAVSO for most of the objects during the whole period of the SiO monitoring, ground-based near-IR data for four sources overlapping with 3 to 5 observed SiO periods, and DIRBE near-IR data covering a significant portion of an SiO period in 10 sources, make this data set a unique reference for comparing optical, NIR and SiO variability in order to elucidate the physical mechanisms that pump SiO masers in evolved stars. The basis for the conclusions obtained in this work comes from a numerical time series analysis of the suitable SiO, optical and NIR light curves in regular variables to obtain precise values of the periods and phase lags between the different curves. This analysis shows evidence that in regular variable evolved stars the three types of emission have the same period and that the SiO maxima happen in phase with NIR maxima and with a phase lag typically between 0.05 and 0.20 with respect to optical maxima. We conclude that in these objects the observational evidence presented in this work favors the radiative pumping of SiO masers against the collisional pumping.

Polarimetric observations of OH masers in proto-planetary nebulae

The 1612 and 1667 MHz OH maser lines have been measured in all four Stokes parameters in 47 proto-planetary nebula (PPN) candidates. Out of 42 objects detected, 40 and 34 are 1612 and 1667 MHz emitters, respectively. The spectral extent of the 1667 MHz line overshoots that of the 1612 MHz line in about 80% of the targets. 52% and 26% of the 1612 and 1667 MHz sources, respectively, show linear polarization in at least some features. Circular polarization is more frequent, occurring in 78% and 32% of sources of the respective OH lines. The percentage polarization is usually small (<15%) reaching up to 50-80% in a few sources. Features of linearly polarized emission are usually weak (0.5-4 Jy) and narrow (0.3-0.5 km s -1 ). The strength of the magnetic field inferred from likely Zeeman pairs in two sources of a few mG is consistent with values reported elsewhere for those classes of objects. An upper limit of the electron density in the envelope of OH17.7-2.0 derived from the difference in the position angle of polarization vectors for the two OH lines is about 1 cm -3 . Distinct profiles of polarization position angle at 1612 and 1667 MHz are seen in about one third of the sources and strongly suggest that the envelopes are permeated by structured magnetic fields. The geometry of the magnetic field is implicated as an important cause of the depolarization found in some PPN candidates. For the subset of targets which show axisymmetric shells in the optical or radio images we found a dominance of magnetic field components which are orthogonal to the long axis of the nebulae. This finding supports the hypothesis that such bipolar lobes are shaped by the magnetic field.