Intense UV Fields Research Articles

Photoevaporation and Close Encounters: How the Environment around Cygnus OB2 Affects the Evolution of Protoplanetary Disks

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks orbiting around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cyg OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun and hosts hundreds of massive stars and thousands of low-mass members, both with and without disks. In this paper, we analyze the spatial variation of the disk fraction (i.e., the fraction of cluster members bearing a disk) in Cyg OB2 and study its correlation with the local values of far-ultraviolet (FUV) and extreme-ultraviolet (EUV) radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV fields and large stellar density. In particular, the FUV radiation dominates disk dissipation timescales in the proximity (i.e., within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars owing to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cyg OB2 and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cyg OB2 are potentially hostile to protoplanetary disks but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.

Isolating Dust and Free–Free Emission in ONC Proplyds with ALMA Band 3 Observations

The Orion Nebula Cluster (ONC) hosts protoplanetary disks experiencing external photoevaporation by the cluster’s intense UV field. These “proplyds” are comprised of a disk surrounded by an ionization front. We present ALMA Band 3 (3.1 mm) continuum observations of 12 proplyds. Thermal emission from the dust disks and free–free emission from the ionization fronts are both detected, and the high-resolution (0.″057) of the observations allows us to spatially isolate these two components. The morphology is unique compared to images at shorter (sub)millimeter wavelengths, which only detect the disks, and images at longer centimeter wavelengths, which only detect the ionization fronts. The disks are small (r d = 6.4–38 au), likely due to truncation by ongoing photoevaporation. They have low spectral indices (α ≲ 2.1) measured between Bands 7 and 3, suggesting the dust emission is optically thick. They harbor tens of Earth masses of dust as computed from the millimeter flux using the standard method although their true masses may be larger due to the high optical depth. We derive their photoevaporative mass-loss rates in two ways: first, by invoking ionization equilibrium and second, by using the brightness of the free–free emission to compute the density of the outflow. We find decent agreement between these measurements and = 0.6–18.4 × 10−7 M ⊙ yr−1. The photoevaporation timescales are generally shorter than the ∼1 Myr age of the ONC, underscoring the known “proplyd lifetime problem.” Disk masses that are underestimated due to being optically thick remains one explanation to ease this discrepancy.

Multiphoton ionization and dissociation of polycyclic aromatic hydrocarbon molecules of astrophysical interest

The photoprocessing of astrophysically relevant small polycyclic aromatic hydrocarbon (PAH) molecules, namely, anthracene, phenanthrene, and pentacene, under intense UV field is experimentally studied to explore the formation of smaller ions upon their dissociative ionization and the effect of their size and structure on the dynamics of dissociative ionization. The molecules are UV processed in the multiphoton regime using a 266-nm nanosecond laser pulse. The dissociative ionization spectra at different laser intensities reveal distinct dissociation dynamics of these PAHs based on their size and structures.

Herschel observations of the circumstellar environments of the Herbig Be stars R Mon and PDS 27

We report and analyse FIR observations of two Herbig Be stars, R Mon and PDS 27, obtained with Herschel's instruments PACS and SPIRE. We construct SEDs and derive the infrared excess. We extract line fluxes from the PACS and SPIRE spectra and construct rotational diagrams in order to estimate the excitation temperature of the gas. We derive CO, [OI] and [CI] luminosities to determine physical conditions of the gas, as well as the dominant cooling mechanism. We confirm that the Herbig Be stars are surrounded by remnants from their parental clouds, with an IR excess that mainly originates in a disc. In R Mon we detect [OI], [CI], [CII], CO (26 transitions), water and OH, while in PDS 27 we only detect [CI] and CO (8 transitions). We attribute the absence of OH and water in PDS 27 to UV photo-dissociation and photo-evaporation. From the rotational diagrams, we find several components for CO: we derive $T_{rot}$ 949$\pm$90 K, 358$\pm$20 K & 77$\pm$12 K for R Mon, 96$\pm$12 K & 31$\pm$4 K for PDS 27 and 25$\pm$8 K & 27$\pm$6 K for their respective compact neighbours. The forsterite feature at 69$\mu$m was not detected in either of the sources, probably due to the lack of (warm) crystalline dust in a flat disc. We find that cooling by molecules is dominant in the Herbig Be stars, while this is not the case in Herbig Ae stars where cooling by [OI] dominates. Moreover, we show that in the Herbig Be star R Mon, outflow shocks are the dominant gas heating mechanism, while in Herbig Ae stars this is stellar. The outflow of R Mon contributes to the observed line emission by heating the gas, both in the central spaxel/beam covering the disc and the immediate surroundings, as well as in those spaxels/beams covering the parabolic shell around it. PDS 27, a B2 star, has dispersed a large part of its gas content and/or destroyed molecules; this is likely given its intense UV field.

High resolution ALMA observations of dense molecular medium in the central regions of active galaxies

AbstractIn the central regions of active galaxies, dense molecular medium are exposed to various types of radiation and energy injections, such as UV, X-ray, cosmic ray, and shock dissipation. With the rapid progress of chemical models and implementation of new-generation mm/submm interferometry, we are now able to use molecules as powerful diagnostics of the physical and chemical processes in galaxies. Here we give a brief overview on the recent ALMA results to demonstrate how molecules can reveal underlying physical and chemical processes in galaxies. First, new detections of Galactic molecular absorption systems with elevated HCO/H13CO+column density ratios are reported, indicating that these molecular media are irradiated by intense UV fields. Second, we discuss the spatial distributions of various types of shock tracers including HNCO, CH3OH and SiO in NGC 253 and NGC 1068. Lastly, we provide an overview of proposed diagnostic methods of nuclear energy sources using ALMA, with an emphasis on the synergy with sensitive mid-infrared spectroscopy, which will be implemented by JWST and SPICA to disentangle the complex nature of heavily obscured galaxies across the cosmic time.

Ammonia observations in the LBV nebula G79.29+0.46

The surroundings of Luminous Blue Variable (LBV) stars are excellent laboratories to study the effects of their high UV radiation, powerful winds, and strong ejection events onto the surrounding gas and dust. The LBV G79.29+0.46 powered two concentric infrared rings which may interact with the infrared dark cloud (IRDC) G79.3+0.3. The Effelsberg 100m telescope was used to observe the NH_3 (1,1), (2,2) emission surrounding G79.29+0.46 and the IRDC. In addition, we observed particular positions in the (3,3) transition toward the strongest region of the IRDC. We report here the first coherent shell-like structure of dense NH_3 gas associated with an evolved massive star. The shell, two or three orders of magnitude more tenuous than the IRDC, is well traced in both ammonia lines, and surrounds the ionized nebula. The NH_3 emission in the IRDC is characterized by a low and uniform rotational temperature (T_rot ~ 10 K) and moderately high opacities in the (1,1) line. The rest of the observed field is spotted by warm or hot zones (T_rot > 30 K) and characterized by optically thin emission of the (1,1) line. The NH_3 abundances are about 10^{-8} in the IRDC, and 10^{-10}-10^{-9} elsewhere. The warm temperatures and low abundances of NH_3 in the shell suggest that the gas is being heated and photo-dissociated by the intense UV field of the LBV star. An outstanding region is found to the south-west (SW) of the LBV star within the IRDC. The NH_3 (3,3) emission at the centre of the SW region reveals two velocity components tracing gas at temperatures > 30K. The northern edge of the SW region agrees with the border of the ring nebula and a region of continuum enhancement; here, the opacity of the (1,1) line and the NH_3 abundance do not decrease as expected in a typical clump of an isolated cold dark cloud. This strongly suggests some kind of interaction between the ring nebula and the IRDC.

MID-INFRARED POLYCYCLIC AROMATIC HYDROCARBON AND H 2 EMISSION AS A PROBE OF PHYSICAL CONDITIONS IN EXTREME PHOTODISSOCIATION REGIONS

Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) and molecular hydrogen emission are a potentially powerful tool to derive physical properties of dense environments irradiated by intense UV fields. We present new, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the high UV-field and dense photodissocation region (PDR) around Monoceros R2, the closest ultracompact \hII region, revealing the spatial structure of ionized gas, PAHs and H$_2$ emissions. Using a PDR model and PAH emission feature fitting algorithm, we build a comprehensive picture of the physical conditions prevailing in the region. We show that the combination of the measurement of PAH ionization fraction and of the ratio between the H$_2$ 0-0 S(3) and S(2) line intensities, respectively at 9.7 and 12.3 $\mu$m, allows to derive the fundamental parameters driving the PDR: temperature, density and UV radiation field when they fall in the ranges $T = 250-1500 $K, $n_H=10^4-10^6$cm$^{-3}$, $G_0=10^3-10^5$ respectively. These mid-IR spectral tracers thus provide a tool to probe the similar but unresolved UV-illuminated surface of protoplanetary disks or the nuclei of starburst galaxies.

Physical conditions in the neutral interstellar medium at z = 2.43 toward Q 2348-011

Aims. We aim at deriving the physical conditions in the neutral gas associated with damped Lyman-α systems using observation and analysis of H2 and C i absorptions. Methods. We obtained a high-resolution VLT-UVES spectrum of the quasar Q 2348−011 over a wavelength range that covers most of the prominent metal and molecular absorption lines from the logN(H i) = 20.50 ± 0.10 damped Lyman-α system at zabs = 2.4263. We detected H2 in this system and measured column densities of H2 ,C i ,C i ∗ ,C i ∗∗ , Si ii ,P ii ,S ii ,F eii ,a nd Niii. From the column density ratios and, in particular, the relative populations of H2 rotational and C i fine-structure levels, we derived the physical conditions in the gas (relative abundances, dust-depletion, particle density, kinetic temperature, and ionising flux) and discuss physical conditions in the neutral phase. Results. Molecular hydrogen was detected in seven components in the first four rotational levels (J = 0–3) of the vibrational ground state. Absorption lines of H2 J = 4 (resp. J = 5) rotational levels are detected in six (resp. two) of these components. This leads to a total molecular fraction of log f �− 1.69 +0.37 −0.58 . Fourteen components are needed to reproduce the metal-line profiles. The overall metallicity is found to be −0.80, −0.62, −1.17 ± 0.10 for, respectively, [Si/H], [S/H] and [Fe/H]. We confirm the earlier findings that there is a correlation between logN(Fe ii)/N(S ii )a nd logN(Si ii)/N(S ii) from different components indicative of a dust-depletion pattern. Surprisingly, however, the depletion of metals onto dust in the H2 components is not large in this system: [Fe/S] = −0.8 to −0.1. The gas in H2-bearing components is found to be cold but still hotter than similar gas in our Galaxy (T > 130 K, instead of typically 80 K) and dense (n ∼ 100−200 cm −3 ). There is an anti-correlation (R = −0.97) between the logarithm of the photo-absorption rate, log β0 ,a nd logN(H2)/N(C i) derived for each H2 component. We show that this is mostly due to shielding effects and imply that the photo-absorption rate β0 is a good indicator of the physical conditions in the gas. We find that the gas is immersed in an intense UV field, about one order of magnitude higher than in the solar vicinity. These results suggest that the gas in H2-bearing DLAs is clumpy, and star-formation occurs in the associated object.

Photon-dominated Chemistry in the Nucleus of M82: Widespread HOC + Emission in the Inner 650 Parsec Disk

The nucleus of M82 has been mapped in several 3 and 1 mm lines of CN, HCN, C2H, c-C3H2, CH3C2H, HC3N, and HOC+ using the IRAM 30 m telescope. These species have been purposely selected as good tracers of photon-dominated chemistry. We have measured [CN]/[HCN] ~ 5 in the inner 650 pc galaxy disk. Furthermore, we have detected the HOC+ 1 → 0 line with an intensity similar to that of the H13CO+ 1 → 0 line. This implies an [HCO+]/[HOC+] ratio of ~40. These results corroborate the existence of a giant photodissociation region (PDR) in the nucleus of M82. In fact, the low [HCO+]/[HOC+] ratio can only be explained if the nucleus of M82 is formed by small (r < 0.02-0.2 pc) and dense (n ~ a few times 104-105 cm-3) clouds immersed in an intense UV field (G0 ~ 104 in units of the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.

A Self-Consistent Model for the Broadband Spectrum of Sagittarius A Eastat the Galactic Center

Sgr A East is a very prominent elongated shell structure surrounding (although off-centered from) the Galactic nucleus. Its energetics (~4×1052 ergs), based on the power required to carve out the radio synchrotronremnant within the surrounding dense molecular cloud, appear to be extreme compared to the total energy (~1051 ergs) released in a typical supernova (SN) explosion. Yet it shares several characteristics in common with SN remnants (SNRs), the most significant of which is the ~0.1-10 GeV γ-ray spectrum measured by EGRET, if we associate the Galactic center source 2EG J1746-2852 with this nonthermal shell. We here show that the highest energy component in Sgr A East's spectrum, like that of SNRs, can be fitted with the γ-rays produced in π0 decays. Further, we demonstrate in a self-consistent manner that the leptons released in the associated π± decays produce an e± distribution that can mimic a power law with index ~3, like that inferred from the VLA data for this source. These relativistic electrons and positrons also radiate by bremsstrahlung and inverse Compton scattering with the intense IR and UV fields from the nucleus. We show that the overall emissivity calculated in this way may account for Sgr A East's broadband spectrum ranging from GHz frequencies all the way to TeV energies, where Whipple has thus far set an upper limit to the flux corresponding to a 2.5 σ signal above the noise.

Far-Infrared [C II] Properties of Low-Metallicity Galaxies

The line flux ratio of the [C II] 158 μm fine-structure (2P3/2 → 2P3/2) emission to CO rotational emission is a probe of the intensity of the interstellar UV radiation in galaxies. The largest values for the [C II]/CO (J = 1-0) line ratios are found in starburst galaxies, none of which, however, show values larger than 6 ×103 (Stacey et al. 1991). The exception ([CII]/CO ≃ 2 × 104) are galaxies of Magellanic type which have low metallicity, an intense interstellar UV field, and irregular morphology (Table 1).Mochizuki et al. (1994) showed that the two conditions, a low metallicity and an intense UV field are expected to result in the large [C II]/CO ratios in the Magellanic types. These conditions are likely to be common in more massive galaxies as well, but at an early epoch in their evolution. This suggests that bright [C II] emission can be a good indicator of young galaxies. However, the metal-poor galaxies previously observed in the [C II] line were exclusively Magellanic types. Observations of different types of dwarf galaxies are necessary to help us understand the physical conditions in the interstellar medium of low-metallicity systems.

First Detection of 492 [CLC]GHz[/CLC] [C [CSC]i[/CSC]] Emission from the Large Magellanic Cloud

The 3 P1 3 3 P0 fine-structure transition of neutral atomic carbon [C I] was observed with the Antarctic Submillimeter Telescope and Remote Observatory (ASTyRO) toward two star-forming regions in the Large Magellanic Cloud: N159 and 30 Doradus. The [C I] line is weak in the vicinity of 30 Dor, a region with a uniquely hard and intense UV field. The I[C I]yICO ratio in N159 is enhanced by a factor?2 compared to the Milky Way Galaxy, a result attributable to the lower metallicity of the Large Magellanic Cloud. Subject headings: galaxies: ISM—ISM: atoms—ISM: individual (N159, 30 Doradus)—Magellanic Clouds— radio lines: ISM

Molecular and atomic hydrogen line emission from star-forming galaxies

view Abstract Citations (99) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular and Atomic Hydrogen Line Emission from Star-forming Galaxies Puxley, P. J. ; Hawarden, T. G. ; Mountain, C. M. Abstract Models of the generation of molecular and atomic hydrogen line emission from star-forming regions are discussed. These take into account the variations in the efficiency with which fluorescent H_2_ emission is produced as the ratio of the incident soft UV to the gas density changes (the lower this ratio, the higher the efficiency). The models deal with hot stars embedded within molecular clouds, large compact clusters of young stars generating intense UV fields, and emission from clouds bathed in a relatively diffuse UV field. The results from the models are compared with the ratio of the observed intensities of the H_2_ v = 1-0 S(1) and Brγ lines from the nuclear regions for a sample of 29 galaxies whose optical spectra are dominated by H II regions. Unlike previous investigators, we find that the scatter in the ratio of the two lines is quite small, suggesting a common mechanism for the H_2_ excitation. The line ratios are found to be consistent with H_2_ emission from photodissociation zones and in most of the sample can be understood in terms of ensembles of hot stars intimately mixed with molecular clouds, and perhaps forming blisters at their surface, although a few galaxies require much higher ratios of the UV intensity to the gas density, conditions which imply the presence of large numbers of massive stars in compact clusters. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169386 Bibcode: 1990ApJ...364...77P Keywords: Atomic Spectra; Emission Spectra; Hydrogen; Molecular Spectra; Star Formation; Starburst Galaxies; Astronomical Models; Star Clusters; Stellar Mass; Ultraviolet Radiation; Astrophysics; GALAXIES: INTERSTELLAR MATTER; INTERSTELLAR: MOLECULES; RADIATION MECHANISMS; STARS: FORMATION full text sources ADS | data products NED (29) SIMBAD (28) Related Materials (1) Erratum: 1991ApJ...372..733P

Fluorescent HD emission from photodissociation regions

Computations are presented of the IR fluorescent HD emission spectrum that is produced in cold isothermal static photodissociation regions at the surfaces of interstellar molecular clouds exposed to UV radiation. A scaling equation is derived which relates the HD line intensities to the gas density, the intensity of the incident UV radiation, the grain-surface molecular-formation rate coefficient, and the cosmic-ray ionization rate. Fluorescent HD emission lines may be detectable from dense photodissociation regions with hydrogen densities equal to about 5 x 10 to the 6th/cu cm which are exposed to intense UV fields. 36 refs.

The Populations of H2 Vibrational States in Intense UV Fields Near O-B Stars

In most places where molecular hydrogen exists in the interstellar medium, it will be found in the ground vibrational and ground electronic state. This will not be so, however, near 0 or early B stars where, in the region just beyond the ionization boundary, populations will be determined by UV fields up to 105 times more intense than the mean interstellar value (4 × 10−16 ergs cm−3 s−1 = 1 Habing unit). The H2 absorbs Lyman-Werner band photons longwards of λ91 nm and subsequent decays to the ground electronic state may lead to dissociation (vibrational continuum) or to one of 14 vibrationally excited states. Molecules in these states have lifetimes of order 1010 s and, in the intense fields, will be exposed to further Lyman-Werner excitation. The probability of dissociation is therefore greatly enhanced by this ‘multiple excitation’, since the number of lines available to vibrationally excited H2 is many times that available to ground-state H2 (Shull, 1978).