Searcharxiv⌕ Search

arXiv subjects

Neal J. Evans II

Publications and source records attributed to Neal J. Evans II.

214 records · Page 12Linked to original sources
arXiv2001

Photon Dominated Regions in Low UV Fields: A Study of The Peripheral Region of L1204/S140

We have carried out an in-depth study of the peripheral region of the molecular cloud L1204/S140, where the far ultraviolet radiation and the density are relatively low. Our observations test theories of photon-dominated regions (PDRs) in a regime that has been little explored. CII 158 micron and OI 63 micron lines are detected by ISO at all 16 positions along a 1-dimensional cut in right ascension. Emission from molecular hydrogen rotational transitions, at 28 and 17 micron, was also detected at several positions. The CII, OI, and molecular hydrogen intensities along the cut show much less spatial variation than do the rotational lines of CO and other CO isotopes. The average CII and OI intensities and their ratio are consistent with models of PDRs with low FUV radiation (Go) and density. The best-fitting model has Go about 15 and density, n about 1000 per cubic cm. Standard PDR models underpredict the intensity in the H2 rotational lines by up to an order of magnitude. This problem has also been seen in bright PDRs and attributed to factors, such as geometry and gas-grain drift, that should be much less important in the regime studied here. The fact that we see the same problem in our data suggests that more fundamental solutions, such as higher H2 formation rates, are needed. Also, in this regime of low density and small line width, the OI line is sensitive to the radiative transfer and geometry. Using the ionization structure of the models, a quantitative analysis of timescales for ambipolar diffusion in the peripheral regions of the S140 cloud is consistent with a theory of photoionization-regulated star formation. Observations of CII in other galaxies differ both from those of high Go PDRs in our galaxy and from the low Go regions we have studied. ~

astro-ph↗
arXiv2001

Dust Continuum Observations of Massive Star Forming Regions

We have observed 51 high mass star forming cores associated with water masers at 350 micron. The spectral energy distributions (SEDs) and dust continuum normalized radial intensity profiles were modeled for 28 sources using a one-dimensional dust radiative transfer code assuming a power law density distribution in the envelope n = n_0 (r/r_0)^{-p}. The best fit density power law exponent, p, ranged from 1.0 to 2.5 with a mean value of p = 1.72. We determined the dust masses for the modeled cores and found a mean mass of 209 M_{sun}.

astro-ph↗
arXiv2001

Dust and Molecular Emission from the S235 Star Forming Region

We present the results of a multi-faceted study of the high-mass star-forming region S235. With the CalTech Submillimeter Observatory (CSO), we have observed and/or mapped over fifty molecular transitions and 350 micron dust emission in an attempt to learn more about the chemistry, kinematics, and mass distribution within this rich region. We have found two dust cores that are not resolved in our molecular line observations and suggest that one of these is a less evolved star-forming core.

astro-ph↗
arXiv2001

Early Phases and Initial Conditions for Massive Star Formation

Our knowledge of the initial conditions and early stages of high mass star formation is very limited. We will review recent surveys of regions in the early stages of massive star formation using molecular tracers of high density and dust continuum emission and consider the status of evolutionary schemes. Comparison to the situation for low mass, relatively isolated star formation will be used to illustrate the outstanding issues in massive star formation. The problem of initial conditions is particularly acute because there is a lack of observational evidence for regions capable of forming massive stars BEFORE star formation actually begins. By analogy with the Pre-Protostellar Cores (PPCs) studied for low-mass star formation, one might call such regions Pre-Proto-cluster Cores (PPclCs). We will conclude with some speculation about what such cores might look like and possibilities for their detection.

astro-ph↗
arXiv2001

Tracing the Mass during Low-Mass Star Formation. II. Modelling the Submillimeter Emission from Pre-Protostellar Cores

We have modeled the emission from dust in pre-protostellar cores, including a self-consistent calculation of the temperature distribution for each input density distribution. Model density distributions include Bonnor-Ebert spheres and power laws. The Bonnor-Ebert spheres fit the data well for all three cores we have modeled. The dust temperatures decline to very low values ($\Td \sim 7$ K) in the centers of these cores, strongly affecting the dust emission. Compared to earlier models that assume constant dust temperatures, our models indicate higher central densities and smaller regions of relatively constant density. Indeed, for L1544, a power-law density distribution, similar to that of a singular, isothermal sphere, cannot be ruled out. For the three sources modeled herein, there seems to be a sequence of increasing central condensation, from L1512 to L1689B to L1544. The two denser cores, L1689B and L1544, have spectroscopic evidence for contraction, suggesting an evolutionary sequence for pre-protostellar cores.

astro-ph↗
arXiv2000

Tracing the Mass during Low-Mass Star Formation. I. Submillimeter Continuum Observations

We have obtained 850 and 450 micron continuum maps of 21 low mass cores with SED's ranging from pre-protostellar to Class I (18K < Tbol < 370K), using SCUBA at the JCMT. In this paper we present the maps, radial intensity profiles, and photometry. Pre-protostellar cores do not have power-law intensity profiles, whereas the intensity profiles of Class 0 and Class I sources can be fitted with power laws over a large range of radii. A substantial number of sources have companion sources within a few arcminutes (2 out of 5 pre-protostellar cores, 9 out of 16 Class 0/I sources). The mean separation between sources is 10800 AU. The median separation is 18000 AU including sources without companions as a lower limit. The mean value of the spectral index between 450 and 850 microns is 2.8 +- 0.4, with PPCs having slightly lower spectral indices (2.5 +- 0.4). The mean mass of the sample, based on the dust emission in a 120 arcsecond aperture, is 1.1 +- 0.9 solar masses. For the sources fitted by power-law intensity distributions (I_nu(b)/I_nu(0) = (b/b_0) ^ m), the mean value of m is 1.52 +- 0.45 for Class 0 and I sources at 850 microns and 1.44 +- 0.25 at 450 microns. Based on a simple analysis, assuming the emission is in the Rayleigh-Jeans limit and that T_d(r) ~ r ^ {-0.4}, these values of m translate into power-law density distributions (n ~ r ^ {-p}) with p ~ 2.1. However, we show that this result may be changed by more careful consideration of effects such as beam size and shape, finite outer radii, more realistic T_d(r), and failure of the Rayleigh-Jeans approximation.

astro-ph↗
arXiv2000

How to Identify Pre-Protostellar Cores

We have observed the HCO+ J=3-2 line toward 17 starless cores selected from the list of Ward-Thompson et al. (1994). Six of these cores have line asymmetries indicative of collapse. The excess of blue-skewed profiles over red-skewed profiles is at least as large as that found in samples of Class 0 and early Class I sources. The observed line profiles have the same narrow linewidths and small peak temperatures predicted for young sources in evolutionary models, but the blue/red ratios, like those of older sources, are higher than models predict. The infall signature also occurs over large scales, suggesting that these cores have overall inward motions. We have divided these starless cores into two groups based on the continuum photometry of Ward-Thompson et al. and our HCO+ data. We find stronger HCO+ emission among the cores detected in the submillimeter and all the blue-skewed line profiles are in this group, supporting the suggestion of Ward-Thompson et al. that these are the pre-protostellar cores.

astro-ph↗
arXiv2000

Studies of Low-Mass Star Formation with ALMA

ALMA will revolutionize the study of star formation by providing a combination of angular resolution and sensitivity that far exceeds that of present instruments. I will focus on studies of relatively isolated cores that are forming low-mass stars. There is a general paradigm for the formation of such stars, and there are detailed theoretical predictions for the evolution of the density and velocity fields for different assumptions about the initial conditions. Because the theory is well developed, observational tests are particularly revealing. The primary probes of physical conditions in these regions are discussed and the sensitivity of ALMA to these probes is shown and compared to the current state of the art. The consequences for the ALMA requirements are discussed.

astro-ph↗
arXiv1999

Does Infall End Before the Class I Stage?

We have observed HCO+ J=3-2 toward 16 Class I sources and 18 Class 0 sources, many of which were selected from Mardones et al. (1997). Eight sources have profiles significantly skewed to the blue relative to optically thin lines. We suggest six sources as new infall candidates. We find an equal "blue excess" among Class 0 and Class I sources after combining this sample with that of Gregersen et al. (1997). We used a Monte Carlo code to simulate the temporal evolution of line profiles of optically thick lines of HCO+, CS and H2CO in a collapsing cloud and found that HCO+ had the strongest asymmetry at late times. If a blue-peaked line profile implies infall, then the dividing line between the two classes does not trace the end of the infall stage.

astro-ph↗
arXiv1999

The Impact of the Massive Young Star GL 2591 on its Circumstellar Material: Temperature, Density and Velocity Structure

The temperature, density and kinematics of the gas and dust surrounding the luminous young stellar object GL~2591 are investigated on scales as small as 100 AU, probed by 4.7 micron absorption spectroscopy, to over 60,000 AU, probed by single-dish submillimeter spectroscopy. These two scales are connected by interferometric 86-226 GHz images of size 60,000 AU and resolution 2000 AU in continuum and molecular lines. The data are used to constrain the physical structure of the envelope and investigate the influence of the young star on its immediate surroundings.

astro-ph↗
arXiv1997

New Protostellar Collapse Candidates: An HCO+ Survey of the Class 0 Sources

We have observed 23 Class 0 sources in the HCO+ J=4-3 and 3-2 lines. The mean bolometric temperature of the 16 sources with well-determined values is 44 K and the mean luminosity is 5.7 L_sun, excluding two sources of considerably higher luminosity. Nine sources, including three sources previously suggested to be collapsing, have the correct (blue) spectral line asymmetry for infall in both lines. Three sources have the opposite (red) asymmetry in both lines, and one source, L1157, has a red asymmetry in HCO+ J=4-3 and a blue asymmetry in J=3-2. The rest have no significant or consistent asymmetry. The H13CO+ J=4-3 and 3-2 lines were also observed to find the velocity of the ambient gas, and sources with an interesting line asymmetry were mapped. A Monte Carlo code was used to produce an evolutionary sequence of collapsing cloud models of the HCO+ J=4-3 and 3-2 lines and to compare various diagnostics of the resulting line profiles. The same code was used to compare infall models to the observations in one source, L1527. The results were consistent with previous collapse models. Based on integrated intensity maps of the line peaks and wings, as well as the velocity of the H13CO+ line, we select six of the nine sources with a blue line asymmetry as good candidates for protostellar collapse. Further evidence is needed to establish that infall is taking place. The HCO+ spectra are not conclusive because bipolar outflows produce strong emission, which can confuse the issue in any individual source. However, the predominance of blue asymmetries over red asymmetries is not naturally explained in outflow models, whereas it is expected in collapse models.

astro-ph↗
arXiv1996

Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers

We have observed 150 regions of massive star formation, selected originally by the presence of a water maser, in the J = 5-4, 3-2, and 2-1 transitions of CS, and 49 regions in the same transitions of C$^{34}$S. Over 90% of the 150 regions were detected in the J = 2-1 and 3-2 transitions of CS and 75% were detected in the J=5-4 transition. We have combined the data with the J = 7-6 data from our original survey (Plume et al. 1992) to determine the density by analyzing the excitation of the rotational levels. Using Large Velocity Gradient (LVG) models, we have determined densities and column densities for 71 of these regions. The gas densities are very high (the mean log of the density is 5.9), but much less than the critical density of the J=7-6 line. Small maps of 25 of the sources in the J = 5-4 line yield a mean diameter of 1.0 pc. The mean virial mass is 3800 solar masses. The mean ratio of bolometric luminosity to virial mass (L/M) is 190, about 50 times higher than estimates using CO emission, suggesting that star formation is much more efficient in the dense gas probed in this study. The gas depletion time for the dense gas is roughly 1.3 x 10^7 yr. We find no statistically significant linewidth--size or density--size relationships in our data. Instead, both linewidth and density are larger for a given size than would be predicted by the usual relationships. We find that the linewidth increases with density, the opposite of what would be predicted by the usual arguments. We estimate that the luminosity of our Galaxy (excluding the inner 400 pc) in the CS J = 5-4 transition is 15 to 23 L_sun, considerably less than the luminosity in this line within the central 100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to CS luminosity is higher in M82 than in any cloud in our sample.

astro-ph↗
arXiv1995

Observation of Infrared and Radio Lines of Molecules toward GL2591 and Comparison to Physical and Chemical Models

We have observed rovibrational transitions of acetylene and HCN near 13 microns in absorption toward GL2591. We also observed rotational lines of CS, HCN, H2CO, and HCO+. The combined data are analyzed in terms of models with a cloud envelope with density gradients and discrete regions of hot, dense gas, probably near the infrared source. The abundance of HCN is enhanced by a factor of 400 in the gas producing the infrared absorption, in agreement with chemical models which involve depletion of molecules onto grains and subsequent sublimation when temperatures are raised.

astro-ph↗
arXiv1993

Exteremely High Velocity Outflows

Extremely high velocity (EHV) wings, with full widths of 72 to 140 km/s, are seen on the CO J=3-2 lines toward W3 IRS 5, GL 490, NGC 2071, W28 A2, GL 2591, S140, and Cepheus A. The results of our survey suggest that EHV wings are common around infrared sources of moderate to high luminosity (500 to 4x10^5 Lsun) in dense regions. Line ratios imply that the EHV gas is usually optically thin and warm. Characteristic velocities range from 20 to 40 km/s, yielding timescales of 1600-4200 yr. Since most sources in this study are producing some ionizing photons, these short timescales suggest that neutral winds coexist with ionizing photons. We examined two possible sources for the EHV CO emission: a neutral stellar wind; and swept-up or entrained molecular gas. Neither can be ruled out. If the high-velocity (HV) gas is swept up by a momentum-conserving stellar wind traced by the extremely high velocity CO emission, most of the C in the winds from luminous objects cannot be in CO. If the EHV and HV forces are equal, the fraction of C in a form other than CO increases with source luminosity and with the production rate of ionizing photons.

astro-ph↗