Searcharxiv⌕ Search

arXiv subjects

S. Casassus

Publications and source records attributed to S. Casassus.

At least 55 records · Page 3Linked to original sources
arXiv2013

The Science Cases for Building a Band 1 Receiver Suite for ALMA

We present the various science cases for building Band 1 receivers as part of ALMA's ongoing Development Program. We describe the new frequency range for Band 1 of 35-52 GHz, a range chosen to maximize the receiver suite's scientific impact. We first describe two key science drivers: 1) the evolution of grains in protoplanetary disks and debris disks, and 2) molecular gas in galaxies during the era of re-ionization. Studies of these topics with Band 1 receivers will significantly expand ALMA's Level 1 Science Goals. In addition, we describe a host of other exciting continuum and line science cases that require ALMA's high sensitivity and angular resolution. For example, ALMA Band 1 continuum data will probe the Sunyaev-Zel'dovich Effect in galaxy clusters, Very Small Grains and spinning dust, ionized jets from young stars, spatial and flaring studies of Sgr A*, the acceleration sites of solar flares, pulsar wind nebulae, radio supernovae, and X-ray binaries. Furthermore, ALMA Band 1 line data will probe chemical differentiation in cloud cores, complex carbon chain molecules, extragalactic radio recombination lines, masers, magnetic fields through Zeeman effect measurements, molecular outflows from young stars, the co-evolution of star formation and active galactic nuclei, and the molecular content of galaxies at z ~ 3. ALMA provides similar to better sensitivities than the JVLA over 35-50 GHz, with differences increasing with frequency. ALMA's smaller antennas and shorter baselines, greater number of baselines, and single-dish capabilities, however, give it a significant edge for observing extended emission, making wide-field maps (mosaics), or attaining high image fidelity, as required by the described science cases.

astro-ph.IM↗
arXiv2013

Near-Infrared Imaging Polarimetry of HD142527

HD 142527 is a pre-transition disk with strong evidence for on-going planet formation. Recent observations show a disrupted disk with spiral arms, a dust-depleted inner cavity and the possible presence of gas streams driving gas from the outer disk towards the central star. We aim to derive the morphology of the disk, as well as the distribution and properties of the dust at its surface. We have obtained polarized differential images of HD 142527 at $H$ and $Ks$ bands with NaCo at the VLT. Combining these images with classical PSF-subtraction, we are able to derive the polarization degree of this disk. At $H$ band the polarization degree of the disk varies between 10% and 25%. This result cannot be reproduced by dust distributions containing highly porous material. The polarization is better matched by distributions of compact particles, with maximum sizes at least up to a few microns, in agreement with previous observations. We also observe two regions of low emission (nulls) in total and in polarized intensity. In particular, one of these nulls is at roughly the same position as the maximum of the horse-shoe shape observed in sub-millimeter continuum emission ALMA Band-7 (345 GHz) observations. We discuss the possible link between both features.

astro-ph.SR↗
arXiv2013

Constraints on free-free emission from Anomalous Microwave Emission Sources in the Perseus Molecular Cloud

We present observations performed with the Green Bank Telescope at 1.4 and 5 GHz of three strips coincident with the anomalous microwave emission features previously identified in the Perseus molecular cloud at 33 GHz with the Very Small Array. With these observations we determine the level of the low frequency (~1 - 5 GHz) emission. We do not detect any significant extended emission in these regions and we compute conservative 3σupper limits on the fraction of free-free emission at 33 GHz of 27%, 12%, and 18% for the three strips, indicating that the level of the emission at 1.4 and 5 GHz cannot account for the emission observed at 33 GHz. Additionally, we find that the low frequency emission is not spatially correlated with the emission observed at 33 GHz. These results indicate that the emission observed in the Perseus molecular cloud at 33 GHz, is indeed in excess over the low frequency emission, hence confirming its anomalous nature.

astro-ph.GA↗
arXiv2012

The Science Case for Building a Band 1 Receiver Suite for ALMA

We present a set of compelling science cases for the ALMA Band 1 receiver suite. For these cases, we assume in tandem the updated nominal Band 1 frequency range of 35-50 GHz with a likely extension up to 52 GHz; together these frequencies optimize the Band 1 science return. The scope of the science cases ranges from nearby stars to the re-ionization edge of the Universe. Two cases provide additional leverage on the present ALMA Level One Science Goals and are seen as particularly powerful motivations for building the Band 1 Receiver suite: (1) detailing the evolution of grains in protoplanetary disks, as a complement to the gas kinematics, requires continuum observations out to ~35 GHz (~9mm); and (2) detecting CO 3-2 line emission from galaxies like the Milky Way during the epoch of re-ionization, i.e., 6 < z < 10, also requires Band 1 receiver coverage. The range of Band 1 science is wide, however, and includes studies of very small dust grains in the ISM, pulsar wind nebulae, radio supernovae, X-ray binaries, the Galactic Center (i.e., Sgr A*), dense cloud cores, complex carbon-chain molecules, masers, magnetic fields in the dense ISM, jets and outflows from young stars, distant galaxies, and galaxy clusters (i.e., the Sunyaev-Zel'dovich Effect). A comparison of ALMA and the Jansky VLA (JVLA) at the same frequencies of Band 1 finds similar sensitivity performance at 40-50 GHz, with a slight edge for ALMA at higher frequencies (e.g., within a factor of 2 for continuum observations). With its larger number of instantaneous baselines, however, ALMA Band 1data will have greater fidelity than those from the JVLA at similar frequencies.

astro-ph.IM↗
arXiv2012

The dynamically disrupted gap in HD 142527

The vestiges of planet formation have been observed in debris disks harboring young and massive gaseous giants. The process of giant planet formation is terminated by the dissipation of gas in the protoplanetary disk. The gas-rich disk around HD142527 features a small inner disk, a large gap from \sim10 to \sim140AU, and a massive outer disk extending out to \sim300AU. The gap could have been carved-out by a giant planet. We have imaged the outer regions of this gap using the adaptive-optics camera NICI on Gemini South. Our images reveal that the disk is dynamically perturbed. The outer boundary of the roughly elliptical gap appears to be composed of several segments of spiral arms. The stellar position is offset by 0.17+-0.02" from the centroid of the cavity, consistent with earlier imaging at coarser resolutions. These transient morphological features are expected in the context of disk evolution in the presence of a perturbing body located inside the cavity. We perform hydro-dynamical simulations of the dynamical clearing of a gap in a disk. A 10Mjup body in a circular orbit at r = 90AU, perturbs the whole disks, even after thousands of orbits. By then the model disk has an eccentric and irregular cavity, flanked by tightly wound spiral arms, but it is still evolving far from steady state. A particular transient configuration that is a qualitative match to HD142527 is seen at 1.7Myr.

astro-ph.SR↗
arXiv2012

A multi-wavelength investigation of RCW175: an HII region harboring spinning dust emission

Using infrared, radio continuum and spectral observations, we performed a detailed investigation of the HII region RCW175. We determined that RCW175, which actually consists of two separate HII regions, G29.1-0.7 and G29.0-0.6, is located at a distance of 3.2+/-0.2 kpc. Based on the observations we infer that the more compact G29.0-0.6 is less evolved than G29.1-0.7 and was possibly produced as a result of the expansion of G29.1-0.7 into the surrounding interstellar medium. We compute a star formation rate for RCW175 of (12.6+/-1.9)x10^{-5} M_{\sun}/yr, and identified 6 possible young stellar object candidates within its vicinity. Additionally, we estimate that RCW175 contains a total dust mass of 215+/-53 M_{\sun}. RCW175 has previously been identified as a source of anomalous microwave emission (AME), an excess of emission at cm wavelengths often attributed to electric dipole radiation from the smallest dust grains. We find that the AME previously detected in RCW175 is not correlated with the smallest dust grains (polycyclic aromatic hydrocarbons or small carbonaceous dust grains), but rather with the exciting radiation field within the region. This is a similar result to that found in the Perseus molecular cloud, another region which harbors AME, suggesting that the radiation field may play a pivotal role in the production of this new Galactic emission mechanism. Finally, we suggest that these observations may hint at the importance of understanding the role played by the major gas ions in spinning dust models.

astro-ph.GA↗
arXiv2011

Planck early results. XX. New light on anomalous microwave emission from spinning dust grains

Anomalous microwave emission (AME) has been observed by numerous experiments in the frequency range ~10-60 GHz. Using Planck maps and multi-frequency ancillary data, we have constructed spectra for two known AME regions: the Perseus and Rho Ophiuchi molecular clouds. The spectra are well fitted by a combination of free-free radiation, cosmic microwave background, thermal dust, and electric dipole radiation from small spinning dust grains. The spinning dust spectra are the most precisely measured to date, and show the high frequency side clearly for the first time. The spectra have a peak in the range 20-40 GHz and are detected at high significances of 17.1 sigma for Perseus and 8.4 sigma for Rho Ophiuchi. In Perseus, spinning dust in the dense molecular gas can account for most of the AME; the low density atomic gas appears to play a minor role. In Rho Ophiuchi, the ~30 GHz peak is dominated by dense molecular gas, but there is an indication of an extended tail at frequencies 50-100 GHz, which can be accounted for by irradiated low density atomic gas. The dust parameters are consistent with those derived from other measurements. We have also searched the Planck map at 28.5 GHz for candidate AME regions, by subtracting a simple model of the synchrotron, free-free, and thermal dust. We present spectra for two of the candidates; S140 and S235 are bright HII regions that show evidence for AME, and are well fitted by spinning dust models.

astro-ph.GA↗
arXiv2011

The Structure of NGC 1976 in the Radio Range, v2

High angular resolution radio continuum images of NGC 1976 (M42, Orion A) at frequency=330 MHz (wavelength=91 cm), 1.5 GHz (20 cm) and 10.6 GHz (2.8 cm), have been aligned, placed on a common grid, smoothed to common resolutions of 80" (=0.16 pc at 420 pc) and 90" (=0.18 pc) and compared on a position-by-position basis. The results are not consistent with a single value of Te. The best fit to the continuum data is a multi-layer model based on radio recombination line (RRL) data with a monotonic variation, from Te=8500K in the higher intensity, more compact region at the rear of NGC 1976 to Te =6000K in the low intensity, extended region in the foreground. An estimate of temperature fluctuations toward the peak from this model yields t2=0.003. This is a factor of 10 lower than fluctuation values from optical collisionally excited line data.

astro-ph.GA↗
arXiv2011

The Structure of NGC 1976 in the Radio Range

High angular resolution radio continuum images of NGC 1976 (M42, Orion A) at frequency=330 MHz (wavelength=91 cm), 1.5 GHz (20 cm) and 10.6 GHz (2.8 cm), have been aligned, placed on a common grid, smoothed to common resolutions of 80" (=0.16 pc at 420 pc) and 90" (=0.18 pc) and compared on a position-by-position basis. The results are not consistent with a single value of Te. Rather, there is a significant variation, from Te =6000K in the low intensity, extended region to Te=8500K in the higher intensity, more compact region. The best fit to the data is a multi-layer model obtained from radio recombination line (RRL) data. An estimate of temperature fluctuations from the model yields t2=0.003. This is a factor of 10 lower than fluctuation values from optical O++ line data.

physics.space-ph↗
arXiv2011

Spitzer characterisation of dust in an anomalous emission region: the Perseus cloud

Anomalous microwave emission is known to exist in the Perseus cloud. One of the most promising candidates to explain this excess of emission is electric dipole radiation from rapidly rotating very small dust grains, commonly referred to as spinning dust. Photometric data obtained with the Spitzer Space Telescope have been reprocessed and used in conjunction with the dust emission model DUSTEM to characterise the properties of the dust within the cloud. This analysis has allowed us to constrain spatial variations in the strength of the interstellar radiation field ($χ_\mathrm{ISRF}$), the mass abundances of the PAHs and VSGs relative to the BGs (Y$_\mathrm{PAH}$ and Y$_\mathrm{VSG}$), the column density of hydrogen (N$_\mathrm{H}$) and the equilibrium dust temperature (T$_\mathrm{dust}$). The parameter maps of Y$_\mathrm{PAH}$, Y$_\mathrm{VSG}$ and $χ_\mathrm{ISRF}$ are the first of their kind to be produced for the Perseus cloud, and we used these maps to investigate the physical conditions in which anomalous emission is observed. We find that in regions of anomalous emission the strength of the ISRF, and consequently the equilibrium temperature of the dust, is enhanced while there is no significant variation in the abundances of the PAHs and the VSGs or the column density of hydrogen. We interpret these results as an indication that the enhancement in $χ_\mathrm{ISRF}$ might be affecting the properties of the small stochastically heated dust grains resulting in an increase in the spinning dust emission observed at 33 GHz. This is the first time that such an investigation has been performed, and we believe that this type of analysis creates a new perspective in the field of anomalous emission studies, and represents a powerful new tool for constraining spinning dust models.

astro-ph.GA↗
arXiv2011

Dust-correlated cm-wavelength continuum emission on translucent clouds ζ Oph and LDN 1780

The diffuse cm-wave IR-correlated signal, the "anomalous" CMB foreground, is thought to arise in the dust in cirrus clouds. We present Cosmic Background Imager (CBI) cm-wave data of two translucent clouds, ζ Oph and LDN 1780 with the aim of characterising the anomalous emission in the translucent cloud environment. In ζ Oph, the measured brightness at 31 GHz is 2.4σ higher than an extrapolation from 5 GHz measurements assuming a free-free spectrum on 8 arcmin scales. The SED of this cloud on angular scales of 1{\odot} is dominated by free-free emission in the cm-range. In LDN 1780 we detected a 3 σ excess in the SED on angular scales of 1{\odot} that can be fitted using a spinning dust model. In this cloud, there is a spatial correlation between the CBI data and IR images, which trace dust. The correlation is better with near-IR templates (IRAS 12 and 25 μm) than with IRAS 100 μm, which suggests a very small grain origin for the emission at 31 GHz. We calculated the 31 GHz emissivities in both clouds. They are similar and have intermediate values between that of cirrus clouds and dark clouds. Nevertheless, we found an indication of an inverse relationship between emissivity and column density, which further supports the VSGs origin for the cm-emission since the proportion of big relative to small grains is smaller in diffuse clouds.

astro-ph.GA↗
arXiv2010

Structure and Composition of Two Transitional Circumstellar Disks in Corona Australis

The late stages of evolution of the primordial circumstellar disks surrounding young stars are poorly understood, yet vital to constrain theories of planet formation. We consider basic structural models for the disks around two ~10 Myr-old members of the nearby RCrA association, RX J1842.9-3532 and RX J1852.3-3700. We present new arcsecond-resolution maps of their 230 GHz continuum emission from the Submillimeter Array and unresolved CO(3-2) spectra from the Atacama Submillimeter Telescope Experiment. By combining these data with broadband fluxes from the literature and infrared fluxes and spectra from the catalog of the Formation and Evolution of Planetary Systems (FEPS) Legacy program on the Spitzer Space Telescope, we assemble a multiwavelength data set probing the gas and dust disks. Using the Monte Carlo radiative transfer code RADMC to model simultaneously the SED and millimeter continuum visibilities, we derive basic dust disk properties and identify an inner cavity of radius 16 AU in the disk around RX J1852.3-3700. We also identify an optically thin 5 AU cavity in the disk around RX J1842.9-3532, with a small amount of optically thick material close to the star. The molecular line observations suggest an intermediate disk inclination in RX J1842.9-3532, consistent with the continuum emission. In combination with the dust models, the molecular data allow us to derive a lower CO content than expected, suggesting that the process of gas clearing is likely underway in both systems, perhaps simultaneously with planet formation.

astro-ph.SR↗
arXiv2010

IR-correlated 31 GHz radio emission from Orion East

Lynds dark cloud LDN1622 represents one of the best examples of anomalous dust emission, possibly originating from small spinning dust grains. We present Cosmic Background Imager (CBI) 31 GHz data of LDN1621, a diffuse dark cloud to the north of LDN1622 in a region known as Orion East. A broken ring with diameter g\approx 20 arcmin of diffuse emission is detected at 31 GHz, at \approx 20-30 mJy beam$^{-1}$ with an angular resolution of \approx 5 arcmin. The ring-like structure is highly correlated with Far Infra-Red emission at $12-100 μ$m with correlation coefficients of r \approx 0.7-0.8, significant at $\sim10σ$. Multi-frequency data are used to place constraints on other components of emission that could be contributing to the 31 GHz flux. An analysis of the GB6 survey maps at 4.85 GHz yields a $3σ$ upper limit on free-free emission of 7.2 mJy beam$^{-1}$ ($\la 30 per cent of the observed flux) at the CBI resolution. The bulk of the 31 GHz flux therefore appears to be mostly due to dust radiation. Aperture photometry, at an angular resolution of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of IRAS maps and the {\it WMAP} 5-year W-band map at 93.5 GHz. A single modified blackbody model was fitted to the data to estimate the contribution from thermal dust, which amounts to $\sim$ 10 per cent at 31 GHz. In this model, an excess of 1.52\pm 0.66 Jy (2.3σ) is seen at 31 GHz. Future high frequency $\sim$ 100-1000 GHz data, such as those from the {\it Planck} satellite, are required to accurately determine the thermal dust contribution at 31 GHz. Correlations with the IRAS $100 μ$m gave a coupling coefficient of $18.1\pm4.4 μ$K (MJy/sr)$^{-1}$, consistent with the values found for LDN1622.

astro-ph.GA↗
arXiv2008

Anomalous Microwave Emission from the HII region RCW175

We present evidence for anomalous microwave emission in the RCW175 \hii region. Motivated by 33 GHz $13\arcmin$ resolution data from the Very Small Array (VSA), we observed RCW175 at 31 GHz with the Cosmic Background Imager (CBI) at a resolution of $4\arcmin$. The region consists of two distinct components, G29.0-0.6 and G29.1-0.7, which are detected at high signal-to-noise ratio. The integrated flux density is $5.97\pm0.30$ Jy at 31 GHz, in good agreement with the VSA. The 31 GHz flux density is $3.28\pm0.38$ Jy ($8.6σ$) above the expected value from optically thin free-free emission based on lower frequency radio data and thermal dust constrained by IRAS and WMAP data. Conventional emission mechanisms such as optically thick emission from ultracompact \hii regions cannot easily account for this excess. We interpret the excess as evidence for electric dipole emission from small spinning dust grains, which does provide an adequate fit to the data.

astro-ph↗
arXiv2007

Bayesian Image Reconstruction Based on Voronoi Diagrams

We present a Bayesian Voronoi image reconstruction technique (VIR) for interferometric data. Bayesian analysis applied to the inverse problem allows us to derive the a-posteriori probability of a novel parameterization of interferometric images. We use a variable Voronoi diagram as our model in place of the usual fixed pixel grid. A quantization of the intensity field allows us to calculate the likelihood function and a-priori probabilities. The Voronoi image is optimized including the number of polygons as free parameters. We apply our algorithm to deconvolve simulated interferometric data. Residuals, restored images and chi^2 values are used to compare our reconstructions with fixed grid models. VIR has the advantage of modeling the image with few parameters, obtaining a better image from a Bayesian point of view.

astro-ph↗
arXiv2007

A centimetre-wave excess over free-free emission in planetary nebulae

We report a centimetre-wave (cm-wave, 5-31GHz) excess over free-free emission in PNe. Accurate 31 and 250GHz measurements show that the 31GHz flux densities in our sample are systematically higher than the level of optically thin free-free continuum extrapolated from 250GHz. The 31GHz excess is observed, within one standard deviation, in all 18 PNe with reliable 31 and 250GHz data, and is significant in 9 PNe. The only exception is the peculiar object M2-9, whose radio spectrum is that of an optically thick stellar wind. On average the fraction of non-free-free emission represents 51% of the total flux density at 31GHz, with a scatter of 11%. The average 31-250GHz spectral index of our sample is = -0.43+-0.03 (in flux density, with a scatter of 0.14). The 31--250 GHz drop is reminiscent of the anomalous foreground observed in the diffuse ISM by CMB anisotropy experiments. The 5--31 GHz spectral indices are consistent with both flat spectra and spinning dust emissivities, given the 10% calibration uncertainty of the comparison 5GHz data. But a detailed study of the objects with the largest cm-excess, including the low frequency data available in the literature, shows that present spinning dust models cannot alone explain the cm-excess in PNe. Although we have no definitive interpretation of our data, the least implausible explanation involves a synchrotron component absorbed by a cold nebular screen. We give flux densities for 37 objects at 31GHz, and for 26 objects at 250GHz.

astro-ph↗
arXiv2007

Interstellar 12C/13C from CH+ absorption lines: Results from an extended survey

The 12C/13C isotope ratio in the interstellar medium (ISM), and its evolution with time, is an important tracer of stellar yields. Spatial variations of this ratio can be used to study mixing in the ISM. We want to determine this ratio and its spatial variations in the local ISM from CH+ absorption lines in the optical towards early-type stars. The aim is to determine the average value for the local ISM and study possible spatial variations. We observed a large number of early-type stars with Feros to extend the sample of suitable target stars for CH+ isotope studies. The best suited targets were observed with Uves with higher signal-to-noise ratio and spectral resolution to determine the isotope ratio from the interstellar CH+ lines. This study significantly expands the number of 13CH+ detections. We find an average ratio of = 76.27 +- 1.94 or, for f = 1/R, = (120.46 +- 3.02) 10^{-4}. The scatter in f is 6.3 sigma( ). This findings strengthens the case for chemical inhomogeneity in the local ISM, with important implications for the mixing in the ISM. Given the large scatter, the present-day value in the ISM is not significantly larger than the solar value, which corresponds to the local value 4.5 Gyr ago.

astro-ph↗