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Register a new userSMA Imaging of the Maser Emission from the H30$alpha$ Radio Recombination Line in MWC349A
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We used the Submillimeter Array to map the angular distribution of the H30$alpha$ recombination line (231.9 GHz) in the circumstellar region of the peculiar star MWC349A. The resolution was $1farcs2$, but because of high signal-to-noise ratio we measured the positions of all maser components to accuracies better than $0farcs01$, at a velocity resolution of $1 kms$. The two strongest maser components (called high velocity components) at velocities near -14 and $32 kms$ are separated by $0farcs048 pm 0farcs001$ (60 AU) along a position angle of $102 pm 1arcdeg$. The distribution of maser emission at velocities between and beyond these two strongest components were also provided. The continuum emission lies at the center of the maser distribution to within 10 mas. The masers appear to trace a nearly edge-on rotating disk structure, reminiscent of the water masers in Keplerian rotation in the nuclear accretion disk of the galaxy NGC4258. However, the maser components in MWC349A do not follow a simple Keplerian kinematic prescription with $v sim r^{-1/2}$, but have a larger power law index. We explore the possibility that the high velocity masers trace spiral density or shock waves. We also emphasize caution in the interpretation of relative centroid maser positions where the maser is not clearly resolved in position or velocity, and we present simulations that illustrate the range of applicability of the centroiding method.
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The Submillimeter Array (SMA) has been used to image the emission from radio recombination lines of hydrogen at subarcsecond angular resolution from the young high-mass star MWC349A in the H26$alpha$, H30$alpha$, and H31$alpha$ transitions at 353, 232, and 211 GHz, respectively. Emission was seen over a range of 80 kms-1 in velocity and 50~mas (corresponding to 60~AU for a distance of 1200 pc). The emission at each frequency has two distinct components, one from gas in a nearly edge-on annular disk structure in Keplerian motion, and another from gas lifted off the disk at distances of up to about 25~AU from the star. The slopes of the position-velocity (PV) curves for the disk emission show a monotonic progression of the emission radius with frequency with relative radii of $0.85pm0.04$, 1, and $1.02pm0.01$ for the H26$alpha$, H30$alpha$, and H31$alpha$ transitions, respectively. This trend is consistent with theoretical excitation models of maser emission from a region where the density decreases with radius and the lower transitions are preferentially excited at higher densities. The mass is difficult to estimate from the PV diagrams because the wind components dominate the emission at the disk edges. The mass estimate is constrained to be only in the range of 10--30 solar masses. The distribution of the wind emission among the transitions is surprisingly different, which reflects its sensitivity to excitation conditions. The wind probably extracts significant angular momentum from the system.
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 0farcs15 (1650 au) and 0farcs04 (440 au), respectively. The 3.6~cm data reveal new morphological detail in the ultracompact ion{H}{2} 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 ion{H}{2} regions (sources A, B2 and G2) all show broad kinematic linewidths, with $Delta$V$_{FWHM}gtrsim$40~km~s$^{-1}$. A multi-line analysis indicates that broad linewidths remain after correcting for pressure broadening effects, suggesting the presence of supersonic flows. Substantial changes in linewidth over the 21 year time baseline at both 3.6 cm and 7 mm are found for source G2. At 3.6 cm, the linewidth of G2 changed from 31.7$pm$1.8 km s$^{-1}$ to 55.6$pm$2.7 km s$^{-1}$, an increase of $+$23.9$pm$3.4 km s$^{-1}$. The G2 source was previously reported to have shown a 3.6~cm continuum flux density decrease of 40% between 1994 and 2015. This source sits near the center of a very young bipolar outflow whose variability may have produced these changes.
We present a (sub)millimeter line survey of the methanol maser outflow located in the massive star-forming region DR21(OH) carried out with the Submillimeter Array (SMA) at 217/227 GHz and 337/347 GHz. We find transitions from several molecules towards the maser outflow such as CH$_3$OH, H$_2$CS, C$^{17}$O, H$^{13}$CO$^+$ and C$^{34}$S. However, with the present observations, we cannot discard the possibility that some of the observed species such as C$^{17}$O, C$^{34}$S, and H$_2$CS, might be instead associated with the compact and dusty continuum sources located in the MM2 region. Given that most of transitions correspond to methanol lines, we have computed a rotational diagram with CASSIS and a LTE synthetic spectra with XCLASS for the detected methanol lines in order to estimate the rotational temperature and column density in small solid angle of the outflow where enough lines are present. We obtain a rotational temperature of $28pm 2.5$K and a column density of $6.0pm 0.9 times 10^{15}$ cm$^{-2}$. These values are comparable to those column densities/rotational temperatures reported in outflows emanating from low-mass stars. Extreme and moderate physical conditions to excite the maser and thermal emission coexist within the CH$_3$OH flow. Finally, we do not detect any complex molecules associated with the flow, e.g., CH3OCHO, (CH3)2CO, and CH$_3$CH$_2$CN.
We have detected C91$alpha$ (8.5891 GHz) emission toward 4 ultra-compact HII regions (UCHII s; W49G, J, L & C) in the W49 North massive star forming region with the Very Large Array (VLA) at 3arcsec resolution. No carbon line emission was detected toward UCHII s W49F, A, O, S and Q at this frequency to a 3$sigma$ level of 2 mJy. We also observed the same region in the C75$alpha$ line (15.3 GHz) with no detection at a 3$sigma$ level of 6 mJy with a 1arcsec.7 beam. Detection of line emission toward these sources add supporting data to the earlier result of ocite{retal05a}Roshi et al (2005a) that many UCHII s have an associated photo-dissociation region (PDR). Similarity of the LSR velocities of carbon recombination lines and H$_2$CO absorption toward UCHII s in W49 North suggests that the PDRs reside in the dense interface zone surrounding these HII regions. Combining the observed carbon line parameters at 8.6 GHz with the upper limits on line emission at 15.3 GHz, we obtain constraints on the physical properties of the PDRs associated with W49G and J. The upper limit on the number density of hydrogen molecule obtained from carbon line models is $sim$ $5 times 10^6$ cmthree.
We present Submillimeter Array (SMA) 1.35 mm subarcsecond angular resolution observations toward the LkH{alpha} 234 intermediate-mass star-forming region. The dust emission arises from a filamentary structure of $sim$5 arcsec ($sim$4500 au) enclosing VLA 1-3 and MM 1, perpendicular to the different outflows detected in the region. The most evolved objects are located at the southeastern edge of the dust filamentary structure and the youngest ones at the northeastern edge. The circumstellar structures around VLA 1, VLA 3, and MM 1 have radii between $sim$200 and $sim$375 au and masses in the $sim$0.08-0.3 M$_{odot}$ range. The 1.35 mm emission of VLA 2 arises from an unresolved (r$< 135$ au) circumstellar disk with a mass of $sim$0.02 M$_{odot}$. This source is powering a compact ($sim$4000 au), low radial velocity ($sim$7 km s$^{-1}$) SiO bipolar outflow, close to the plane of the sky. We conclude that this outflow is the large-scale counterpart of the short-lived, episodic, bipolar outflow observed through H$_2$O masers at much smaller scales ($sim $180 au), and that has been created by the accumulation of the ejection of several episodic collimated events of material. The circumstellar gas around VLA 2 and VLA 3 is hot ($sim$130 K) and exhibits velocity gradients that could trace rotation. There is a bridge of warm and dense molecular gas connecting VLA 2 and VLA 3. We discuss the possibility that this bridge could trace a stream of gas between VLA 3 and VLA 2, increasing the accretion rate onto VLA 2 to explain why this source has an important outflow activity.
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