104 research outputs found
New H_(2)O masers in Seyfert and FIR bright galaxies: III. The southern sample
Context. A relationship between the water maser detection rate and far infrared (FIR) flux densities was established as a result of two 22 GHz maser surveys in a complete sample of galaxies (Dec > −30°) with flux densities of >50 Jy and >30 Jy.
Aims. We attempted to discover new maser sources and investigate the galaxies hosting the maser spots by extending previous surveys to southern galaxies with particular emphasis on the study of their nuclear regions.
Methods. A sample of 12 galaxies with Dec 50 Jy was observed with the 70-m telescope of the Canberra deep space communication complex (CDSCC) at Tidbinbilla (Australia) in a search for water maser emission. The average 3σ noise level of the survey was 15 mJy for a 0.42 km s^(−1) channel, corresponding to a detection threshold of ∼0.1 L_☉ for the isotropic maser luminosity at a distance of 25 Mpc.
Results. Two new detections are reported: a kilomaser with an isotropic luminosity L_(H_(2)O) ~ 5 L_☉ in NGC 3620 and a maser with about twice this luminosity in the merger system NGC 3256. The detections have been followed-up by continuum and spectral line interferometric observations with the Australia Telescope Compact Array (ATCA). In NGC 3256, a fraction (about a third) of the maser emission originates in two hot spots associated with star formation activity, which are offset from the galactic nuclei of the system. The remaining emission may originate in weaker centres of maser activity distributed over the central 50". For NGC 3620, the water maser is coincident with the nuclear region of the galaxy. Our continuum observations indicate that the nature of the nuclear emission is probably linked to particularly intense star formation. Including the historical detection in NGC 4945, the water maser detection rate in the southern sample is 15% (3/20), consistent with the northern sample. The high rate of maser detections in the complete all-sky FIR sample (23%, 15/65) confirms the existence of a link between overall FIR flux density and maser phenomena. A relation between H_(2)O and OH masers in the FIR sample is also discussed
VLBI observations of the H2O gigamaser in TXS2226-184
Outside the Milky Way, the most luminous H2O masers at 22 GHz, called
'megamasers' because of their extreme luminosity with respect to the Galactic
and extragalactic H2O masers associated with star formation, are mainly
detected in active galactic nuclei. In the case of the H2O maser detected in
the nuclear region of the galaxy TXS2226-184 for the first time the term
'gigamaser' was used. However, the origin of this very luminous H2O maser
emission has never been investigated into details. We study the nature of the
H2O gigamaser in TXS2226-184 by measuring for the first time its absolute
position at mas resolution, by comparing the morphology and characteristics of
the maser emission at the VLBI scales after about 20 years, and by trying to
detect its polarized emission. We observed the maser emission towards
TXS2226-184 three times: the very first one with the VLBA (epoch 2017.45) and
the next two times with the EVN (epochs 2017.83 and 2018.44). The first two
epochs were observed in phase-reference mode, while the last epoch was observed
in full-polarization mode but not in phase-reference mode to increase the
on-source integration time. We also retrieved and analyzed the VLBA archival
data at 22 GHz of TXS2226-184 observed in 1998.40. We detected 6 H2O maser
features in epoch 2017.45 (VLBA), one in epoch 2017.83 (EVN), and two in epoch
2018.44 (EVN). All of them but one are red-shifted with respect to the systemic
velocity of TXS2226-184, we detected only one blue-shifted maser feature and it
is the weakest one. For the first time, we were able to measure the absolute
position of the H2O maser features with errors below 1 mas. No linear and
circular polarization was detected. We were able to associate the H2O maser
features in TXS2226-184 with the most luminous radio continuum clump reported
in the literature.Comment: 13 pages, 10 figures, 3 tables, accepted for publication by Astronomy
& Astrophysic
The magnetic field at milliarcsecond resolution around IRAS20126+4104
IRAS20126+4104 is a well studied B0.5 protostar that is surrounded by a ~1000
au Keplerian disk and is where a large-scale outflow originates. Both 6.7-GHz
CH3OH masers and 22-GHz H2O masers have been detected toward this young stellar
object. The CH3OH masers trace the Keplerian disk, while the H2O masers are
associated with the surface of the conical jet. Recently, observations of dust
polarized emission (350 um) at an angular resolution of 9 arcseconds (~15000
au) have revealed an S-shaped morphology of the magnetic field around
IRAS20126+4104. The observations of polarized maser emissions at milliarcsecond
resolution (~20 au) can make a crucial contribution to understanding the
orientation of the magnetic field close to IRAS20126+4104. This will allow us
to determine whether the magnetic field morphology changes from arcsecond
resolution to milliarcsecond resolution. The European VLBI Network was used to
measure the linear polarization and the Zeeman splitting of the 6.7-GHz CH3OH
masers toward IRAS20126+4104. The NRAO Very Long Baseline Array was used to
measure the linear polarization and the Zeeman splitting of the 22-GHz H2O
masers toward the same region. We detected 26 CH3OH masers and 5 H2O masers at
high angular resolution. Linear polarization emission was observed toward three
CH3OH masers and toward one H2O maser. Significant Zeeman splitting was
measured in one CH3OH maser (\Delta V_{Z}=-9.2 +/- 1.4 m/s). No significant (5
sigma) magnetic field strength was measured using the H2O masers. We found that
in IRAS20126+4104 the rotational energy is less than the magnetic energy.Comment: 9 pages, 5 figures, 2 tables, accepted by Astronomy & Astrophysic
EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions III. The flux-limited sample
Theoretical simulations and observations at different angular resolutions
have shown that magnetic fields have a central role in massive star formation.
Like in low-mass star formation, the magnetic field in massive young stellar
objects can either be oriented along the outflow axis or randomly. Measuring
the magnetic field at milliarcsecond resolution (10-100 au) around a
substantial number of massive young stellar objects permits determining with a
high statistical significance whether the direction of the magnetic field is
correlated with the orientation of the outflow axis or not. In late 2012, we
started a large VLBI campaign with the European VLBI Network to measure the
linearly and circularly polarized emission of 6.7 GHz methanol masers around a
sample of massive star-forming regions. This paper focuses on the first seven
observed sources, G24.78+0.08, G25.65+1.05, G29.86-0.04, G35.03+0.35,
G37.43+1.51, G174.20-0.08, and G213.70-12.6. For all these sources, molecular
outflows have been detected in the past. We detected a total of 176 methanol
masing cloudlets toward the seven massive star-forming regions, 19% of which
show linearly polarized emission. The methanol masers around the massive young
stellar object MM1 in G174.20-0.08 show neither linearly nor circularly
polarized emission. The linear polarization vectors are well ordered in all the
other massive young stellar objects. We measured significant Zeeman splitting
toward both A1 and A2 in G24.78+0.08, and toward G29.86-0.04 and G213.70-12.6.
By considering all the 19 massive young stellar objects reported in the
literature for which both the orientation of the magnetic field at
milliarcsecond resolution and the orientation of outflow axes are known, we
find evidence that the magnetic field (on scales 10-100 au) is preferentially
oriented along the outflow axes.Comment: 17 pages, 10 figures, 9 tables, accepted by Astronomy & Astrophysics.
arXiv admin note: text overlap with arXiv:1306.633
Velocity and magnetic fields within 1000 AU from a massive YSO
We want to study the velocity and magnetic field morphology in the vicinity
(<1000 AU) of a massive young stellar object (YSO), at very high spatial
resolution (10-100 AU). We performed milli-arcsecond polarimetric observations
of the strong CH3OH maser emission observed in the vicinity of an O-type YSO,
in G023.01-00.41. We have combined this information with the velocity field of
the CH3OH masing gas previously measured at the same angular resolution. We
analyse the velocity and magnetic fields in the reference system defined by the
direction of the molecular outflow and the equatorial plane of the hot
molecular core at its base, as recently observed on sub-arcsecond scales. We
provide a first detailed picture of the gas dynamics and magnetic field
configuration within a radius of 2000 AU from a massive YSO. We have been able
to reproduce the magnetic field lines for the outer regions (>600 AU) of the
molecular envelope, where the magnetic field orientation shows a smooth change
with the maser cloudlets position (0.2 degree/AU). Overall, the velocity field
vectors well accommodate with the local, magnetic field direction, but still
show an average misalignment of 30 degrees. We interpret this finding as the
contribution of a turbulent velocity field of about 3.5 km/s, responsible for
braking up the alignment between the velocity and magnetic field vectors. We do
resolve different gas flows which develop both along the outflow axis and
across the disk plane, with an average speed of 7 km/s. In the direction of the
outflow axis, we establish a collimation of the gas flow, at a distance of
about 1000 AU from the disk plane. In the disk region, gas appears to stream
outward along the disk plane for radii greater than 500-600 AU, and inward for
shorter radii.Comment: 7 pages, 4 figures, 1 table, accepted by Astronomy & Astrophysic
Magnetic field measurements at milliarcsecond resolution around massive young stellar objects
Magnetic fields have only recently been included in theoretical simulations
of high-mass star formation. The simulations show that magnetic fields can play
a crucial role not only in the formation and dynamics of molecular outflows,
but also in the evolution of circumstellar disks. Therefore, new measurements
of magnetic fields at milliarcsecond resolution close to massive young stellar
objects (YSOs) are fundamental for providing new input for numerical
simulations and for understanding the formation process of massive stars. The
polarized emission of 6.7 GHz CH3OH masers allows us to investigate the
magnetic field close to the massive YSO where the outflows and disks are
formed. Recently, we have detected with the EVN CH3OH maser polarized emission
towards 10 massive YSOs. From a first statistical analysis we have found
evidence that magnetic fields are primarily oriented along the molecular
outflows. To improve our statistics we are carrying on a large observational
EVN campaign for a total of 19 sources, the preliminary results of the first
seven sources are presented in this contribution. Furthermore, we also describe
our efforts to estimate the Lande' g-factors of the CH3OH maser transition to
determine the magnetic field strength from our Zeeman-splitting measurements.Comment: Accepted for publication in the proceeding of the "12th European VLBI
Network Symposium and Users Meeting", eds Tarchi et al. PoS(EVN 2014)04
Planar infall of CH3OH gas around Cepheus A HW2
Aims: In order to test the nature of an (accretion) disk in the vicinity of
Cepheus A HW2, we measured the three-dimensional velocity field of the CH3OH
maser spots, which are projected within 1000au of the HW2 object, with an
accuracy of the order of 0.1km/s. Methods: We made use of the European VLBI
Network (EVN) to image the 6.7GHz CH3OH maser emission towards Cepheus A HW2
with 4.5 milli-arcsecond resolution (3au). We observed at three epochs spaced
by one year between 2013 and 2015. During the last epoch, on mid-march 2015, we
benefited from the new deployed Sardinia Radio Telescope. Results: We show that
the CH3OH velocity vectors lie on a preferential plane for the gas motion with
only small deviations of 12+/-9 degrees away from the plane. This plane is
oriented at a position angle of 134 degrees east of north, and inclined by 26
degrees with the line-of-sight, closely matching the orientation of the
disk-like structure previously reported by Patel et al.(2005). Knowing the
orientation of the equatorial plane, we can reconstruct a face-on view of the
CH3OH gas kinematics onto the plane. CH3OH maser emission is detected within a
radius of 900au from HW2, and down to a radius of about 300au, the latter
coincident with the extent of the dust emission at 0.9mm. The velocity field is
dominated by an infall component of about 2km/s down to a radius of 300au,
where a rotational component of 4km/s becomes dominant. We discuss the nature
of this velocity field and the implications for the enclosed mass. Conclusions:
These findings bring direct support to the interpretation that the high-density
gas and dust emission, surrounding Cepheus A HW2, trace an accretion disk.Comment: 9 pages, 4 figures, 2 tables, accepted by Astronomy & Astrophysic
EVN observations of 6.7-GHz methanol maser polarization in massive star-forming regions II. First statistical results
Magnetic fields have only recently been included in theoretical simulations
of high-mass star formation. The simulations show that magnetic fields play an
important role in the formation and dynamics of molecular outflows. Masers, in
particular 6.7-GHz CH3OH masers, are the best probes of the magnetic field
morphologies around massive young stellar objects on the smallest scales of
10-100 AU. This paper focuses on 4 massive young stellar objects,
IRAS06058+2138-NIRS1, IRAS22272+6358A, S255-IR, and S231, which complement our
previous 2012 sample (the first EVN group). From all these sources, molecular
outflows have been detected in the past. Seven of the European VLBI Network
antennas were used to measure the linear polarization and Zeeman-splitting of
the 6.7-GHz CH3OH masers in the star-forming regions in this second EVN group.
We detected a total of 128 CH3OH masing cloudlets. Fractional linear
polarization (0.8%-11.3%) was detected towards 18% of the CH3OH masers in our
sample. The linear polarization vectors are well ordered in all the massive
young stellar objects. We measured significant Zeeman-splitting in
IRAS06058+2138-NIRS1 (DVz=3.8+/-0.6 m/s) and S255-IR (DVz=3.2+/-0.7 m/s). By
considering the 20 massive young stellar objects towards which the morphology
of magnetic fields was determined by observing 6.7-GHz CH3OH masers in both
hemispheres, we find no evident correlation between the linear distributions of
CH3OH masers and the outflows or the linear polarization vectors. On the other
hand, we present first statistical evidence that the magnetic field (on scales
10-100 AU) is primarily oriented along the large-scale outflow direction.
Moreover, we empirically find that the linear polarization fraction of
unsaturated CH3OH masers is P_l<4.5%.Comment: 13 pages, 8 figures, 7 tables, accepted by Astronomy & Astrophysic
Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732
Context. The importance of the magnetic field in high-mass-star formation is
not yet fully clear and there are still many open questions concerning its role
in the accretion processes and generation of jets and outflows. In the past few
years, masers have been successfully used to probe the magnetic field
morphology and strength at scales of a few au around massive protostars, by
measuring linear polarisation angles and Zeeman splitting. The massive
protostar IRAS 18089-1732 is a well studied high-mass-star forming region,
showing a hot core chemistry and a disc-outflow system. Previous SMA
observations of polarised dust revealed an ordered magnetic field oriented
around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic
field in the dense region probed by 6.7 GHz methanol maser observations and
compare it with observations in dust continuum polarisation, to investigate how
the magnetic field in the compact maser region relates to the large-scale field
around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz
of IRAS 18089-1732 and we analysed the polarised emission by methanol masers.
Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz
methanol maser region is consistent with the magnetic field constrained by the
SMA dust polarisation observations. A tentative detection of circularly
polarised line emission is also presented. Conclusions. We found that the
magnetic field in the maser region has the same orientation as in the disk.
Thus the large-scale field component, even at the au scale of the masers,
dominates over any small-scale field fluctuations. We obtained, from the
circular polarisation tentative detection, a field strength along the line of
sight of 5.5 mG which appeared to be consistent with the previous estimates.Comment: 12 pages, 7 figures, accepted for publication in A&
EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions
The role of magnetic fields in the formation of high-mass stars is still
under debate, and recent measurements of their orientation and strength by
using polarized maser emissions are contributing new insights. Masers
polarization, in particular of the 6.7-GHz methanol masers, are one of the best
probes of the magnetic field morphologies around massive protostars.
Determining the magnetic field morphology around an increasing number of
massive protostars at milliarcsecond resolution by observing 6.7-GHz methanol
masers is crucial to better understand the role of magnetic fields in massive
star formation.The First EVN Group consists of 4 massive star-forming
complexes: W51, W48, IRAS18556+0138, and W3(OH). These contain well-studied
\hii ~regions from some of which molecular bipolar outflows were also detected
(W51-e2, G35.20-0.74N). Nine of the European VLBI Network antennas were used to
measure the linear polarization and Zeeman-splitting of the 6.7-GHz methanol
masers in the star-forming regions of the First EVN Group. We detected a total
of 154 CH3OH masers, one third of these towards W3(OH). Fractional linear
polarization (1.2-11.5%) was detected towards 55 masers. The linear
polarization vectors are well-ordered in all the massive star-forming regions.
We measured significant Zeeman-splitting in 3 massive star-forming regions
(W51, W48, and W3(OH)) revealing a range of separations -3.5 m/s<\Delta
V_{z}<3.8 m/s with the smallest |\Delta V_{z}|=0.4m/s. We were also able to
compare our magnetic field results with those obtained from submillimeter
wavelength dust observation in W51 and show that the magnetic field at low and
high resolutions are in perfect agreement.Comment: 15 pages, 11 figures, 5 tables, accepted by Astronomy & Astrophysic
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