arXiv subjects

T. G. Phillips

Publications and source records attributed to T. G. Phillips.

At least 55 records · Page 3Linked to original sources

arXiv2009

Are $^{12}$CO lines good indicators of the star formation rate in galaxies?

In this paper, we investigate the relevance of using the $^{12}$CO line emissions as indicators of star formation rates (SFR). For the first time, we present this study for a relatively large number of $^{12}$CO transitions (12) as well as over a large interval in redshift (from z$\sim$0 to z$\sim$6). For the nearby sources (D$\leq$10 Mpc), we have used homogeneous sample of $^{12}$CO data provided by Bayet et al. (2004, 2006), mixing observational and modelled line intensities. For higher-z sources (z $\geq$ 1), we have collected $^{12}$CO observations from various papers and have completed the data set of line intensities with model predictions which we also present in this paper. Finally, for increasing the statistics, we have included recent $^{12}$CO(1-0) and $^{12}$CO(3-2) observations of intermediate-z sources. Linear regressions have been calculated for identifying the tightest SFR-$^{12}$CO line luminosity relationships. We show that the \emph{total} $^{12}$CO, the $^{12}$CO(5-4), the $^{12}$CO(6-5) and the $^{12}$CO(7-6) luminosities are the best indicators of SFR (as measured by the far-infrared luminosity). Comparisons with theoretical approaches from Krumholz and Thompson (2007) and Narayanan et al. (2008) are also performed in this paper. Although in general agreement, the predictions made by these authors and the observational results we present here show small and interesting discrepancies. In particular, the slope of the linear regressions, for J$_{upper}\geq$ 4 $^{12}$CO lines are not similar between theoretical studies and observations. On one hand, a larger high-J $^{12}$CO data set of observations might help to better agree with models, increasing the statistics. On the other hand, theoretical studies extended to high redshift sources might also reduce such discrepancies.

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arXiv2008

Tentative detection of phosphine in IRC+10216

The J,K = 1,0-0,0 rotational transition of phosphine (PH3) at 267 GHz has been tentatively identified with a T_MB = 40 mK spectral line observed with the IRAM 30-m telescope in the C-star envelope IRC+10216. A radiative transfer model has been used to fit the observed line profile. The derived PH3 abundance relative to H2 is 6 x 10^(-9), although it may have a large uncertainty due to the lack of knowledge about the spatial distribution of this species. If our identification is correct, it implies that PH3 has a similar abundance to that reported for HCP in this source, and that these two molecules (HCP and PH3) together take up about 5 % of phosphorus in IRC+10216. The abundance of PH3, as that of other hydrides in this source, is not well explained by conventional gas phase LTE and non-LTE chemical models, and may imply formation on grain surfaces.

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arXiv2007

350 Micron Observations of Local Luminous Infrared Galaxies and the Temperature Dependence of the Emissivity Index

We report 350micron observations of 18 nearby luminous infrared galaxies (LIRGs), using the Submillimeter High Angular Resolution Camera II (SHARC-II) mounted on the Caltech Submillimeter Observatory (CSO) 10.4m telescope. Combining our 350micron flux measurements with the existing far-infrared (far-IR) and submillimeter (submm) data, we fit a single-temperature model to the spectral energy distributions (SEDs), and find the dust temperatures, emissivity indices and far-IR luminosities having sample medians of Td = 39.4+-7.9K, beta = 1.6+-0.3 and log(Lfir/Lsolar) = 11.2+-0.6. An empirical inverse Td-beta correlation, best described by Td = 9.86E9^(1/(4.63 + beta)), is established for the local LIRG sample, which we argue can be explained by the intrinsic interdependence between the dust temperature and grain emissivity index as physical parameters, as well as variations in grain properties in the the interstellar medium (ISM).

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arXiv2006

A survey of submillimeter C and CO lines in nearby galaxies

The submillimeter CO lines are formed in warm and dense molecular gas and are therefore sensitive to the physical conditions whereas the CO (J=1-0) line is a tracer of the total molecular gas mass. In order to be able to compare the properties of molecular gas in nearby and distant galaxies, we have observed C and CO submillimeter lines (including the 12CO(6-5) and 12CO(7-6) lines) in a sample of nearby galaxies using the CSO. We have obtained a complete view of the CO cooling curve (also called CO spectral energy distribution) by combining the submillimeter CSO data with previous observations found in the literature. We made use of Large Velocity Gradient (LVG) models to analyse the observed CO cooling curve, predict CO line intensities from J=1-0 to J=15-14 in the studied galaxies, and derive the physical properties of the warm and dense molecular gas : the kinetic temperature (TK); the gas density (n(H2)); the N(12CO)/$Δv$. The predictions for the line intensities and for the total CO cooling power, obtained from LVG modelling have been compared with predictions from Photo Dissociation Regions (PDR) models. We show how the CO SED varies according to the galaxy star forming activity. For active nuclei, the peak is located near the 12CO(6-5) or 12CO(7-6) rotational lines, while, for normal nuclei, most of the energy is carried by the 12CO(4-3) and 12CO(5-4) lines. Whatever the spectral type of the nucleus, the observed C cooling rate is lower than the observed CO cooling rate (by a factor of around 4). The CO cooling curve of nearby starburst galaxies (e.g. NGC 253) has a quite similar shape to the CO cooling curve of distant galaxies. Therefore, the CO cooling curves are useful diagnostics for the star forming activity in distant objects.

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arXiv2006

SHARC-2 350 Micron Observations of Distant Submillimeter Selected Galaxies

We present 350 micron observations of 15 Chapman et al. submillimeter galaxies (SMGs) with radio counterparts and optical redshifts. We detect 12 and obtain sensitive upper limits for three, providing direct, precise measurements of their far-infrared luminosities and characteristic dust temperatures. With these, we verify the linear radio--far-infrared correlation at redshifts of z ~ 1--3 and luminosities of 10^11--10^13 L_sun, with a power-law index of 1.02+-0.12 and rms scatter of 0.12 dex. However, either the correlation constant q or the dust emissivity index beta is lower than measured locally. The best fitting q ~ 2.14 is consistent with SMGs being predominantly starbust galaxies, without significant AGN contribution, at far-infrared wavelengths. Gas-to-dust mass ratios are estimated at 54+-13 (kappa_850 / 0.15 m^2 kg^-1), depending on the absoption efficiency kappa, with intrinsic dispersion ~40% around the mean value. Dust temperatures consistent with 34.6+-3 K (beta/1.5)^-0.71, at z ~ 1.5--3.5, suggest that far-infrared photometric redshifts may be viable, and perhaps accurate to 10% < dz/(1+z), for up to 80% of the SMG population in this range, if the above temperature characterizes the full range of SMGs. However, observed temperature evolution of T_d ~ (1+z) is also plausible and could result from selection effects. From the observed luminosity-temperature (L-T) relation, L ~ T_obs^(2.82+-0.29), we derive scaling relations for dust mass versus dust temperature, and we identify expressions to inter-relate the observed quantities. These suggest that measurements at a single wavelength, in the far-infrared, submillimeter or radio wave bands, might constrain dust temperatures and far-infrared luminosities for most SMGs with redshifts at z ~ 0.5--4.

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arXiv2006

Deuterium enhancement in H3+ in prestellar cores

Deuterium enhancement of monodeuterated species has been recognized for more than 30 years as a result of the chemical fractionation that results from the difference in zero point energies of deuterated and hydrogenated molecules. The key reaction is the deuteron exchange in the reaction between HD, the reservoir of deuterium in dark interstellar clouds, and the H3+ molecular ion, leading to the production of the H2D+ molecule, and the low temperature in dark interstellar clouds favors this production. Furthermore, the presence of multiply deuterated species have incited our group to proceed further and consider the subsequent reaction of H2D+ with HD, leading to D2H+ (first detected by Vastel et al. 2004), which can further react with HD to produce D3+. In prestellar cores, where CO was found to be depleted (Bacmann et al. 2003), this production should be increased, as CO would normally destroy H3+. The first model including D2H+ and D3+ (Roberts, Herbst & Millar 2003) predicted that these molecules should be as abundant as H2D+ (see contribution by H. Roberts). The first detection of the D2H+ was made possible by the recent laboratory measurement by Hirao & Amano (2003) for the frequency of the fundamental line of the para-D2H+ (see contribution by T. Amano). Here we present observations of H2D+ and D2H+ towards a sample of dark clouds and prestellar cores and show how the distribution of ortho-H2D+ (1_1,0-1_1,1) can trace the deuterium factory in prestellar cores. We also present how future instrumentation will improve our knowledge concerning the deuterium enhancement of H3+.

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arXiv2006

The Distribution of Ortho-H_2D+(1_1,0 - 1_1,1) in L1544: Tracing the Deuteration Factory in Prestellar Cores

Prestellar cores are unique laboratories for studies of the chemical and physical conditions preceding star formation. We observed the prestellar core L1544 in the fundamental transition of ortho-H2D+ (1_1,0-1_1,1) at different positions over 100", and found a strong correlation between its abundance and the CO depletion factor. We also present a tentative detection of the fundamental transition of para-D2H+ (1_1,0-1_0,1) at the dust emission peak. Maps in N2H+, N2D+, HCO+ and DCO+ are used, and interpreted with the aid of a spherically symmetric chemical model that predicts the column densities and abundances of these species as a function of radius. The correlation between the observed deuterium fractionation of H3+, N2H+ and HCO+ and the observed integrated CO depletion factor across the core can be reproduced by this chemical model. In addition a simpler model is used to study the H2D+ ortho-to-para ratio. We conclude that, in order to reproduce the observed ortho-H2D+ observations, the grain radius should be larger than 0.3 microns.

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arXiv2004

The submillimeter C and CO lines in Henize 2-10 and NGC 253

The purpose of this paper is to describe a method for determining a cooling template for galaxies, using nearby galaxies, and applicable to future observations of distant galaxies. We observed two starburst galaxies (NGC 253 and Henize 2-10) with the Caltech Submillimeter Observatory in the rotational lines of carbon monoxide 12CO(J=3-2), (J=6-5) and (J=7-6) for both, and also 12CO(J=4-3) and 13CO(J=3-2) for Henize 2-10 and in the 3P2-3P1 fine-structure transitions of atomic carbon [CI] at 809 GHz for NGC 253. Some of these observations have been made previously, but the present multitransition study (including data found in the literature) is the most complete to date for the two galaxies. From these observations, we have derived the properties of the warm and dense molecular gas in the galaxy nuclei. We used an LTE analysis and an LVG radiative transfer model to determine physical conditions of the interstellar medium in both sources and predicted integrated line properties of all CO transitions up to 12CO(15-14). A PDR model has also been used and here the data are well fitted (within 20 %) by a model cloud with a gas density of n(H)= 8.0 +- 1.0 * 10E5 cm-3 and an incident FUV flux of chi = 20000 for Henize 2-10. For NGC 253, we deduced n(H) = 3.0 +- 0.5 * 10E5 cm-3 and chi = 20000 for the modelled cloud. The physical properties of warm gas and CO cooling curves of the target galaxies are compared with those measured for the nucleus of the Milky Way and the Cloverleaf QSO. The gas properties and CO cooling curve are similar for the two starburst galaxies and the Cloverleaf QSO while the Milky Way nucleus exhibits lower excitation molecular gas.

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arXiv2004

The slowly expanding envelope of CRL618 probed with HC3N rotational ladders

Lines from HC3N and isotopic substituted species in ground and vibrationally excited states produce crowded millimeter and submillimeter wave spectra in the C-rich protoplanetary nebula CRL618. The complete sequence of HC3N rotational lines from J=9-8 to J=30-29 has been observed with the IRAM 30m telescope toward this object. Lines from a total of 15 different vibrational states (including the fundamental), with energies up to 1100 cm^-1, have been detected for the main HC3N isotopomer. In addition, the CSO telescope has been used to complement this study in the range J=31-30 to J=39-38, with detections in five of these states, all of them below 700 cm^-1. Vibrationally excited HC3N rotational lines exhibit P-Cygni profiles at 3 mm, evolving to pure emission lineshapes at shorter wavelengths. This evolution of the line profile shows little dependence on the vibrational state from which they rotational lines arise. The absorption features are formed against the continuum emission, which has been successfully characterized in this work due to the large frequency coverage. The HC3N column density in front of the the continuum source has been determined by comparing the output of an array of models to the data. The best fits are obtained for column densities in the range 2.0-3.5 10$^17 cm^-2, consistent with previous estimates from ISO data, and TK in the range 250 to 275 K, in very good agreement with estimates made from the same ISO data.

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arXiv2004

Detection of D2H+ in the Dense Interstellar Medium

The 692 GHz para ground-state line of D2H+ has been detected at the Caltech Submillimeter Observatory towards the pre-stellar core 16293E. The derived D2H+ abundance is comparable to that of H2D+, as determined by observations of the 372 GHz line of ortho-H2D+. This is an observational verification of recent theoretical predictions (Roberts, Herbst & Millar 2003), developed to explain the large deuteration ratios observed in cold, high-density regions of the interstellar medium associated with low mass pre-stellar cores and protostars. This detection confirms expectations that the multiply deuterated forms of H3+ were missing factors of earlier models. The inclusion of D2H+ and D3+ in the models leads to predictions of higher values of the D/H ratio in the gas phase.

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arXiv2004

Line-of-sight distribution of water towards the SgrB2 complex

Here we report the detection, with the Caltech Submillimeter Observatory, of the 894-GHz HDO(111-000) transition, observed in absorption against the background continuum emission of the SgrB2 cores M and N. Radiative transfer modeling of this feature, together with the published data set of mm and submm HDO and H2O-18 transitions, suggests that ground-state absorption features from deuterated and non-deuterated water trace different gas components along the line of sight. In particular, while the HDO line seems to be produced by the large column densities of gas located in the SgrB2 warm envelope, the H2O-18 ground-state transition detected by SWAS and KAO at 548 GHz (Neufeld et al. 2000; Zmuidzinas et al. 1995) is instead a product of the hot, diffuse, thin gas layer lying in the foreground of the SgrB2 complex.

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arXiv2003

First detection of doubly deuterated hydrogen sulfide

This work was carried out with using the Caltech Submillimeter Observatory and presents the observational study of HDS and D2S towards a sample of Class 0 sources, and dense cores. We report the first detection of doubly deuterated hydrogen sulfide (D2S) in two dense cores and analyze the chemistry of these molecules aiming to help understand the deuteration processes in the interstellar medium. The observed values of the D2S/HDS ratio, and upper limits, require an atomic D/H ratio in the accreting gas of 0.1-1. The study presented in this Letter supports the hypothesis that formaldehyde, methanol and hydrogen sulfide are formed on the grain surfaces, during the cold pre-stellar core phase, where the CO depleted gas has large atomic D/H ratios. The high values for the D/H ratios are consistent with the predictions of a recent gas-phase chemical model that includes H3+ and its deuterated isotopomers, H2D+, D2H+ and D3+ (Roberts et al. 2003).

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arXiv2003

The line-of-sight distribution of water in the SgrB2 complex

We report the detection, with the Caltech Submillimeter Observatory, of the 894-GHz HDO(111-000) transition, observed in absorption against the background continuum emission of the SgrB2 cores M and N. Radiative transfer modeling of this feature, together with the published data set of mm and submm HDO and H2O-18 transitions, suggests that ground-state absorption features from deuterated and non-deuterated water trace different gas components along the line of sight. In particular, while the HDO line seems to be produced by the large column densities of gas located in the SgrB2 warm envelope, the H2O-18 ground-state transition detected by SWAS and KAO at 548 GHz (Neufeld et al. 2000; Zmuidzinas et al. 1995a) is instead a product of the hot, diffuse, thin gas layer lying in the foreground of the SgrB2 complex.

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arXiv2002

Future Mm/Submm Instrumentation and Science Opportunities: Example of Deuterated Molecules

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportunities, in the field of astrochemistry, will increase by a huge factor. This will be enhanced by the new receivers with greater bandwidth and sensitivity. The new opportunities will be in the area of astrochemistry of distant objects, through greater sensitivity, or new spectral ranges due to the platforms above the Earth's atmosphere. Various aspects of new spectral ranges are discussed, with emphasis on H2O lines, features previously hidden under H2O or O2 lines, light hydrides and particularly on deuterium in molecules. Recently, multiply deuterated species have been detected, e.g. ND3, in cold dense regions of the interstellar medium. It is argued here that it is possible that so much deuterium could be trapped, by the fractionation process, into heavy molecules such as ND3, etc..., and species such as H2D+ and possibly D2H+, that D and HD might be depleted. This would be the mechanism for the large dispersion of [D]/[H] values found in the interstellar medium. Light molecules (hydrides and deuterides) generally have large fundamental rotation frequencies, often lying in the HIFI bands. The deuterides are a specially suitable case, because the species exist mainly in cold dense regions, where the molecules are in the ground states and THz observations will best be carried out by absorption spectroscopy against background dust continuum sources such as Sgr B2 and W49N.

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arXiv2002

Triply deuterated ammonia in NGC 1333

The Caltech Submillimeter Observatory has detected triply deuterated ammonia, ND3, through its 10a-00s transition near 310 GHz. Emission is found in the NGC 1333 region, both towards IRAS 4A and a position to the South-East where DCO+ peaks. In both cases, the hyperfine ratio indicates that the emission is optically thin. Column densities of ND3 are 3--6 x 10^11 cm^-2 for T_ex=10 K and twice as high for T_ex=5 K. Using a Monte Carlo radiative transfer code and a model of the structure of the IRAS source with temperature and density gradients, the estimated ND3 abundance is 3.2 x 10^-12 if ND3/H2 is constant throughout the envelope. In the more likely case that ND3/H2D+ is constant, ND3/H2 peaks in the cold outer parts of the source at a value of 1.0 x 10^-11. To reproduce the observed NH3/ND3 abundance ratio of ~1000, grain surface chemistry requires an atomic D/H ratio of ~0.15 in the gas phase, >10 times higher than in recent chemical models. More likely, the deuteration of NH3 occurs by ion-molecule reactions in the gas phase, in which case the data indicate that deuteron transfer reactions are much faster than proton transfers.

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arXiv2000

Submillimeter and Far-InfraRed Experiment (SAFIRE): A PI class instrument for SOFIA

SAFIRE is a versatile imaging Fabry-Perot spectrograph covering 145 to 655 microns, with spectral resolving powers ranging over 5-10,000. Selected as a "PI" instrument for the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA), SAFIRE will apply two-dimensional pop-up bolometer arrays to provide background-limited imaging spectrometry. Superconducting transition edge bolometers and SQUID multiplexers are being developed for these detectors. SAFIRE is expected to be a "First Light" instrument, useable during the initial SOFIA operations. Although a PI instrument rather than a "Facility Class" science instrument, it will be highly integrated with the standard SOFIA planning, observation, and data analysis tools.

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arXiv1999

Physical conditions in shocked regions of Orion from ground-based observations of H_2O

We present observations of the 5$_{1,5}\to4_{2,2}$ transition of water vapor at 325.15 GHz taken with the CSO telescope towards Orion IRc2. The emission is more extended than that of other molecular species such as CH$_3$OH. However, it is much less extended than the emission of water vapor at 183.31 GHz reported by Cernicharo et al (1994). A comparison of the line intensities at 325.15 GHz and 183.31 GHz puts useful constraints on the density and temperature of the emitting regions and allows an estimate of H_2O abundance, x(H_2O), of $\simeq10^{-4}$ in the Plateau and $\simeq10^{-6}-10^{-5}$ in the Ridge.

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arXiv1998

Plans for a 10-m Submillimeter-wave Telescope at the South Pole

A 10 meter diameter submillimeter-wave telescope has been proposed for the NSF Amundsen-Scott South Pole Station. Current evidence indicates that the South Pole is the best submillimeter-wave telescope site among all existing or proposed ground-based observatories. Proposed scientific programs place stringent requirements on the optical quality of the telescope design. In particular, reduction of the thermal background and offsets requires an off-axis, unblocked aperture, and the large field of view needed for survey observations requires shaped optics. This mix of design elements is well-suited for large scale (square degree) mapping of line and continuum radiation from submillimeter-wave sources at moderate spatial resolutions (4 to 60 arcsecond beam size) and high sensitivity (milliJansky flux density levels). the telescope will make arcminute angular scale, high frequency Cosmic Microwave Background measurements from the best possible ground-based site, using an aperture which is larger than is currently possible on orbital or airborne platforms. Effective use of this telescope will require development of large (1000 element) arrays of submillimeter detectors which are background-limited when illuminated by antenna temperatures near 50 K.

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