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

VALES: IV. Exploring the transition of star formation efficiencies between normal and starburst galaxies using APEX/SEPIA Band-5 and ALMA at low redshift

In this work we present new APEX/SEPIA Band-5 observations targeting the CO ($J=2\text{-}1$) emission line of 24 Herschel-detected galaxies at $z=0.1-0.2$. Combining this sample {with} our recent new Valpara\'iso ALMA Line Emission Survey (VALES), we investigate the star formation efficiencies (SFEs = SFR/$M_{\rm H_{2}}$) of galaxies at low redshift. We find the SFE of our sample bridges the gap between normal star-forming galaxies and Ultra-Luminous Infrared Galaxies (ULIRGs), which are thought to be triggered by different star formation modes. Considering the $\rm SFE'$ as the SFR and the $L'_{\rm CO}$ ratio, our data show a continuous and smooth increment as a function of infrared luminosity (or star formation rate) with a scatter about 0.5 dex, instead of a steep jump with a bimodal behaviour. This result is due to the use of a sample with a much larger range of sSFR/sSFR$_{\rm ms}$ using LIRGs, with luminosities covering the range between normal and ULIRGs. We conclude that the main parameters controlling the scatter of the SFE in star-forming galaxies are the systematic uncertainty of the $\alpha_{\rm CO}$ conversion factor, the gas fraction and physical size.Comment: 9pages, 7 figures, 1 table, accepted for publication in MNRA

Magnetic field regulated infall on the disc around the massive protostar Cepheus A HW2

We present polarization observations of the 6.7-GHz methanol masers around the massive protostar Cepheus A HW2 and its associated disc. The data were taken with the Multi-Element Radio Linked Interferometer Network. The maser polarization is used to determine the full three-dimensional magnetic field structure around Cepheus A HW2. The observations suggest that the masers probe the large scale magnetic field and not isolated pockets of a compressed field. We find that the magnetic field is predominantly aligned along the protostellar outflow and perpendicular to the molecular and dust disc. From the three-dimensional magnetic field orientation and measurements of the magnetic field strength along the line of sight, we are able to determine that the high density material, in which the masers occurs, is threaded by a large scale magnetic field of ~23 mG. This indicates that the protostellar environment at ~1000 AU from Cepheus A HW2 is slightly supercritical (lambda approximately 1.7) and the relation between density and magnetic field is consistent with collapse along the magnetic field lines. Thus, the observations indicate that the magnetic field likely regulates accretion onto the disc. The magnetic field dominates the turbulent energies by approximately a factor of three and is sufficiently strong to be the crucial component stabilizing the massive accretion disc and sustaining the high accretion rates needed during massive star-formation.Comment: 10 pages, 6 figures; accepted for publication in MNRAS. High resolution version can be found at http://www.astro.uni-bonn.de/~wouter/papers/papers.shtm

Dynamics of the 6.7 and 12.2 GHz methanol masers around Cepheus A HW2

The 6.7 GHz methanol maser is exclusively associated with high-mass star formation. However, it remains unclear what structures harbour the methanol masers. Cepheus A is one of the closest regions of massive star formation, making it an excellent candidate for detailed studies. We determine the dynamics of maser spots in the high-mass star-forming region Cepheus A in order to infer where and when the maser emission occurs. Very long baseline interferometry (VLBI) observations of the 6.7 and 12.2 GHz methanol masers allows for mapping their spatial and velocity distribution. Phase-referencing is used to determine the astrometric positions of the maser emission, and multi-epoch observations can reveal 3D motions. The 6.7 GHz methanol masers are found in a filamentary structure over ~1350 AU, straddling the waist of the radio jet HW2. The positions agree well with previous observations of both the 6.7 and 12.2 GHz methanol masers. The velocity field of the maser spots does not show any sign of rotation, but is instead consistent with an infall signature. The 12.2 GHz methanol masers are closely associated with the 6.7 GHz methanol masers, and the parallax that we derive confirms previous measurements. We show that the methanol maser emission very likely arises in a shock interface in the equatorial region of Cepheus A HW2 and presents a model in which the maser emission occurs between the infalling gas and the accretion disk/process.Comment: 9 pages, 5 figures; accepted for publication in Astronomy and Astrophysic

Distribution and excitation of thermal methanol in 6.7 GHz maser bearing star-forming regions. I. The nearby source Cepheus A

Context. Candidate high-mass star-forming regions can be identified through the occurrence of 6.7 GHz methanol masers. In these sources the methanol abundance of the gas must be enhanced, because the masers require a considerable methanol path length. The place and time of origin of this enhancement is not well known. Similarly, it is debated in which of the physical components of the high-mass star-forming region the masers are located.Aims. The aim of this study is to investigate the distribution and excitation of the methanol gas around Cep A and to describe the physical conditions of the region. In addition the large-scale abundance distribution is determined to understand the morphology and kinematics of star-forming regions in which methanol masers occur.Methods. The spatial distribution of methanol is studied by mapping the line emission, as well as the column density and excitation temperature, which are estimated using rotation diagrams. For a limited number of positions the parameters are checked with non-LTE models. Furthermore, the distribution of the methanol abundance is derived in comparison with archival dust continuum maps.Results. Methanol is detected over a 0.3x0.15 pc area centred on the Cep A HW2 source and shows an outflow signature. Most of the gas can be characterized by a moderately warm rotation temperature (30-60 K). At the central position two velocity components are detected with different excitation characteristics, the first related to the large-scale outflow. The second component, uniquely detected at the central location, is probably associated with the maser emission on much smaller scales of 2 ''. A detailed analysis reveals that the highest densities and temperatures occur for these inner components. In the inner region the dust and gas are shown to have different physical parameters.Conclusions. Abundances of methanol in the range 10(-9)-10(-7) are inferred, with the abundance peaking at the maser position. The geometry of the large-scale methanol is in accordance with previous determinations of the Cep A geometry, in particular those from methanol masers. The dynamical and chemical time-scales are consistent with a scenario where the methanol originates in a single driving source associated with the HW2 object and the masers in its equatorial region.</p

Intermittent maser flare around the high mass young stellar object G353.273+0.641 I: data & overview

We have performed VLBI and single-dish monitoring of 22 GHz H$_{2}$O maser emission from the high mass young stellar object G353.273+0.641 with VERA (VLBI Exploration of Radio Astrometry) and Tomakamai 11-m radio telescope. Two maser flares have been detected, separated almost two years. Frequent VLBI monitoring has revealed that these flare activities have been accompanied by structural change of the prominent shock front traced by H2O maser alignments. We have detected only blue-shifted emissions and all maser features have been distributed within very small area of 200 $\times$ 200 au$^{2}$ in spite of wide velocity range (> 100 km s$^{-1}$). The light curve shows notably intermittent variation and suggests that the H$_{2}$O masers in G353.273+0.641 are excited by episodic radio jet. The time-scale of \sim2 yr and characteristic velocity of \sim500 km s$^{-1}$ also support this interpretation. Two isolated velocity components of C50 (-53 \pm 7 km s$^{-1}$) and C70 (-73 \pm 7 km s$^{-1}$) have shown synchronised linear acceleration of the flux weighted V_{\rmn{LSR}} values (\sim-5 km s$^{-1}$ yr$^{-1}$) during the flare phase. This can be converted to the lower-limit momentum rate of 1.1 \times 10$^{-3}$ M_{\sun} km s$^{-1}$ yr$^{-1}$. Maser properties are quite similar to that of IRAS 20126+4104 especially. This corroborates the previous suggestion that G353.273+0.641 is a candidate of high mass protostellar object. The possible pole-on geometry of disc-jet system can be suitable for direct imaging of the accretion disc in this case.Comment: 13 pages, 5 figures accepted for publication in MNRA

GASP. X: APEX detection of molecular gas in the tails and in the disks of ram-pressure stripped galaxies

Jellyfish galaxies in clusters are key tools to understand environmental processes at work in dense environments. The advent of Integral Field Spectroscopy has recently allowed to study a significant sample of stripped galaxies in the cluster environment at z$\sim 0.05$, through the GAs Stripping Phenomena in galaxies with MUSE (GASP) survey. However, optical spectroscopy can only trace the ionized gas component through the H$_{\alpha}$ emission that can be spatially resolved on kpc scale at this redshift. The complex interplay between the various gas phases (ionized, neutral, molecular) is however yet to be understood. We report here the detection of large amounts of molecular gas both in the tails and in the disks of 4 jellyfish galaxies from the GASP sample with stellar masses $\sim 3.5\times 10^{10}-3\times 10^{11} M_{\odot}$, showing strong stripping. The mass of molecular gas that we measure in the tails amounts to several $10^9 M_{\odot}$ and the total mass of molecular gas ranges between 15 and 100 \% of the galaxy stellar mass. The molecular gas content within the galaxies is compatible with the one of normal spiral galaxies, suggesting that the molecular gas in the tails has been formed in-situ. We find a clear correlation between the ionized gas emission $\rm H\alpha$ and the amount of molecular gas. The CO velocities measured from APEX data are not always coincident with the underlying $\rm H\alpha$ emitting knots, and the derived Star Formation Efficiencies appear to be very low.Comment: 14 pages, 7 figures, Submitted to MNRA

A log N(HI) = 22.6 DLA in a dark gamma-ray burst: the environment of GRB 050401

The optical afterglow spectrum of GRB050401 (at z=2.8992+/-0.0004) shows the presence of a DLA, with log(nHI)=22.6+/-0.3. This is the highest column density ever observed in a DLA, and is about five times larger than the strongest DLA detected so far in any QSO spectrum. From the optical spectrum, we also find a very large Zn column density, allowing us to infer an abundance of [Zn/H]=-1.0+/-0.4. These large columns are supported by the X-ray spectrum from Swift-XRT which shows a column density (in excess of Galactic) of log(nH)=22.21^{+0.06}_{-0.08} assuming solar abundances (at z=2.9). The comparison of this X-ray column density, which is dominated by absorption due to alpha-chain elements, and the HI column density derived from the Ly-alpha absorption line, allows us to derive a metallicity for the absorbing matter of [alpha/H]=-0.4+/-0.3. The optical spectrum is reddened and can be well reproduced with a power-law with SMC extinction, where A_V=0.62+/-0.06. But the total optical extinction can also be constrained in a way which is independent of the shape of the extinction curve: from the optical-to-X-ray spectral energy distribution we find, 0.5<~A_V<~4.5. However, even this upper limit, independent of the shape of the extinction curve, is still well below the dust column that is inferred from the X-ray column density, i.e. A_V=9.1^{+1.4}_{-1.5}. This discrepancy might be explained by a small dust content with high metallicity (low dust-to-metals ratio). `Grey' extinction cannot explain the discrepancy since we are comparing the metallicity to a measurement of the total extinction (without reference to the reddening). Little dust with high metallicity may be produced by sublimation of dust grains or may naturally exist in systems younger than a few hundred Myr.Comment: 28 pages, 5 figures, accepted for ApJ, scheduled for November 20 issue, missing author adde