Hot Stars and Cool Clouds: The Photodissociation Region M16 (original) (raw)
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Photoionized and Photodissociated Regions around Main‐Sequence Stars
The Astrophysical Journal, 1998
Within a molecular cloud, the strong ultraviolet radiation Ðeld produced by newly formed stars dissociates and ionizes the surrounding molecular gas. The radiative Ñux depends on the e †ective temperature and metallicity of the star. Using the most recent line-blanketed atmosphere models from Kurucz, we obtain the rates of ionizing and dissociating photons from stars with e †ective temperatures of 7.5 ] 103 to 5 ] 104 K, and for metallicities between 0.01 times solar and solar. With a radiative transfer model, we then compute the basic structures and sizes of the photoionized and photodissociated regions produced by stars embedded in a molecular gas with uniform densities. Absorption of the UV Ñux by dust decreases the mass of H II and H I produced within the cloud, and its e †ects are taken into account in our model. We also discuss the constraints imposed by photodissociated regions on the number of intermediate-and high-mass stars that can form in molecular clouds.
PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars
2022
Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the Universe, from the era of vigorous star formation at redshifts of 1-3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter- and circumstellar media including diffuse clouds, proto-planetary disks and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the James Webb ...
H ii REGION G46.5-0.2: THE INTERPLAY BETWEEN IONIZING RADIATION, MOLECULAR GAS, AND STAR FORMATION
The Astronomical Journal, 2015
Hii regions are particularly interesting because they can generate dense layers of gas and dust, elongated columns or pillars of gas pointing towards the ionizing sources, and cometary globules of dense gas, where triggered star formation can occur. Understanding the interplay between the ionizing radiation and the dense surrounding gas is very important to explain the origin of these peculiar structures, and hence to characterize triggered star formation. G46.5-0.2 (G46), a poorly studied galactic HII region located at about 4 kpc, is an excellent target to perform this kind of studies. Using public molecular data extracted from the Galactic Ring Survey (13 CO J=1-0) and from the James Clerk Maxwell Telescope data archive (12 CO, 13 CO, C 18 O J=3-2, HCO + and HCN J=4-3), and infrared data from the GLIMPSE and MIPSGAL surveys, we perform a complete study of G46, its molecular environment and the young stellar objects placed around it. We found that G46, probably excited by an O7V star, is located close to the edge of the GRSMC G046.34-00.21 molecular cloud. It presents a horseshoe morphology opening in direction of the cloud. We observed a filamentary structure in the molecular gas likely related to G46 and not considerable molecular emission towards its open border. We found that about 10 ′ towards the southwest of G46 there are some pillar-like features, shining at 8 µm and pointing towards the Hii region open border. We propose that the pillar-like features were carved and sculpted by the ionizing flux from G46. We found several young stellar objects likely embedded in the molecular cloud grouped in two main concentrations: one, closer to the G46 open border consisting of Class II type sources, and other one mostly composed by Class I type YSOs located just ahead the pillars-like features, strongly suggesting an age gradient in the YSOs distribution.
Monthly Notices of the Royal Astronomical Society, 2010
We report the results of visual spectroscopy, mid-infrared (MIR) mapping and photometry, and near infrared (NIR) photometry of two candidate symbiotic stars (IPHAS J193108.67+164950.5 and IPHAS J193709.65+202655.7) associated with extended MIR emission. Our analysis of the continua of these sources shows that they are likely to represent Class I-II young stellar objects (YSOs) in which most of the IR emission arises from circumstellar disks, and for which the physical characteristics (stellar temperatures, radii, masses and luminosities) are similar. The extended emission is characterised by a substantial increase in fluxes and dimensions to longer MIR wavelengths. This is likely to arise as a result of emission by polycyclic aromatic hydrocarbons (PAHs) within extended photodissociation regimes (PDRs), centred upon more compact ionized regions responsible for much of the shorter wave emission. Such dual emission structures are characteristic of those observed in many compact HII regions. Finally, we note that the clouds have asymmetrical structures and windswept morphologies, conceivably indicative of shock interaction with external winds. Where this is the case, then it is possible that the YSOs are located in regions of triggered star-formation.
Astronomy & Astrophysics, 2013
Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope. Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages. Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H2O, OH, and atomic lines. For four targets the spectra span the entire 55-200 μm region. The PACS field-of-view covers ~47" with the resolution of 9.4". Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H2O 818-707 63.3 μm line (Eu/kB = 1071 K) is detected in 7 sources. CO transitions from J = 14-13 up to J = 49 - 48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~350 K and ~700 K. H2O has typical excitation temperatures of ~150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on &≥10,000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H2O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [O I] fluxes. In contrast, [O I] and OH often peak together at the central position. Conclusions. The PACS data probe at least two physical components. The H2O and CO emission very likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, most likely resulting from H2O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [O I] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H2O and CO, with the fraction contributed by [O I] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state.
We present new observations of the major starforming region N 66 in the Small Magellanic Cloud and of its surroundings, which add to those presented in Contursi et al. (2000, Paper I). High-sensitivity CO observations allowed the detection of molecular gas associated with the H ii region, for which a high-resolution image in [O iii ] λ 5007 is presented. We also present images in the v(1-0) S(1) line of H 2 at 2.12 µm and in the adjacent continuum. This material reveals an interesting photodissociation region. We show that the molecular gas that has not yet been photodissociated by the UV radiation of the stars is in hot, dense clumps with a very small surface filling factor. We discovered several embedded stars or stellar associations, and suggest that three successive stellar generations have already taken place in less than 3 million years.
Near-infrared spectroscopic analysis of hot massive stars
2004
1.33 Observations of young massive stars 1.3.11 High-Mass Protostellar Objects 1.3.22 Ultracompact H II regions 1.3.33 High excitation compact H II "blobs" 1.44 Diagnostics from near-infrared spectroscopy 1.55 Outline of this thesis 1.5.11 Empirical diagnostics 1.5.22 Predictions 1.5.33 Application to NGC2024 1.5.44 Conclusions and prospects for near-infrared spectroscopy 22 An atlas of 2.4 to 4.1 /xm ISO/SWS spectra of early-type stars 2.11 Introduction 2.22 Observations 2.2.11 The ISO/SWS sample 2.2.22 The CGS4/UKIRT sample 2.2.33 Atlas 2.33 Identification and measurement of spectral lines 2.3.11 Overview of lines in the 2.4 to 4.1 /xm region 2.3.22 Of supergiants and WR 147 l l Contents Contents 2.3.33 Equivalent width 40 2.44 Line trends and spectral classification of O and B stars 2.4.11 B supergiants and bright giants 2.4.22 B dwarfs and giants 45 2.4.33 O stars 47 2.55 B stars with emission lines 48 2.66 Summary 50 2.77 Appendix: EW measurements 33 Hydrogen infrared recombination lines as a diagnostic tool for the geometryy of the circumstellar material of hot stars 3.11 Introduction 3.22 The sample of stars and the ISO spectra 3.33 Line flux ratio diagnostic 3.44 Conclusion 44 Modelling the near-infrared lines of O-type stars 4.11 Introduction 4.22 The grid of models 4.2.11 Limitations of the models 4.33 Line trends 4.3.11 Hydrogen lines 4.3.22 Hen lines 4.3.33 He I lines 4.44 Comparison with observations 4.4.11 Lines of carbon and nitrogen 4.55 Diagnostics 4.5.11 Spectral Type 4.5.22 Surface gravity 4.5.33 Wind density 4.66 Conclusion 55 Identification of the ionising source of NGC2024 97 102 5.3.33 The position of IRS2b in the HRD 105 5.44 Discussion 107 n n Contents Contents 66 The peculiar circumstellar environment of NGC2024-IRS2 111 6.11 Introduction 6.22 Observations 6.2.11 Observations and data reduction 6.2.22 Line properties and identifications 6.33 The spectral energy distribution 6.3.11 Contribution of IRS2 to the observed SED 6.3.22 Extinction 6.44 Diagnostic tools 6.4.11 Radio slope 6.4.22 Infrared hydrogen recombination lines 6.4.33 CO band-heads 6.55 Discussion 6.5.11 Model A: a gaseous disk 6.5.22 Model B: a stellar wind and optically thin dust 6.5.33 Model C: a wind plus a gaseous disk 6.5.44 The nature of NGC2024/IRS2 6.66 Conclusion 6.77 Appendix: Literature data
The Astrophysical Journal, 2011
We present an analysis of Spitzer-IRS observations of H 2 O, OH, HCN, C 2 H 2 and CO 2 emission, and Keck-NIRSPEC observations of CO emission, from a diverse sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections and strengths of most mid-IR molecular emission features are correlated with each other, suggesting a common origin and similar excitation conditions for this mid-infrared line forest. Aside from the remarkable differences in molecular line strengths between T Tauri, Herbig Ae/Be and transitional disks discussed in Pontoppidan et al. (2010b), we note that the line detection efficiency is anti-correlated with the 13/30 µm SED spectral slope, which is a measure of the degree of grain settling in the disk atmosphere. We also note a correlation between detection efficiency and Hα equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H 2 O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L ⋆. However, significant sample variation, especially in molecular line ratios, remains, and its origin has yet to be explained. LTE models of the H 2 O emission show that line strength is primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H 2 O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near ∼ 10 18 cm −2 and 450 K for all sources, suggesting a high abundance of H 2 O in many planet-forming regions. Other molecules have a range of excitation temperatures from ∼ 500 − 1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of ∼ 10 −3 for all molecules, with the exception of CO, for which n(CO)/n(H 2 O)∼1. However, LTE fitting caveats and differences in the way thermochemical modeling results are reported make comparisons with such models difficult, and highlight the need for additional observations coupled with the use of line-generating radiative transfer codes.
2014
Since the launch of the Herschel Space Observatory, our understanding about the photo-dissociation regions (PDR) has taken a step forward. In the bandwidth of the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) on board Herschel, ten CO rotational transitions, including J=4-3 to J=13-12, and three fine structure lines, including [CI] 609, [CI] 370, and [NII] 250 micron, are covered. In this paper, we present our findings from the FTS observations at the nuclear region of M83, based on the spatially resolved physical parameters derived from the CO spectral line energy distribution (SLED) map and the comparisons with the dust properties and star-formation tracers. We discuss (1) the potential of using [NII] 250 and [CI] 370 micron as star-formation tracers; (2) the reliability of tracing molecular gas with CO; (3) the excitation mechanisms of warm CO; (4) the possibility of studying stellar feedback by tracing the thermal pressure of molec...