arXiv subjects

R. L. Plambeck

Publications and source records attributed to R. L. Plambeck.

17 recordsLinked to original sources

arXiv2017

What is the Hidden Depolarization Mechanism in Low Luminosity AGN?

Millimeter wavelength polarimetry of accreting black hole systems can provide a tomographic probe of the accretion flow on a wide range of linear scales. We searched for linear polarization in two low luminosity active galactic nuclei (LLAGN), M81 and M84, using the Combined Array for Millimeter Astronomy (CARMA) and the Submillimeter Array (SMA). We find upper limits of $\sim 1 - 2\%$ averaging over the full bandwidth and with a rotation measure (RM) synthesis technique. These low polarization fractions, along with similar low values for LLAGN M87 and 3C84, suggest that LLAGN have qualitatively different polarization properties than radio-loud sources and Sgr A*. If the sources are intrinsically polarized and then depolarized by Faraday rotation then we place lower limits on the RM of a few times $10^7\, {\rm rad\, m^{-2}}$ for the full bandwidth case and $\sim 10^9\, {\rm rad\, m^{-2}}$ for the RM synthesis analysis. These limits are inconsistent with or marginally consistent with expected accretion flow properties. Alternatively, the sources may be depolarized by cold electrons within a few Schwarzschild radii from the black hole, as suggested by numerical models.

astro-ph.HE↗

arXiv2017

ALMA Images of the Orion Hot Core at 349 GHz

We present ALMA images of the dust and molecular line emission in the Orion Hot Core at 349 GHz. At 0.2" angular resolution the images reveal multiple clumps in an arc ~ 1" east of Orion Source I, the protostar at the center of the Kleinmann-Low Nebula, and another chain of peaks from IRc7 towards the southwest. The molecular line images show narrow filamentary structures at velocities > 10 km/s away from the heavily resolved ambient cloud velocity ~5 km/s. Many of these filaments trace the SiO outflow from Source I, and lie along the edges of the dust emission. Molecular line emission at excitation temperatures 300--2000 K, and velocities > 10 km/s from the ambient cloud, suggest that the Hot Core may be heated in shocks by the outflow from Source I or from the BN/Source I explosion. The spectral line observations also reveal a remarkable molecular ring, ~ 2" south of Source I, with a diameter ~ 600 AU. The ring is seen in high excitation transitions of HC3N, HCN v2=1, and SO2. An impact of ejecta from the BN/Source I explosion with a dense dust clump could result in the observed ring of shocked material.

astro-ph.GA↗

arXiv2016

ALMA Observations of Orion Source I at 350 and 660 GHz

Orion Source I (`SrcI') is the protostar at the center of the Kleinmann-Low Nebula. ALMA observations of SrcI with 0.2" angular resolution were made at 350 and 660 GHz to search for the H26alpha and H21alpha hydrogen recombination lines and to measure the continuum flux densities. The recombination lines were not detected, ruling out the possibility that SrcI is a hypercompact HII region. The deconvolved size of the continuum source is approximately 0.23 x 0.07" (100 x 30 AU); it is interpreted as a disk viewed almost edge-on. Optically thick thermal emission from ~500 K dust is the most plausible source of the continuum, even at frequencies as low as 43 GHz; the disk mass is most likely in the range 0.02 to 0.2 Mo. A rich spectrum of molecular lines is detected, mostly from sulfur- and silicon-rich molecules like SO, SO2, and SiS, but also including vibrationally excited CO and several unidentified transitions. Lines with upper energy levels E_U > 500 K appear in emission and are symmetric about the source's LSR velocity of 5 km/sec, while lines with E_U < 500 K appear as blueshifted absorption features against the continuum, indicating that they originate in outflowing gas. The emission lines exhibit a velocity gradient along the major axis of the disk that is consistent with rotation around a 5-7 Mo central object. The relatively low mass of SrcI and the existence of a 100 AU disk around it are difficult to reconcile with the model in which SrcI and the nearby Becklin-Neugebauer Object were ejected from a multiple system 500 years ago.

astro-ph.GA↗

arXiv2016

Multiwavelength study of the flaring activity of Sgr A* in 2014 February-April

The supermassive black hole Sgr A* is located at the Milky Way center. We studied its flaring activity close to the DSO/G2 pericenter passage to constrain the physical properties and origin of the flares. Simultaneous/coordinated observations were made in 2014 Feb-Apr with XMM-Newton, HST/WFC3, VLT/SINFONI, VLA and CARMA. We detected 2 X-ray and 3 NIR flares on Mar. 10 and Apr. 2 with XMM-Newton and HST and 2 NIR flares on Apr. 3 and 4 with VLT. The Mar. 10 X-ray flare has a long rise and a rapid decay. Its NIR counterpart peaked 4320s before the X-ray peak implying a variation in the X-ray-to-NIR flux ratio. This flare may be a single flare where change in the flux ratio is explained by the adiabatic compression of a plasmon or 2 close flares with simultaneous X-ray/NIR peaks. We observed an increase in the rising radio flux density on Mar. 10 with the VLA. It could be the delayed emission from a NIR/X-ray flare preceding our observation. The Apr. 2 X-ray flare occurred for HST in the Earth occultation of Sgr A*. We thus only observed the start of its NIR counterpart. After the occultation, we observed the decay phase of a bright NIR flare with no X-ray counterpart. On Apr. 3, 2 CARMA flares were observed. The 1rst one may be the delayed emission of a VLT NIR flare. We thus observed 7 NIR flares whose 3 have an X-ray counterpart. We studied the physical parameters of the flaring region for each NIR flare but none of the possible radiative processes can be ruled out for the X-ray flares creation. Our X-ray flaring rate is consistent with those observed in the 2012 Chandra XVP campaign. No increase in the flaring activity was thus triggered close to the DSO/G2 pericenter passage. Moreover, higher X-ray flaring rates had already been observed with no increase in the quiescent level. There is thus no direct link between an X-ray flaring-rate increase and an accretion-rate change. (abridged)

astro-ph.HE↗

arXiv2015

Millimeter polarisation of the protoplanetary nebula OH 231.8+4.2: A follow-up study with CARMA

In order to investigate the characteristics and influence of the magnetic field in evolved stars, we performed a follow-up investigation of our previous submillimeter analysis of the proto-planetary nebula (PPN) OH 231.8+4.2 (Sabin et al. 2014), this time at 1.3mm with the CARMA facility in polarisation mode for the purpose of a multi-scale analysis. OH 231.8+4.2 was observed at ~2.5" resolution and we detected polarised emission above the 3-sigma threshold (with a mean polarisation fraction of 3.5 %). The polarisation map indicates an overall organised magnetic field within the nebula. The main finding in this paper is the presence of a structure mostly compatible with an ordered toroidal component that is aligned with the PPN's dark lane. We also present some alternative magnetic field configuration to explain the structure observed. These data complete our previous SMA submillimeter data for a better investigation and understanding of the magnetic field structure in OH 231.8+4.2.

astro-ph.SR↗

arXiv2014

Testing Magnetic Field Models for the Class 0 Protostar L1527

For the Class 0 protostar, L1527, we compare 131 polarization vectors from SCUPOL/JCMT, SHARP/CSO and TADPOL/CARMA observations with the corresponding model polarization vectors of four ideal-MHD, non-turbulent, cloud core collapse models. These four models differ by their initial magnetic fields before collapse; two initially have aligned fields (strong and weak) and two initially have orthogonal fields (strong and weak) with respect to the rotation axis of the L1527 core. Only the initial weak orthogonal field model produces the observed circumstellar disk within L1527. This is a characteristic of nearly all ideal-MHD, non-turbulent, core collapse models. In this paper we test whether this weak orthogonal model also has the best agreement between its magnetic field structure and that inferred from the polarimetry observations of L1527. We found that this is not the case; based on the polarimetry observations the most favored model of the four is the weak aligned model. However, this model does not produce a circumstellar disk, so our result implies that a non-turbulent, ideal-MHD global collapse model probably does not represent the core collapse that has occurred in L1527. Our study also illustrates the importance of using polarization vectors covering a large area of a cloud core to determine the initial magnetic field orientation before collapse; the inner core magnetic field structure can be highly altered by a collapse and so measurements from this region alone can give unreliable estimates of the initial field configuration before collapse.

astro-ph.SR↗

arXiv2014

Probing the Parsec-scale Accretion Flow of 3C 84 with Millimeter Polarimetry

We report the discovery of Faraday rotation toward radio source 3C 84, the active galactic nucleus in NGC1275 at the core of the Perseus Cluster. The rotation measure (RM), determined from polarization observations at wavelengths of 1.3 and 0.9 mm, is (8.7 +/- 2.3) x 10^5 radians/m^2, among the largest ever measured. The RM remained relatively constant over a 2 year period even as the intrinsic polarization position angle wrapped through a span of 300 degrees. The Faraday rotation is likely to originate either in the boundary layer of the radio jet from the nucleus, or in the accretion flow onto the central black hole. The accretion flow probably is disk-like rather than spherical on scales of less than a parsec, otherwise the RM would be even larger.

astro-ph.GA↗

arXiv2013

Interferometric Upper Limits on Millimeter Polarization of the Disks around DG Tau, GM Aur, and MWC 480

Millimeter-wavelength polarization measurements offer a promising method for probing the geometry of magnetic fields in circumstellar disks. Single dish observations and theoretical work have hinted that magnetic field geometries might be predominantly toroidal, and that disks should exhibit millimeter polarization fractions of 2-3%. While subsequent work has not confirmed these high polarization fractions, either the wavelength of observation or the target sources differed from the original observations. Here we present new polarimetric observations of three nearby circumstellar disks at 2" resolution with the Submillimeter Array (SMA) and the Combined Array for Research in Millimeter Astronomy (CARMA). We reobserve GM Aur and DG Tau, the systems in which millimeter polarization detections have been claimed. Despite higher resolution and sensitivity at wavelengths similar to the previous observations, the new observations do not show significant polarization. We also add observations of a new HAeBe system, MWC 480. These observations demonstrate that a very low (<0.5%) polarization fraction is probably common at large (>100 AU) scales in bright circumstellar disks. We suggest that high-resolution observations may be worthwhile to probe magnetic field structure on linear distances smaller than the disk scale height, as well as in regions closer to the star that may have larger MRI-induced magnetic field strengths.

astro-ph.SR↗

arXiv2013

The Ionized Circumstellar Envelopes of Orion Source I and the Becklin Neugebauer Object

The 229 GHz (lambda 1.3mm) radio emission from Orion-KL was mapped with up to 0.14" angular resolution with CARMA, allowing measurements of the flux densities of Source I ('SrcI') and the Becklin-Neugebauer Object (BN), the 2 most massive stars in this region. We find integrated flux densities of 310 +/- 45 mJy for SrcI and 240 +/- 35 mJy for BN. SrcI is optically thick even at 229 GHz. No trace of the H30alpha recombination line is seen in its spectrum, although the v_2=1, 5(5,0)-6(4,3) transition of H2O, 3450 K above the ground state, is prominent. SrcI is elongated at position angle 140 degrees, as in 43 GHz images. These results are most easily reconciled with models in which the radio emission from SrcI arises via the H- free-free opacity in a T < 4500 K disk, as considered by Reid et al. (2007). By contrast, the radio spectrum of BN is consistent with p+/e- free-free emission from a dense (n_e ~ 5x10^7 cm^{-3}), but otherwise conventional, hypercompact HII region. The source is becoming optically thin at 229 GHz, and the H30alpha recombination line, at VLSR = 23.2 +/- 0.5 km/sec, is prominent in its spectrum. A Lyman continuum flux of 5x10^{45} photons/sec, consistent with that expected from a B star, is required to maintain the ionization. Supplementary 90 GHz observations were made to measure the H41alpha and H42alpha recombination lines toward BN. Published 43 and 86 GHz data suggest that SrcI brightened with respect to BN over the 15 year period from 1994 to 2009.

astro-ph.SR↗

arXiv2009

Tracing the Bipolar Outflow from Orion Source I

Using CARMA, we imaged the 87 GHz SiO v=0 J=2-1 line toward Orion-KL with 0.45 arcsec angular resolution. The maps indicate that radio source I drives a bipolar outflow into the surrounding molecular cloud along a NE--SW axis, in agreement with the model of Greenhill et al. (2004). The extended high velocity outflow from Orion-KL appears to be a continuation of this compact outflow. High velocity gas extends farthest along a NW--SE axis, suggesting that the outflow direction changes on time scales of a few hundred years.

astro-ph.GA↗

arXiv2009

Dynamically Driven Evolution of the Interstellar Medium in M51

We report the highest-fidelity observations of the spiral galaxy M51 in CO emission, revealing the evolution of giant molecular clouds (GMCs) vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (so-called GMAs) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics --their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the inter-arm region and into the next spiral arm passage.

astro-ph.CO↗

arXiv2008

A Resolved Ring of Debris Dust Around the Solar Analog HD 107146

We present resolved images of the dust continuum emission from the debris disk around the young (80-200 Myr) solar-type star HD 107146 with CARMA at $λ$1.3 mm and the CSO at $λ$350 $μ$m. Both images show that the dust emission extends over an $\sim$10\arcsec diameter region. The high resolution (3\arcsec) CARMA image further reveals that the dust is distributed in a partial ring with significant decrease in flux inward of 97 AU. Two prominent emission peaks appear within the ring separated by $\sim$140 degrees in position angle. The morphology of the dust emission is suggestive of dust captured into a mean motion resonance, which would imply the presence of a planet at an orbital radius of $\sim$45-75 AU.

astro-ph↗

arXiv2008

Proplyds and Massive Disks in the Orion Nebula Cluster Imaged with CARMA and SMA

[Abridged] We imaged a 2' x 2' region of the Orion Nebula cluster in 1.3 mm wavelength continuum emission with the recently commissioned Combined Array for Research in Millimeter Astronomy (CARMA) and with the Submillimeter Array (SMA). Our mosaics include >250 known near-IR cluster members, of which 36 are so-called "proplyds" that have been imaged previously with the Hubble Space Telescope. We detected 40 sources in 1 mm continuum emission, and several of them are spatially resolved with our observations. Dust masses inferred for detected sources range from 0.01 to 0.5 Msun, and the average disk mass for undetected sources is estimated to be ~0.001 Msun, approximately an order of magnitude smaller than the minimum mass solar nebula. Most stars in the ONC thus do not appear to currently possess sufficient mass in small dust grains to form Jupiter-mass (or larger) planets. Comparison with previous results for younger and older regions indicates that massive disks evolve significantly on ~Myr timescales. We also show that the percentage of stars in Orion surrounded by disks more massive than ~0.01 Msun is substantially lower than in Taurus, indicating that environment has an impact on the disk mass distribution. Finally, we explore potential correlations of disk mass with stellar mass and location within the cluster.

astro-ph↗

arXiv2003

Magnetic Field Morphology of Orion-IRc2 from 86 GHz SiO Maser Polarization Images

In an attempt to probe the magnetic field morphology near the massive young star Orion-IRc2, we mapped the linear polarization of its J=2-1 SiO masers, in both the v=0 and v=1 vibrational levels, with 0.5 arcsec resolution. The intense v=1 masers are confined to a narrow zone 40 AU from the star. Their polarization position angles vary significantly on time scales of years. For the v=1 masers the stimulated emission rate R is likely to exceed the Zeeman splitting gΩdue to any plausible magnetic field; in this case the maser polarization need not correlate with the field direction. The much weaker v=0 masers in the ground vibrational level lie 100-700 AU from IRc2, in what appears to be a flared disk. Their fractional polarizations are as high as 50%. The polarization position angles vary little across the line profile or the emission region, and appear to be stable in time. The position angle, P.A. = 80 degrees, we measure for the J=2-1 masers differs by 70 degrees from that measured for the J=1-0 SiO transition, possibly because of Faraday rotation in the foreground, Orion A, HII region. A rotation measure RM = 3.3 \times 10^4 rad m$^{-2}$ is required to bring the J=2-1 and J=1-0 position angles into concordance. The intrinsic polarization position angle for both transitions is then 57 degrees, parallel to the plane of the putative disk. Probably the magnetic field threads the disk poloidally. There is little evidence for a pinched or twisted field near the star.

astro-ph↗

arXiv2003

High resolution imaging of CO outflows in OMC-2 and OMC-3

A large scale, high resolution map of CO(1-0) emission toward the OMC-2 and OMC-3 star forming regions is presented. The map is a mosaic of 46 fields using the Berkeley-Illinois-Maryland Array (BIMA) and covers $\sim 10'\times 15'$ at $\sim 10" $ resolution. These data are combined with singledish FCRAO observations and analyzed to identify and determine the properties of nine protostellar outflows. The BIMA data alone almost completely resolve out the cloud emission at central velocities and only recover 1/20 of the flux in the high velocity gas showing that outflows are generally broadly dispersed over $\sim 1'$ angular scales. All nine identified outflows emanate from known Class 0 or borderline Class 0/I sources, are associated with knots of shocked \h2\ emission, and have short dynamical times. It is suggested that only the youngest, most spatially compact, and energetic outflows have been found and that more distributed high velocity gas undetected by BIMA is due to older outflows continuing through the Class I phase of protostellar evolution. The mechanical energy injection rate into the cloud is estimated to be $\sim 1.5 L_\odot$ which is comparable to the turbulent energy dissipation rate. Outf`lows appear capable, therefore, of sustaining cloud turbulence but a high starformation rate is required implying a short cloud lifetime $\leq 5$ `Myr.

astro-ph↗

arXiv2002

SiO Observations of the NGC 1333 IRAS 2A Protostellar Jet

We present 86.8 GHz SiO J=2-1 observations of the molecular outflow from the protostellar source NGC 1333 IRAS 2A. Silicon monoxide is a sensitive tracer of shocks in molecular gas, and the emission clearly traces terminal bowshocks at the ends of the outflow lobes, as well as emission knots and a source coincident with the protostar. Our three-field mosaic from the BIMA interferometer covers the entire outflow length, including both lobes. The most prominent emission feature, the eastern bowshock, shows SiO at the leading edge, clearly displaced from the maximum seen in CO. This is in contrast to recent observations by Chandler & Richer (2001) of the HH 211 jet, where SiO traced primarily internal working surfaces and was less extended than the CO. Our results are consistent with prompt entrainment at the leading edge of the IRAS 2A bowshocks.

astro-ph↗

arXiv1998

High Resolution Millimeter-Wave Mapping of Linearly Polarized Dust Emission: Magnetic Field Structure in Orion

We present 1.3 and 3.3 mm polarization maps of Orion-KL obtained with the BIMA array at approximately 4 arcsec resolution. Thermal emission from magnetically aligned dust grains produces the polarization. Along the Orion ``ridge'' the polarization position angle varies smoothly from about 10 degrees to 40 degrees, in agreement with previous lower resolution maps. In a small region south of the Orion ``hot core,'' however, the position angle changes by 90 degrees. This abrupt change in polarization direction is not necessarily the signpost of a twisted magnetic field. Rather, in this localized region processes other than the usual Davis-Greenstein mechanism might align the dust grains with their long axes parallel with the field, orthogonal to their normal orientation.

astro-ph↗