Context. Polarized continuum emission at millimeter-to-submillimeter wavelengths is usually attributed to thermal emission from dust grains aligned through radiative torques with the magnetic field. ...However, recent theoretical work has shown that under specific conditions polarization may arise from self-scattering of thermal emission and by radiation fields from a nearby stellar object. Aims. We use multi-frequency polarization observations of a circumbinary disk to investigate how the polarization properties change at distinct frequency bands. Our goal is to discern the main mechanism responsible for the polarization through comparison between our observations and model predictions for each of the proposed mechanisms. Methods. We used the Atacama Large Millimeter/submillimeter Array to perform full polarization observations at 97.5 GHz (Band 3), 233 GHz (Band 6) and 343.5 GHz (Band 7). The ALMA data have a mean spatial resolution of 28 AU. The target is the Class I object BHB07-11, which is the youngest object in the Barnard 59 protocluster. Complementary Karl G. Jansky Very Large Array observations at 34.5 GHz were also performed and revealed a binary system at centimetric continuum emission within the disk. Results. We detect an extended and structured polarization pattern that is remarkably consistent between the three bands. The distribution of polarized intensity resembles a horseshoe shape with polarization angles following this morphology. From the spectral index between Bands 3 and 7, we derived a dust opacity index β ~ 1 consistent with maximum grain sizes larger than expected to produce self-scattering polarization in each band. The polarization morphology and the polarization levels do not match predictions from self-scattering. On the other hand, marginal correspondence is seen between our maps and predictions from a radiation field model assuming the brightest binary component as main radiation source. Previous molecular line data from BHB07-11 indicates disk rotation. We used the DustPol module of the ARTIST radiative transfer tool to produce synthetic polarization maps from a rotating magnetized disk model assuming combined poloidal and toroidal magnetic field components. The magnetic field vectors (i.e., the polarization vectors rotated by 90°) are better represented by a model with poloidal magnetic field strength about three times the toroidal one. Conclusions. The similarity of our polarization patterns among the three bands provides a strong evidence against self-scattering and radiation fields. On the other hand, our data are reasonably well reproduced by a model of disk with toroidal magnetic field components slightly smaller than poloidal ones. The residual is likely to be due to the internal twisting of the magnetic field due to the binary system dynamics, which is not considered in our model.
With the GREAT receiver at the Stratospheric Observatory for Infrared Astronomy (SOFIA), nine massive molecular clumps have been observed in the ammonia 32+−22− line at 1.8 THz in a search for ...signatures of infall. The sources were selected from the ATLASGAL submillimeter dust continuum survey of our Galaxy. Clumps with high masses covering a range of evolutionary stages based on their infrared properties were chosen. The ammonia line was detected in all sources, leading to five new detections and one confirmation of a previous detection of redshifted absorption in front of their strong THz continuum as a probe of infall in the clumps. These detections include two clumps embedded in infrared dark clouds. The measured velocity shifts of the absorptions compared to optically thin C17O (3–2) emission are 0.3–2.8 km s-1, corresponding to fractions of 3% to 30% of the free-fall velocities of the clumps. The ammonia infall signature is compared with complementary data of different transitions of HCN, HNC, CS, and HCO+, which are often used to probe infall because of their blue-skewed line profiles. The best agreement with the ammonia results is found for the HCO+ (4–3) transitions, but the latter is still strongly blended with emission from associated outflows. This outflow signature is far less prominent in the THz ammonia lines, which confirms it as a powerful probe of infall in molecular clumps. Infall rates in the range from 0.3 to 16 × 10-3 M⊙/yr were derived with a tentative correlation with the virial parameters of the clumps. The new observations show that infall on clump scales is ubiquitous through a wide range of evolutionary stages, from L/M covering about ten to several hundreds.
We present the Heterodyne Receiver Array CO Line Extragalactic Survey, an atlas of CO emission from 18 nearby galaxies that are also part of The H I Nearby Galaxy Survey and the Spitzer Infrared ...Nearby Galaxies Survey. We used the HERA multipixel receiver on the IRAM 30-m telescope to map the CO J = 2 -> 1 line over the full optical disk (defined by the isophotal radius r 25) of each target, at 13'' angular resolution and 2.6 km s-1 velocity resolution. Here we describe the observations and reduction of the data and show channel maps, azimuthally averaged profiles, integrated intensity maps, and peak intensity maps. The implied H2 masses range from 7 X 106 to 6 X 109 M , with four low metallicity dwarf irregular galaxies yielding only upper limits. In the cases where CO is detected, the integrated H2-to-H I ratios range from 0.02 to 1.13 and H2-to-stellar mass ratios from 0.01 to 0.25. Exponential scale lengths of the CO emission for our targets are in the range 0.8-3.2 kpc, or 0.2 ± 0.05r 25. The intensity-weighted mean velocity of CO matches that of H I very well, with a 1s scatter of only 6 km s-1. The CO J = 2 -> 1/J = 1 -> 0 line ratio varies over a range similar to that found in the Milky Way and other nearby galaxies, ~0.6-1.0, with higher values found in the centers of galaxies. The typical line ratio, ~0.8, could be produced by optically thick gas with an excitation temperature of ~10 K.
We report on the young massive clump (G35.20w) in W48 that previous molecular line and dust observations have revealed to be in the very early stages of star formation. Based on virial analysis, we ...find that a strong field of 1640 μG is required to keep the clump in pressure equilibrium. We performed a deep Zeeman effect measurement of the 113 GHz CN (1−0) line towards this clump with the IRAM 30 m telescope. We combine simultaneous fitting of all CN hyperfines with Monte Carlo simulations for a large range in realization of the magnetic field to obtain a constraint on the line-of-sight field strength of −687 ± 420 μG. We also analyze archival dust polarization observations towards G35.20w. A strong magnetic field is implied by the remarkably ordered field orientation that is perpendicular to the longest axis of the clump. Based on this, we also estimate the plane-of-sky component of the magnetic field to be ~740 μG. This allows for a unique comparison of the two orthogonal measurements of magnetic field strength of the same region and at similar spatial scales. The expected total field strength shows no significant conflict between the observed field and that required for pressure equilibrium. By producing a probability distribution for a large range in field geometries, we show that plane-of-sky projections are much closer to the true field strengths than line-of-sight projections. This can present a significant challenge for Zeeman measurements of magnetized structures, even with ALMA. We also show that CN molecule does not suffer from depletion on the observed scales in the predominantly cold and highly deuterated core in an early stage of high-mass star formation and is thus a good tracer of the dense gas.
Context. Short millimeter observations of radio-loud active galactic nuclei (AGN) offer an excellent opportunity to study the physics of their synchrotron-emitting relativistic jets from where the ...bulk of radio and millimeter emission is radiated. On one hand, AGN jets and their emission cores are significantly less affected by Faraday rotation and depolarization than at longer wavelengths. On the other hand, the millimeter emission of AGN is dominated by the compact innermost regions in the jets, where the jet cannot be seen at longer wavelengths due to synchrotron opacity. Aims. We present the first simultaneous dual frequency 86 GHz and 229 GHz polarimetric survey of all four Stokes parameters for a large sample of 211 radio-loud active galactic nuclei, designed to be flux limited at 1 Jy at 86 GHz. Methods. Most of the observations were made in mid-August 2010 using the XPOL polarimeter on the IRAM 30 m millimetric radio telescope. Results. Linear polarization detections above a 3σ median level of ~ 1.0% are reported for 183 sources at 86 GHz and for 23 sources at 229 GHz, where the median 3σ level is ~ 6.0%. We show a clear excess of the linear polarization degree that is detected at 229 GHz with regard to that at 86 GHz by a factor of ~ 1.6. This implies a progressively better ordered magnetic field for blazar jet regions that are located progressively upstream in the jet. We show that the linear polarization angle at 86 and 229 GHz and the jet structural position angle for both quasars and BL Lacs do not show a clear preference to align in either parallel or perpendicular directions. Our variability study with regard to the 86 GHz data from our previous survey points out a large degree of variation. In particular, we report total flux and linear polarization changes in time scales of years by median factors of ~ 1.5 in total flux and ~ 1.7 in linear polarization degree (with maximum variations by factors up to 6.3 and ~ 5, respectively). Moreover, 86% of sources show linear polarization angles evenly distributed with regard to our previous measurements.
FEEDBACK is a SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic ...high mass star-forming regions in the 158 m (1.9 THz) line of C ii and the 63 m (4.7 THz) line of O i. We employ the 14 pixel Low Frequency Array and 7 pixel High Frequency Array upGREAT heterodyne instrument to spectrally resolve (0.24 MHz) these far-infrared fine structure lines. With a total observing time of 96h, we will cover ∼6700 arcmin2 at 14 1) angular resolution for the C ii line and 6 3 for the O i line. The observations started in spring 2019 (Cycle 7). Our aim is to understand the dynamics in regions dominated by different feedback processes from massive stars such as stellar winds, thermal expansion, and radiation pressure, and to quantify the mechanical energy injection and radiative heating efficiency. This is an important science topic because feedback of massive stars on their environment regulates the physical conditions and sets the emission characteristics in the interstellar medium (ISM), influences the star formation activity through molecular cloud dissolution and compression processes, and drives the evolution of the ISM in galaxies. The C ii line provides the kinematics of the gas and is one of the dominant cooling lines of gas for low to moderate densities and UV fields. The O i line traces warm and high-density gas, excited in photodissociations regions with a strong UV field or by shocks. The source sample spans a broad range in stellar characteristics from single OB stars, to small groups of O stars, to rich young stellar clusters, to ministarburst complexes. It contains well-known targets such as Aquila, the Cygnus X region, M16, M17, NGC7538, NGC6334, Vela, and W43 as well as a selection of H ii region bubbles, namely RCW49, RCW79, and RCW120. These C ii maps, together with the less explored O i 63 m line, provide an outstanding database for the community. They will be made publically available and will trigger further studies and follow-up observations.
Context. During the transition from the asymptotic giant branch (AGB) to planetary nebulae (PN), the circumstellar geometry and morphology change dramatically. Another characteristic of this ...transition is the high mass-loss rate, that can be partially explained by radiation pressure and a combination of various factors, such as the stellar pulsation, the dust grain condensation, and opacity in the upper atmosphere. The magnetic field can also be one of the main ingredients that shapes the stellar upper atmosphere and envelope. Aims. Our main goal is to investigate for the first time the spatial distribution of the magnetic field in the envelope of IRC+10216. More generally we intend to determine the magnetic field strength in the circumstellar envelope (CSE) of C-rich evolved stars, compare this field with previous studies for O-rich stars, and constrain the variation of the magnetic field with r the distance to the star’s centre. Methods. We use spectropolarimetric observations of the Stokes V parameter, collected with Xpol on the IRAM-30 m radiotelescope, observing the Zeeman effect in seven hyperfine components of the CN J = 1–0 line. We use the Crutcher et al. (1996, ApJ, 456, 217) method to estimate the magnetic field. For the first time, the instrumental contamination is investigated, through dedicated studies of the power patterns in Stokes V and I in detail. Results. For C-rich evolved stars, we derive a magnetic field strength (B) between 1.6 and 14.2 mG while B is estimated to be 6 mG for the proto-PN (PPN) AFGL618, and an upper value of 8 mG is found for the PN NGC 7027. These results are consistent with a decrease of B as 1/r in the environment of AGB objects, that is, with the presence of a toroidal field. But this is not the case for PPN and PN stars. Our map of IRC+10216 suggests that the magnetic field is not homogeneously strong throughout or aligned with the envelope and that the morphology of the CN emission might have changed with time.
Context. The fine-structure line of atomic oxygen at 63 μm (OI63μm) is an important diagnostic tool in different fields of astrophysics: it is for example predicted to be the main coolant in several ...environments of star-forming regions (SFRs). However, our knowledge of this line relies on observations with low spectral resolution, and the real contribution of each component (photon-dominated region, jet) in the complex environment of SFRs to its total flux is poorly understood. Aims. We investigate the contribution of jet and photon-dominated region emission, and of absorption to the OI63μm line towards the hot gas around the ultra-compact Hii region G5.89–0.39 and study the far-IR line luminosity of the source in different velocity regimes through spectroscopically resolved spectra of atomic oxygen, CII, CO, OH, and H2O. Methods. We mapped G5.89–0.39 in OI63μm and in CO(16–15) with the GREAT receiver onboard SOFIA. We also observed the central position of the source in the ground-state OH 2Π3/2 J = 5/2 → J = 3/2 triplet and in the excited OH 2Π1/2 J = 3/2 → J = 1/2 triplets with SOFIA. These data were complemented with APEX CO(6–5) and CO(7–6) maps and with Herschel/HIFI maps and single-pointing observations in lines of CII, H2O, and HF. Results. The OI spectra in G5.89–0.39 are severely contaminated by absorptions from the source envelope and from different clouds along the line of sight. Emission is detected only at high velocities, and it is clearly associated with the compact north-south outflows traced by extremely high-velocity emission in low-J CO lines. The mass-loss rate and the energetics of the jet system derived from the OI63μm line agree well with previous estimates from CO, thus suggesting that the molecular outflows in G5.89–0.39 are driven by the jet system seen in OI. The far-IR line luminosity of G5.89–0.39 is dominated by OI at high-velocities; the second coolant in this velocity regime is CO, while CII, OH and H2O are minor contributors to the total cooling in the outflowing gas. Finally, we derive abundances of different molecules in the outflow: water has low abundances relative to H2 of 10-8−10-6, and OH of 10-8. Interestingly, we find an abundance of HF to H2 of 10-8, comparable with measurements in diffuse gas. Conclusions. Our study shows the importance of spectroscopically resolved observations of the OI63μm line for using this transition as diagnostic of star-forming regions. While this was not possible until now, the GREAT receiver onboard SOFIA has recently opened the possibility of detailed studies of the OI63μm line to investigate the potential of the transition for probing different environments.
Aims. We aim to investigate the polarization properties of a starless core in an early evolutionary stage. Linear polarization data reveal the properties of the dust grains in the distinct phases of ...the interstellar medium. Our goal is to investigate how the polarization degree and angle correlate with the cloud and core gas. Methods. We use optical, near infrared, and submillimeter polarization observations on the starless object Pipe-109 in the Pipe nebula. Our data cover a physical scale range of 0.08 to 0.4 pc, comprising the dense gas, envelope, and the surrounding cloud. Results. The cloud polarization is well traced by the optical data. The near infrared polarization is produced by a mixed population of grains from the core border and the cloud gas. The optical and near infrared polarization toward the cloud reaches the maximum possible value and saturates with respect to the visual extinction. The core polarization is predominantly traced by the submillimeter data and has a steep decrease with respect to the visual extinction. Modeling of the submillimeter polarization indicates a magnetic field main direction projected onto the plane-of-sky and loss of grain alignment for densities higher than 6 × 104 cm-3 (or AV> 30 mag). Conclusions. The object is immersed in a magnetized medium with a very ordered magnetic field. The absence of internal source of radiation significantly affects the polarization efficiencies in the core, creating a polarization hole at the center of the starless core. This result supports the theory of dust grain alignment via radiative torques
Opening the Treasure Chest in Carina Mookerjea, B.; Sandell, G.; Güsten, R. ...
Astronomy and astrophysics (Berlin),
06/2019, Letnik:
626
Journal Article
Recenzirano
Odprti dostop
Pillars and globules are the best examples of the impact of the radiation and wind from massive stars on the surrounding interstellar medium. We mapped the G287.84-0.82 cometary globule (with the ...Treasure Chest cluster embedded in it) in the South Pillars region of Carina (i) in C II, 63 μm O I, and CO(11–10) using the heterodyne receiver array upGREAT on SOFIA and (ii) in J = 2–1 transitions of CO, 13CO, C18O, and J = 3–2 transitions of H2CO using the APEX telescope in Chile. We used these data to probe the morphology, kinematics, and physical conditions of the molecular gas and the photon-dominated regions (PDRs) in G287.84-0.82. The velocity-resolved observations of C II and O I suggest that the overall structure of the pillar (with red-shifted photoevaporating tails) is consistent with the effect of FUV radiation and winds from η Car and O stars in Trumpler 16. The gas in the head of the pillar is strongly influenced by the embedded cluster, whose brightest member is an O9.5 V star, CPD −59°2661. The emission of the C II and O I lines peak at a position close to the embedded star, while all the other tracers peak at another position lying to the northeast consistent with gas being compressed by the expanding PDR created by the embedded cluster. The molecular gas inside the globule was probed with the J = 2–1 transitions of CO and isotopologs as well as H2CO, and analyzed using a non-local thermodynamic equilibrium model (escape-probability approach), while we used PDR models to derive the physical conditions of the PDR. We identify at least two PDR gas components; the diffuse part (~ 104 cm−3) is traced by C II, while the dense (n ~ 2–8 × 105 cm−3) part is traced by C II, O I, and CO(11–10). Using the F = 2–1 transition of 13C II detected at 50 positions in the region, we derived optical depths (0.9–5), excitation temperatures (80–255 K) of C II, and N(C+) of 0.3–1 × 1019 cm−2. The total mass of the globule is ~1000 M⊙, about half of which is traced by C II. The dense PDR gas has a thermal pressure of 107–108 K cm−3, which is similar to the values observed in other regions.
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