Dense Interstellar Medium Research Articles

The APEX-CHAMP+view of the Orion Molecular Cloud 1 core

A high density portion of the Orion Molecular Cloud 1 (OMC-1) contains the prominent, warm Kleinmann-Low (KL) nebula that is internally powered by an energetic event plus a farther region in which intermediate to high mass stars are forming. Its outside is affected by ultraviolet radiation from the neighboring Orion Nebula Cluster and forms the archetypical photon-dominated region (PDR) with the prominent bar feature. Its nearness makes the OMC-1 core region a touchstone for research on the dense molecular interstellar medium and PDRs. Using the Atacama Pathfinder Experiment telescope (APEX), we have imaged the line emission from the multiple transitions of several carbon monoxide (CO) isotopologues over the OMC-1 core region. Our observations employed the 2x7 pixel submillimeter CHAMP+ array to produce maps (~ 300 arcsec x 350 arcsec) of 12CO, 13CO, and C18O from mid-J transitions (J=6-5 to 8-7). We also obtained the 13CO and C18O J=3-2 images toward this region. The 12CO line emission shows a well-defined structure which is shaped and excited by a variety of phenomena, including the energetic photons from hot, massive stars in the nearby Orion Nebula's central Trapezium cluster, active high- and intermediate-mass star formation, and a past energetic event that excites the KL nebula. Radiative transfer modeling of the various isotopologic CO lines implies typical H2 densities in the OMC-1 core region of ~10^4-10^6 cm^-3 and generally elevated temperatures (~ 50-250 K). We estimate a warm gas mass in the OMC-1 core region of 86-285 solar masses.

Kinetic database for astrochemistry

KIDA (for KInetic Database for Astrochemistry) is a project initiated by different communities in order to 1) improve the interaction between astrochemists and physico-chemists and 2) simplify the work of modeling the chemistry of astrophysical environments. KIDA was designed to group at the same place and with the same format all gas-phase chemical reactions useful for astrochemical simulations of the interstellar medium and planetary atmospheres. From KIDA, a subset of chemical reactions has been extracted for the cold and dense interstellar medium: kida.uva.2001.

ON THE FORMATION OF INTERSTELLAR WATER ICE: CONSTRAINTS FROM A SEARCH FOR HYDROGEN PEROXIDE ICE IN MOLECULAR CLOUDS

Recent surface chemistry experiments have shown that the hydrogenation of molecular oxygen on interstellar dust grains is a plausible formation mechanism, via hydrogen peroxide (H2O2), for the production of water (H2O) ice mantles in the dense interstellar medium. Theoretical chemistry models also predict the formation of a significant abundance of H2O2 ice in grain mantles by this route. At their upper limits, the predicted and experimental abundances are sufficiently high that H2O2 should be detectable in molecular cloud ice spectra. To investigate this further, laboratory spectra have been obtained for H2O2/H2O ice films between 2.5 and 200 micron, from 10 to 180 K, containing 3%, 30%, and 97% H2O2 ice. Integrated absorbances for all the absorption features in low-temperature H2O2 ice have been derived from these spectra. For identifying H2O2 ice, the key results are the presence of unique features near 3.5, 7.0, and 11.3 micron. Comparing the laboratory spectra with the spectra of a group of 24 protostars and field stars, all of which have strong H2O ice absorption bands, no absorption features are found that can definitely be identified with H2O2 ice. In the absence of definite H2O2 features, the H2O2 abundance is constrained by its possible contribution to the weak absorption feature near 3.47 micron found on the long-wavelength wing of the 3 micron H2O ice band. This gives an average upper limit for H2O2, as a percentage of H2O, of 9% +/- 4%. This is a strong constraint on parameters for surface chemistry experiments and dense cloud chemistry models.

Formation ofH3−by radiative association ofH2andH−in the interstellar medium

We develop the theory of radiative association of an atom and a diatomic molecule within a close-coupling framework. We apply it to the formation of H$_3^-$ after the low energy collision (below 0.5 eV) of H$_2$ with H$^-$. Using recently obtained potential energy and permanent dipole moment surfaces of H$_3^-$, we calculate the lowest rovibrational levels of the H$_3^-$ electronic ground state, and the cross section for the formation of H$_3^-$ by radiative association between H$^-$ and ortho- and para-H$_2$. We discuss the possibility for the H$_3^-$ ion to be formed and observed in the cold and dense interstellar medium in an environment with a high ionization rate. Such an observation would be a probe for the presence of H$^-$ in the interstellar medium.

CONSTRAINING THE RATE OF RELATIVISTIC JETS FROM TIDAL DISRUPTIONS USING RADIO SURVEYS

Tidal disruption of stars by massive black holes produce transient accretion flows that flare at optical, UV, and X-ray wavelengths. At late times, these accretion flows may launch relativistic jets that can be detected through the interaction of the jet with the dense interstellar medium of the galaxy. We present an upper limit for the flux density of a radio counterpart to a tidal disruption event detected by GALEX that is a factor of 6 below theoretical predictions. We also examine existing radio surveys for transients with a time scale of 1 year and use these to set a $2 \sigma$ upper limit on the rate of tidal disruption events producing relativistic jets of ~14 x 10^-7 Mpc^-3 y^-1. This rate is an order of magnitude lower than the highest values from theoretical models and is consistent with detection rates from optical and X-ray surveys.

Tycho’s supernova remnant as a source of cosmic rays in our galaxy

The Tycho’s supernova remnant was observed using the SHALON atmospheric Cherenkov telescope at the Tien Shan High-Altitude Observatory. This object has long been considered to be a candidate for sources of cosmic ray hadrons in the Northern Hemisphere. In [1, 2], the Tycho’s properties were described using the nonlinear kinetic model of cosmic ray acceleration in supernovae remnants. The expected γ-ray flux from π0-decay extends to energies > 30 eV, whereas the γ-ray photon flux generated by inverse Compton scattering is cut off above several TeV. Therefore, the detection of γ-rays at energies of 10–80 TeV by the SHALON telescope is an indication of their hadron origin. From the SHALON telescope data, additional information on such parameters of the Tycho’s supernova remnant as the distance and interstellar medium density was obtained within the theory [1, 2].

A past capture event at Sagittarius A* inferred from the fluorescent X-ray emission of Sagittarius B clouds

The fluorescent X-ray emission from neutral iron in the molecular clouds (Sgr B) indicates that the clouds are being irradiated by an external X-ray source. The source is probably associated with the Galactic central black hole (Sgr A*), which triggered a bright outburst one hundred years ago. We suggest that such an outburst could be due to a partial capture of a star by Sgr A*, during which a jet was generated. By constraining the observed flux and the time variability ($\sim$ 10 years) of the Sgr B's fluorescent emission, we find that the shock produced by the interaction of the jet with the dense interstellar medium represents a plausible candidate for the X-ray source emission.

The Ubiquity of Micrometer-Sized Dust Grains in the Dense Interstellar Medium

Cold molecular clouds are the birthplaces of stars and planets, where dense cores of gas collapse to form protostars. The dust mixed in these clouds is thought to be made of grains of an average size of 0.1 micrometer. We report the widespread detection of the coreshine effect as a direct sign of the existence of grown, micrometer-sized dust grains. This effect is seen in half of the cores we have analyzed in our survey, spanning all Galactic longitudes, and is dominated by changes in the internal properties and local environment of the cores, implying that the coreshine effect can be used to constrain fundamental core properties such as the three-dimensional density structure and ages and also the grain characteristics themselves.

HerschelPACS and SPIRE imaging of CW Leonis

Herschel PACS and SPIRE images have been obtained over a 30'x30' area around the well-known carbon star CW Leo (IRC +10 216). An extended structure is found in an incomplete arc of ~22' diameter, which is cospatial with the termination shock due to interaction with the interstellar medium (ISM) as defined by Sahai & Chronopoulos from ultraviolet GALEX images. Fluxes are derived in the 70, 160, 250, 350, and 550 um bands in the region where the interaction with the ISM takes place, and this can be fitted with a modified black body with a temperature of 25+-3 K. Using the published proper motion and radial velocity for the star, we derive a heliocentric space motion of 25.1 km/s. Using the PACS and SPIRE data and the analytical formula of the bow shock structure, we infer a de-projected standoff distance of the bow shock of R0 = (8.0+-0.3)x10^17 cm. We also derive a relative velocity of the star with respect to the ISM of (106.6+-8.7)/sqrt(n_ISM) km/s, where n_ISM is the number density of the local ISM.

CO AND H+3TOWARD MWC 1080, MWC 349, AND LkHα 101

We present high-resolution, near-infrared NIRSPEC observations of the fundamental rovibrational CO and H + R(1,0), R(1,1) u , and Q(1,0) transitions toward three early-type young stars: MWC 1080, MWC 349, and LkHα 101. These observations were performed for the purpose of constraining the physical characteristics of the interstellar material along each line of sight. Toward MWC 1080, we detected strong CO absorption and determined a column density upper limit of 1.4 × 10 14 cm −2 for H + . We infer that there is very little diffuse material along the line of sight toward MWC 1080 and that the CO absorption is consistent with an origin in the dispersing natal cloud. We detected both cold CO and H + toward MWC 349, consistent with a diffuse cloud origin. Similarly, both CO and H + were detected toward LkHα 101. Using a recently revised value for the cosmic ray ionization rate, we conclude that the CO absorption is consistent with a dense cloud origin while the H + could originate in either the dense or diffuse interstellar medium. We also find no evidence for CO fractionation toward LkHα 101 as reported by Goto et al.

Active galactic nuclei activity: self-regulation from backflow

We study the internal circulation within the cocoon carved out by the relativistic jet emanating from an active galactic nucleus (AGN) within the interstellar medium (ISM) of its host galaxy. First, we develop a model for the origin of the internal flow, noticing that a significant increase of large-scale velocity circulation within the cocoon arises as significant gradients in the density and entropy are created near the hotspot (a consequence of Crocco’s vorticity generation theorem). We find simple and accurate approximate solutions for the large-scale flow, showing that a backflow towards the few inner parsec region develops. We solve the appropriate fluid dynamic equations, and we use these solutions to predict the mass inflow rates towards the central regions. We then perform a series of 2D simulations of the propagation of jets using flash 2.5, in order to validate the predictions of our model. In these simulations, we vary the mechanical input power between 1043 and 1045 erg s−1, and assume a Navarro–Frenk–White (NFW) density profile for the dark matter halo, within which an isothermal diffuse ISM is embedded. The backflows which arise supply the central AGN region with very low angular-momentum gas, at average rates of the order of ⁠, the exact value seen to be strongly dependent on the central ISM density (for fixed input jet power). The time-scales of these inflows are apparently weakly dependent on the jet/ISM parameters, and are of the order of ⁠. We then argue that these backflows could (at least partially) feed the AGN, and provide a self-regulatory mechanism of AGN activity, that is not directly controlled by, but instead controls, the star formation rate within the central circumnuclear disc.

Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms

Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic grain surface analogue in dense clouds, i.e., amorphous water ice. Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique. Results. We observe production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur on dust grains, believed to be the site where key processes lead to the molecular diversity and complexity observed in the Universe.

BONDI–HOYLE–LYTTLETON ACCRETION ONTO A PROTOPLANETARY DISK

Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this `tail-end' accretion for planet formation was recently highlighted by Throop & Bally (2008). While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such that outer disk migrates inwards, increasing the inner disk surface density and reducing the outer radius. The accretion flow also triggers the development of spiral density features, and changes to the disk inclination. The mean accretion rate onto the star remains roughly the same with and without the presence of a disk. We discuss the potential impact of this process on planet formation, including the possibility of triggered gravitational instability; inclination differences between the disk and the star; and the appearance of spiral structure in a gravitationally stable system.

SHORT HARD GAMMA-RAY BURSTS AND THEIR AFTERGLOWS

Long duration gamma ray bursts (GRBs) and X-ray flashes (XRFs) are produced by highly- relativistic jets ejected in core-collapse supernova explosions. The origin of short hard gamma-ray bursts (SHBs) has not been established. They may be produced by highly relativistic jets ejected in various processes: mergers of compact stellar objects; large-mass accretion episodes onto compact stars in close binaries or onto intermediate-mass black holes in dense stellar regions; phase transition of compact stars. Natural environments of such events are the dense cores of globular clusters, superstar clusters and young supernova remnants. We have used the cannonball model of GRBs to analyze all Swift SHBs with a well-sampled X-ray afterglow. We show that their prompt gamma-ray emission can be explained by inverse Compton scattering (ICS) of the progenitor's glory light, and their extended soft emission component by ICS of high density light or synchrotron radiation (SR) in a high density interstellar medium within the cluster. The mechanism generating the afterglow is synchrotron radiation outside the cluster. No associated supernova could be detected in the low luminosity nearby GRBs 060614 and 060505. We interpret them as SHBs seen relatively far off axis.

Three-dimensional numerical model of the Omega Nebula (M17): simulated thermal X-ray emission

We present 3D hydrodynamical simulations of the superbubble M17, also known as the Omega Nebula, carried out with the adaptive grid code yguazu-a, which includes radiative cooling. The superbubble is modelled considering the winds of 11 individual stars from the open cluster inside the nebula (NGC 6618), for which there are estimates of the mass-loss rates and terminal velocities based on their spectral types. These stars are located inside a dense interstellar medium, and they are bounded by two dense molecular clouds. We carried out three numerical models of this scenario, considering different line-of-sight positions of the stars (the position in the plane of the sky is known, thus fixed). Synthetic thermal X-ray emission maps are calculated from the numerical models and compared with ROSAT observations of this astrophysical object. Our models successfully reproduce both the observed X-ray morphology and the total X-ray luminosity, without taking into account the thermal conduction effects.

LABORATORY SPECTRA OF THE CO 2 BENDING-MODE FEATURE IN INTERSTELLAR ICE ANALOGUES SUBJECT TO THERMAL PROCESSING

The infrared absorption features of solid carbon dioxide have been detected by space-based observatories in nearly all lines of sight probing the dense interstellar medium (ISM). It has also been shown that the absorption feature of solid CO2 near 658 cm−1 (15.2 μm) should be a sensitive indicator of the physical conditions of the ice (e.g., temperature and composition). However, the profile structure of this feature is not well understood, and previous laboratory studies have concentrated on a limited range of temperatures and compositions for comparisons with observed spectra from both the Infrared Space Observatory and the Spitzer Space Telescope. In the laboratory study described here, the infrared spectra of ices bearing H2O, CH3OH, and CO2 have been measured with systematically varying compositions and temperatures that span the range of the values expected in the ISM. The mid-IR spectra (λ = 2.5–25 μm) were measured for 47 different ice compositions at temperatures ranging from ∼5.5 K to evaporation, at 5 K intervals.

RADIATION FROM THE INTERACTION OF MICROQUASARS WITH THE ISM

Microquasars (MQs) are X-ray binary systems that display relativistic radio jets. These objects constitute a suitable laboratory for testing high energy astrophysical processes still not well understood, such as those present when jets interact with the interstellar medium (ISM). Focusing on the study of the nonthermal contribution from cocoon and bow-shock regions, we explore, under different ISM densities and ages of the jet source, the possibility to detect MQ jet termination regions. We conclude that emission from these regions may be faint, but still detectable in the radio, X-ray, and gamma-ray bands.

A Measurement of the Spatial Distribution of Diffuse TeV Gamma‐Ray Emission from the Galactic Plane with Milagro

Diffuse $\gamma$-ray emission produced by the interaction of cosmic-ray particles with matter and radiation in the Galaxy can be used to probe the distribution of cosmic rays and their sources in different regions of the Galaxy. With its large field of view and long observation time, the Milagro Gamma Ray Observatory is an ideal instrument for surveying large regions of the Northern Hemisphere sky and for detecting diffuse $\gamma$-ray emission at very high energies. Here, the spatial distribution and the flux of the diffuse $\gamma$-ray emission in the TeV energy range with a median energy of 15 TeV for Galactic longitudes between 30$^\circ$ and 110$^\circ$ and between 136$^\circ$ and 216$^\circ$ and for Galactic latitudes between -10$^\circ$ and 10$^\circ$ are determined. The measured fluxes are consistent with predictions of the GALPROP model everywhere except for the Cygnus region ($l\in[65^\circ,85^\circ]$). For the Cygnus region, the flux is twice the predicted value. This excess can be explained by the presence of active cosmic ray sources accelerating hadrons which interact with the local dense interstellar medium and produce gamma rays through pion decay.

Suzaku Observation of the RCW86 Northeastern Shell

This paper reports on Suzaku results concerning the northeast shell of RCW86. With both spatial and spectral analyses, we separated the X-rays into three distinct components: low ($kT_{\rm e}\sim$0.3keV) and high ($kT_{\rm e}\sim$1.8keV) temperature plasmas and a non-thermal component, and discovered that their spatial distributions are different from each other. The low-temperature plasma is dominated at the east rim, whereas the non-thermal emission is brightest at the northeast rim, which is spatially connected from the east. The high-temperature plasma, found to contain the $\sim$6.42keV line (K$\alpha$ of low-ionized iron), is enhanced at the inward region with respect to the east rim, and has no spatial correlation with the non-thermal X-rays (the northeast). This result suggests that the Fe-K$\alpha$ line originates from Fe-rich ejecta heated by reverse shock. A possible scenario to explain these morphologies and spectra is that a fast-moving blast wave in a thin cavity collided with a dense interstellar medium at the east region very recently. As a result, the reverse shock in this interior decelerated, and arrived at the Fe-rich region of the ejecta and heated it. In the northeast rim, on the other hand, the blast wave is still moving fast, and is accelerating electrons causing them to emit strong synchrotron X-rays.

Analytical view of diffusive and convective cosmic ray transport in elliptical galaxies

Context: An analytical solution of the generalized diffusive and convective transport equation is derived to explain the transport of cosmic ray protons within elliptical galaxies. Aims: Cosmic ray transport within elliptical galaxies is an interesting element in understanding the origin of high energetic particles measured on Earth. As probable sources of those high energetic particles, elliptical galaxies show a dense interstellar medium as a consequence of activity in the galactic nucleus or merging events between galaxies. Thus it is necessary for an appropriate description of cosmic ray transport to take the diffusive and convective processes in a dense interstellar environment into account. Here we show that the transport equations can be solved analytically with respect to the given geometry and boundary conditions in position space, as well as in momentum space. Results: The spatial solution is shown using a generalized source of cosmic rays. Additionally, the special case of a jet-like source is illustrated. We present the solution in momentum space with respect to an escape term for cosmic ray protons depending on the spatial shape of the galaxy. For a delta-shape injection function, the momentum solution is obtained analytically. We find that the spectral index measured on Earth can be obtained by appropriately choosing of the strength of Fermi I and Fermi II processes. From these results we calculate the gamma-ray flux from pion decay due to proton-proton interaction to give connection to observations. Additionally we determine the escape-spectrum of cosmic rays. The results show that both spectra are harder than the intrinsic power-law spectrum for cosmic rays in elliptical galaxies.