Continuum Source VLA Research Articles

IGRINS NEAR-IR HIGH-RESOLUTION SPECTROSCOPY OF MULTIPLE JETS AROUND LkHα 234*

ABSTRACT We present the results of high-resolution near-IR spectroscopy toward the multiple outflows around the Herbig Be star LkHα 234 using the Immersion Grating Infrared Spectrograph. Previous studies indicate that the region around LkHα 234 is complex, with several embedded young stellar objects and the outflows associated with them. In simultaneous H- and K-band spectra from HH 167, we detected 5 [Fe ii] and 14 H2 emission lines. We revealed a new [Fe ii] jet driven by radio continuum source VLA 3B. Position–velocity diagrams of the H2 1−0 S(1) λ2.122 μm line show multiple velocity peaks. The kinematics may be explained by a geometrical bow shock model. We detected a component of H2 emission at the systemic velocity (V LSR = −10.2 km s−1) along the whole slit in all slit positions, which may arise from the ambient photodissociation region. Low-velocity gas dominates the molecular hydrogen emission from knots A and B in HH 167, which is close to the systemic velocity; [Fe ii] emission lines are detected farther from the systemic velocity, at V LSR = −100–−130 km s−1. We infer that the H2 emission arises from shocked gas entrained by a high-velocity outflow. Population diagrams of H2 lines imply that the gas is thermalized at a temperature of 2500–3000 K and the emission results from shock excitation.

RADIO OBSERVATIONS OF THE STAR FORMATION ACTIVITIES IN THE NGC 2024 FIR 4 REGION

Star formation activities in the NGC 2024 FIR 4 region were studied by imaging centimeter continuum sources and water maser sources using several archival data sets from the Very Large Array. The continuum source VLA 9 is elongated in the northwest-southeast direction, consistent with the FIR 4 bipolar outflow axis, and has a flat spectrum in the 6.2-3.6 cm interval. The three water maser spots associated with FIR 4 are also distributed along the outflow axis. One of the spots is located close to VLA 9, and another one is close to an X-ray source. Examinations of the positions of compact objects in this region suggest that the FIR 4 cloud core contains a single low-mass protostar. VLA 9 is the best indicator of the protostellar position. VLA 9 may be a radio thermal jet driven by this protostar, and it is unlikely that FIR 4 contains a high-mass young stellar object (YSO). A methanol 6.7 GHz maser source is located close to VLA 9, at a distance of about 100 AU. The FIR 4 protostar must be responsible for the methanol maser action, which suggests that methanol class II masers are not necessarily excited by high-mass YSOs. Also discussed are properties of other centimeter continuum sources in the field of view and the water masers associated with FIR 6n. Some of the continuum sources are radio thermal jets, and some are magnetically active young stars.

3D kinematics of the near-IR HH 223 outflow in L723

In this work we derive the full 3-D kinematics of the near-infrared outflow HH 223, located in the dark cloud Lynds 723 (L723), where a well-defined quadrupolar CO outflow is found. HH 223 appears projected onto the two lobes of the east-west CO outflow. The radio continuum source VLA 2, towards the centre of the CO outflow, harbours a multiple system of low-mass young stellar objects. One of the components has been proposed to be the exciting source of the east-west CO outflow. From the analisys of the kinematics, we get further evidence on the relationship between the near-infrared and CO outflows and on the location of their exciting source. The proper motions were derived using multi-epoch, narrow-band H$_2$ (2.122 $\mu$m line) images. Radial velocities were derived from the 2.122 $\mu$m line of the spectra. Because of the extended (~5 arcmin), S-shaped morphology of the target, the spectra were obtained with the Multi-Object-Spectroscopy (MOS) observing mode using the instrument LIRIS at the 4.2m William Herschel Telescope. To our knowledge, this work is the first time that MOS observing mode has been successfully used in the near infrared range for an extended target.

Flares and proper motions of ground-state OH masers in W75N

The star-forming region W75N hosts bright OH masers that are observed to be variable. We present observations taken in 2008 of the ground-state OH maser transitions with the Very Long Baseline Array (VLBA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN) and with the Nancay Radio Telescope in 2011. Several of the masers in W75N were observed to be flaring, with the brightest 1720-MHz maser in excess of 400 Jy. The 1720-MHz masers appear to be associated with the continuum source VLA 1, unlike the bright flaring 1665- and 1667-MHz masers, which are associated with VLA 2. The 1720-MHz masers are located in an outflow traced by water masers and are indicative of very dense molecular material near the H II region. The magnetic field strengths are larger in the 1720-MHz maser region than in most regions hosting only main-line OH masers. The density falls off along the outflow, and the order of appearance of different transitions of OH masers is consistent with theoretical models. The 1665- and 1667-MHz VLBA data are compared against previous epochs over a time baseline of over 7 years. The median maser motion is 3.5 km/s, with a scatter that is comparable to thermal turbulence. The general pattern of maser proper motions observed in the 1665- and 1667-MHz transitions is consistent with previous observations.

HH 223: a parsec-scale H2outflow in the star-forming region L723

The dark cloud Lynds 723 (L723) is a low-mass star-forming region where one of the few known cases of a quadrupolar CO outflow has been reported. Two recent works have found that the radio continuum source VLA 2, towards the centre of the CO outflow, is actually a multiple system of young stellar objects (YSOs). Several line-emission nebulae that lie projected on the east-west CO outflow were detected in narrow-band Halpha and [SII] images. The spectra of the knots are characteristic of shock-excited gas (Herbig-Haro spectra), with supersonic blueshifted velocities, which suggests an optical outflow also powered by the VLA 2 YSO system of L723. We imaged a field of ~5' X 5' centred on HH 223, which includes the whole region of the quadrupolar CO outflow with nir narrow-band filters . The H2 line-emission structures appear distributed over a region of 5.5' (0.5 pc for a distance of 300 pc) at both sides of the VLA 2 YSO system, with an S-shape morphology, and are projected onto the east-west CO outflow. Most of them were resolved in smaller knotty substructures. The [FeII] emission only appears associated with HH 223. An additional nebular emission from the continuum in Hc and Kc appears associated with HH 223-K1, the structure closest to the VLA 2 YSO system, and could be tracing the cavity walls. We propose that the H2 structures form part of a large-scale near-infrared outflow, which is also associated with the VLA 2 YSO system. The current data do not allow us to discern which of the YSOs of VLA 2 is powering this large scale optical/near-infrared outflow.

Millimeter Imaging of the HH 270 Protostellar Core and Outflow

The HH 270 region was observed in the C18O and the 13 CO J = 1 → 0 lines with angular resolutions ranging from 3'' to 10''. The dense molecular core containing the centimeter continuum source VLA 1 has a mass of ~1.6 M☉. As the 13CO image shows that the dense core is elongated in the direction perpendicular to the outflow axis, the core may be flattened and viewed nearly edge-on. The 13CO spectrum shows a blue-skewed line profile when the imaging is done at a high resolution, suggesting that the infall motion becomes kinematically important on a small scale, within ~2000 AU from the center. The molecular outflow does not show a large line width, and the outflow axis may be close to the plane of the sky. Archival near-IR images reveal that there is a nebulosity associated with VLA 1, and that VLA 1 and HH 270 IRS are separate objects. Comparison of the near-IR and optical images with an accurate positional alignment shows that VLA 1 is the driving source of the HH 270 flow. If HH 270 IRS is not a background star, the HH 270 core may contain a binary system with inhomogeneous spectral classes: VLA 1 may be a Class 0 protostar, and HH 270 IRS may be a young stellar object with a hotter spectral energy distribution.

OH maser disc and magnetic field structure in AFGL 2591

We report on Multi-Element Radio-Linked Interferometer Network (MERLIN) polarization measurements of OH 1665- and 1667-MHz masers in the star-forming region, AFGL 2591. The masers lie in an elliptical region of angular diameter 1.5 arcsec near the continuum source VLA 3, which has been proposed as the source powering the bipolar outflow in this region. The elliptical distribution of masers is orthogonal to the outflow, and is consistent with the OH masers tracing a molecular torus of radius ∼750 au, inclined to the line of sight by 55°. The ring has a velocity gradient that is consistent with rotation about a central mass of 31 M○.. Zeeman splitting indicates a magnetic field ranging from -1.6 to +3.8 mG. The field reverses direction on opposite sides of the maser disc, indicating a toroidal component. Polarization of the OH masers in this source is relatively low, typically ∼50 per cent at 1665 MHz. The linear polarization vectors are predominantly either perpendicular or parallel to the outflow direction. The data are interpreted in terms of a twisted magnetic field model.

Mid-infrared images of the star forming region GGD 14 (IRAS 06084-0611)

Mid-infrared images at 8.7, 9.7 and 12.5 $\mu \rm m$ are presented of the star forming region GGD 14 associated with the source IRAS 06084-0611. In an area of ~30″ around the IRAS position, two mid-infrared sources were found. One was identified with the cometary compact HII region VLA 1 and the second with the faint and unresolved radio continuum source VLA 4. The mid-infrared morphology of VLA 1 is very similar to that of the radio continuum suggesting a direct interaction between the ionized gas and the dust. The infrared source associated with VLA 4 shows a large infrared excess and its derived infrared luminosity is $L_{1-20~\mu \rm m} = 350~L_\odot$, indicating the presence of a young star later than B3. In addition, its spectrum between 2.47 and 11.62 $\mu \rm m$ observed by ISOPHOT shows the presence of infrared emission bands at 3.3, 6.2, 7.7, 8.6 and 11.2 $\mu \rm m$ with relative intensity ratios typical of compact HII regions. Finally, the radio continuum source VLA 2, undetected on our mid-infrared images, shows an infrared luminosity of ~$ 8.5~L_\odot$ confirming its nature of an embedded T-Tauri star.

Determining the period of the long-period activity of the water-vapor maser in W75N

We present the results of our study of the H2O maser emission from the source W75N, which is associated with a star-forming region, between November 1994 and March 1999. The observations were carried out with the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory (Lebedev Physical Institute). The maser emission in 1994–1999 can be represented as a superposition of flares of separate components with a duration from two to six months, which occurred mainly in the radial-velocity range 8–17.5 km s−1. We detected a regular drift of the velocity centroid from 13 to 9 km s−1 and an abrupt change in its velocity from 9 to 5 km s−1, which took place at the initial stage of maser activity. Based on the variability of the total H2O flux in all years of our observations of W75N (from December 1979 through March 1999), we conclude that the long-period variability of the water-vapor maser emission has a period of ∼11.5 years. We give arguments that this variability is mainly associated with the most compact group of maser spots, whose positions coincide with the position of the continuum source VLA 2.

The Powering Source and Origin of the Quadrupolar Molecular Outflow in L723

We present the results of single-dish observations of CS J = 2-1 and J = 3-2 and interferometric observations of CO J = 1-0 toward the center of the quadrupolar molecular outflow in L723. We have detected a compact CS condensation having a size of 0.04 pc and a mass of 0.55 M? toward the northeastern radio continuum source VLA 2 (AER91 2). The CO outflow also shows the distribution centered at VLA 2. These results suggest that the source VLA 2 is the young stellar object that is powering the conspicuous molecular outflow system. On the other hand, there is no enhancement in the CS intensity or the CO outflow distribution toward the southwestern radio continuum source VLA 1 (AER91 1), indicating that the source VLA 1 does not contribute to the morphology of the quadrupolar outflow in L723. The CO distribution observed with the interferometer delineates the western edge of the blue lobe and the northeastern edge of the red lobe revealed in the single-dish map, suggesting that the outflow in L723 is a single bipolar outflow with a wide opening angle of 120?-170? rather than two independent outflows. We found signs of interaction between the blueshifted outflow and the dense ambient gas: (1) there is a compact CS clump blueshifted by ~1 km s-1, the distribution of which shows anticorrelation with the blueshifted CO outflow; (2) both CS and NH3 spectra show the line broadening toward the blueshifted clump; and (3) there is a temperature enhancement at the boundary of the blueshifted clump of CS emission. It is likely that such interaction with the dense ambient gas has the increased opening angle of the outflow, which accounts for the quadrupolar morphology.

The Ammonia Core in L723: Hot Spots at the Center of the Quadrupolar Molecular Outflow

We present the results of VLA NH3 and H2O observations, as well as Haystack 13CO, C18O, and H2O observations toward the center of the peculiar quadrupolar molecular outflow in L723. In NH3 we detected a high-density structure, ~17 (~0.15 pc) long, elongated in roughly the east-west direction, with the radio continuum source VLA 2 located near its center. We find evidence in the ammonia maps for heating and line broadening toward VLA 2, confirming that this source is driving the large lobe pair of the quadrupolar molecular outflow. Additionally, H2O maser emission is detected toward this radio continuum source. A second, very compact ammonia hot spot is observed 10'' west of VLA 2. This hot spot may be heated by a deeply embedded (still undetected) young stellar object that could be the driving source of the more compact pair of molecular outflow lobes. No ammonia emission is detected at the position of the source VLA 1, which is probably a line-of-sight source.