arXiv subjects

Nick Indriolo

Publications and source records attributed to Nick Indriolo.

33 records · Page 2Linked to original sources

arXiv2014

Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen fraction, f(H2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f(H2) in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042+-0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer zeta_H throughout our sample, and find a log-normal distribution with mean log(zeta_H)=-15.75, (zeta_H=1.78x10^-16 s^-1), and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H3+ observations. Ionization rates in the Galactic center tend to be 10--100 times larger than found in the Galactic disk, also in accord with prior studies.

astro-ph.GA↗

arXiv2014

Ubiquitous argonium (ArH$^+$) in the diffuse interstellar medium -- a molecular tracer of almost purely atomic gas

We describe the assignment of a previously unidentified interstellar absorption line to ArH$^+$ and discuss its relevance in the context of hydride absorption in diffuse gas with a low H$_2$ fraction. The column densities along several lines of sight are determined and discussd in the framework of chemical models. The column densities of ArH$^+$ are compared to those of other species, tracing interstellar medium (ISM) components with different H$_2$ abundances. Chemical models are constructed, taking UV radiation and cosmic ray ionization into account. Due to the detection of two isotopologues, $^{36}$ArH$^+$ and $^{38}$ArH$^+$, we are confident about the carrier assignment to ArH$^+$. NeH$^+$ is not detected with a limit of [NeH$^+$]/[ArH$^+$] $\le$ 0.1. The derived column densities agree well with the predictions of chemical models. ArH$^+$ is a unique tracer of gas with a fractional H$_2$ abundance of $10^{-4}- 10^{-3}$ and shows little correlation with H$_2$O$^+$, which traces gas with a fractional H$_2$ abundance of $\approx $0.1. A careful analysis of variations in the ArH$^+$, OH$^+$, H$_2$O$^+$ and HF column densities promises to be a faithful tracer of the distribution of the H$_2$ fractional abundance, providing unique information on a poorly known phase in the cycle of interstellar matter, its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD) simulations of the interstellar medium, and potentially could evolve into a tool to characterize the ISM. Paradoxically, the ArH$^+$ molecule is a better tracer of \new{almost} purely atomic hydrogen gas than H{\sc i} itself, since H{\sc i} can also be present in gas with a significant molecular content, but ArH$^+$ singles out gas that is $>99.9$\% atomic.

astro-ph.GA↗

arXiv2014

Infrared H3+ and CO Studies of the Galactic Core: GCIRS 3 and GCIRS 1W

We have obtained improved spectra of key fundamental band lines of H3+, R(1,1)l, R(3,3)l, and R(2,2)l, and ro-vibrational transitions of CO on sightlines toward the luminous infrared sources GCIRS 3 and GCIRS 1W, each located in the Central Cluster of the Galactic center within several arcseconds of Sgr A*. The spectra reveal absorption occurring in three kinds of gaseous environments: (1) cold dense and diffuse gas associated with foreground spiral/lateral arms; (2) warm and diffuse gas absorbing over a wide and mostly negative velocity range, which appears to fill a significant fraction of the Galaxy's Central Molecular Zone (CMZ); and (3) warm, dense and compact clouds with velocities near +50 km s^-1 probably within 1-2 pc of the center. The absorptions by the first two cloud types are nearly identical for all the sources in the Central Cluster, and are similar to those previously observed on sightlines from Sgr A* to 30 pc east of it. Cloud type (3), which has only been observed toward the Central Cluster, shows distinct differences between the sightlines to GCIRS 3 and GCIRS 1W, which are separated on the sky by only 0.33 pc in projection. We identify this material as part of an inward extension of the Circumnuclear Disk previously known from HCN mapping. Lower limits on the products of the hydrogen ionization rate zeta and the path length L are 2.3 x 10^5 cm s^-1 and 1.5 x 10^3 cm s^-1 for the warm and diffuse CMZ gas and for the warm and dense clouds in the core, respectively. The limits indicate that the ionization rates in these regions are well above 10^-15 s^-1.

astro-ph.GA↗

arXiv2013

Water Absorption from Gas Very Near the Massive Protostar AFGL 2136 IRS 1

We present ground-based observations of the nu_1 and nu_3 fundamental bands of H2O toward the massive protostar AFGL 2136 IRS 1, identifying absorption features due to 47 different ro-vibrational transitions between 2.468 micron and 2.561 micron. Analysis of these features indicates the absorption arises in warm (T=506+-25 K), very dense (n(H2)>5*10^9 cm^-3) gas, suggesting an origin close to the central protostar. The total column density of warm water is estimated to be N(H2O)=(1.02+-0.02)*10^19 cm^-2, giving a relative abundance of N(H2O)/N(H2)~10^-4. Our study represents the first extensive use of water vapor absorption lines in the near infrared, and demonstrates the utility of such observations in deriving physical parameters.

astro-ph.GA↗

arXiv2013

H3+ Spectroscopy and the Ionization Rate of Molecular Hydrogen in the Central Few Parsecs of the Galaxy

We report observations and analysis of infrared spectra of H3+ and CO lines in the Galactic center, within a few parsecs of the central black hole, Sgr A*. We find a cosmic ray ionization rate typically an order of magnitude higher than outside the Galactic center. Notwithstanding, the elevated cosmic ray ionization rate is 4 orders of magnitude too short to match the proton energy spectrum as inferred from the recent discovery of the TeV gamma-ray source in the vicinity of Sgr A*.

astro-ph.GA↗

arXiv2013

Direct Determination of the HF/H2 Abundance Ratio in Interstellar Gas

We report the first detection of the v=1--0, R(0) ro-vibrational transition of HF at 2.499385 microns arising from interstellar gas. The line is seen in absorption toward 3 background sources---HD 154368, Elias 29, and AFGL 2136 IRS 1---all of which have reported H2 column densities determined from observations of H2. This allows for the first direct determination of the HF/H2 abundance ratio. We find values of N(HF)/N(H2)=1.15*10^-8 and 0.69*10^-8 for HD 154368 and Elias 29, respectively. The sight line toward AFGL 2136 IRS 1 also shows absorption from the v=1--0, R(1) transition of HF, indicating warm, dense (n_H > 10^9 cm^-3) gas, likely very close to the central protostar. Ascribing portions of the HF absorption to warm and cold gas, we find N(HF)/N(H2)=(1.7--2.9)*10^-8 and (0.33--0.58)*10^-8 for the two components, respectively. Except for the warm component toward AFGL 2136 IRS 1, all observed HF/H2 ratios are well below N(HF)/N(H2)=3.6*10^-8, the value predicted if nearly all gas-phase fluorine is in the form of HF. Models of fluorine chemistry that account for depletion onto grains are able to reproduce the results toward HD 154368, but not in the cold, dense gas toward AFGL 2136 IRS 1 and Elias 29. Most likely, some combination of simplifying assumptions made in the chemical models are responsible for these discrepancies.

astro-ph.GA↗

arXiv2012

Chemical Analysis of a Diffuse Cloud along a Line of Sight Toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate

Absorption lines from the molecules OH+, H2O+, and H3+ have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH+ and H2O+ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H3+ is used to infer the cosmic-ray ionization rate of H2 in the molecular interior of the cloud, which we find to be zeta_2=(4.8+-3.4)x10^-16 per second. Combining the results from all three species we find an efficiency factor---defined as the ratio of the formation rate of OH+ to the cosmic-ray ionization rate of H---of epsilon=0.07+-0.04, much lower than predicted by chemical models. This is an important step in the future use of OH+ and H2O+ on their own as tracers of the cosmic-ray ionization rate.

astro-ph.GA↗

arXiv2012

Herschel observations of interstellar chloronium

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six sources in the Galaxy. We detected interstellar chloronium absorption in foreground molecular clouds along the sight-lines to the bright submillimeter continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and para-H2-37Cl+ isotopologues were detected, through observations of their 1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio of 3, the observed optical depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the gas phase. We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of ~ 2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2 Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591. The chloronium abundances inferred in this study are typically at least a factor ~10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine. Several explanations for this discrepancy were investigated, but none has proven satisfactory, and thus the large observed abundances of chloronium remain puzzling.

astro-ph.GA↗

arXiv2011

Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+

Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (zeta_2) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H3+ observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7+-1.3)x10^-16 s^-1<zeta_2<(10.6+-8.2)x10^-16 s^-1 with a mean value of zeta_2=(3.5^+5.3_-3.0)x10^-16 s^-1. Upper limits (3 sigma) derived from non-detections of H3+ are as low as zeta_2<0.4x10^-16 s^-1. These low upper-limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in zeta_2 between different diffuse cloud sight lines. A study of zeta_2 versus N_H (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when N_H increases to values typical of dense molecular clouds. Both the difference in ionization rates between diffuse and dense clouds and the variation of zeta_2 among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.

astro-ph.GA↗

arXiv2011

On the Ortho:Para Ratio of H3+ in Diffuse Molecular Clouds

The excitation temperature T_01 derived from the relative intensities of the J = 0 (para) and J = 1 (ortho) rotational levels of H2 has been assumed to be an accurate measure of the kinetic temperature in interstellar environments. In diffuse molecular clouds, the average value of T_01 is ~70 K. However, the excitation temperature T(H3+) derived from the (J,K) = (1,1) (para) and (1,0) (ortho) rotational levels of H3+ has been observed to be ~30 K in the same types of environments. In this work, we present observations of H3+ in three additional diffuse cloud sight lines for which H2 measurements are available, showing that in 4 of 5 cases T_01 and T(H3+) are discrepant. We then examine the thermalization mechanisms for the ortho:para ratios of H3+ and H2, concluding that indeed T_01 is an accurate measure of the cloud kinetic temperature, while the ortho:para ratio of H3+ need not be thermal. By constructing a steady-state chemical model taking into account the nuclear-spindependence of reactions involving H3+, we show that the ortho:para ratio of H3+ in diffuse molecular clouds is likely governed by a competition between dissociative recombination with electrons and thermalization via reactive collisions with H2.

astro-ph.GA↗

arXiv2010

Investigating the Cosmic-Ray Ionization Rate Near the Supernova Remnant IC 443 Through H3+ Observations

Observational and theoretical evidence suggests that high-energy Galactic cosmic rays are primarily accelerated by supernova remnants. If also true for low-energy cosmic rays, the ionization rate near a supernova remnant should be higher than in the general Galactic interstellar medium (ISM). We have searched for H3+ absorption features in 6 sight lines which pass through molecular material near IC 443---a well-studied case of a supernova remnant interacting with its surrounding molecular material---for the purpose of inferring the cosmic-ray ionization rate in the region. In 2 of the sight lines (toward ALS 8828 and HD 254577) we find large H3+ column densities, N(H3+)~3*10^14 cm^-2, and deduce ionization rates of zeta_2~2*10^-15 s^-1, about 5 times larger than inferred toward average diffuse molecular cloud sight lines. However, the 3 sigma upper limits found for the other 4 sight lines are consistent with typical Galactic values. This wide range of ionization rates is likely the result of particle acceleration and propagation effects, which predict that the cosmic-ray spectrum and thus ionization rate should vary in and around the remnant. While we cannot determine if the H3+ absorption arises in post-shock (interior) or pre-shock (exterior) gas, the large inferred ionization rates suggest that IC 443 is in fact accelerating a large population of low-energy cosmic rays. Still, it is unclear whether this population can propagate far enough into the ISM to account for the ionization rate inferred in diffuse Galactic sight lines.

astro-ph.HE↗

arXiv2010

Constraining the Environment of CH+ Formation with CH3+ Observations

The formation of CH+ in the interstellar medium has long been an outstanding problem in chemical models. In order to probe the physical conditions of the ISM in which CH+ forms, we propose the use of CH3+ observations. The pathway to forming CH3+ begins with CH+, and a steady state analysis of CH3+ and the reaction intermediary CH2+ results in a relationship between the CH+ and CH3+ abundances. This relationship depends on the molecular hydrogen fraction, f_H2, and gas temperature, T, so observations of CH+ and CH3+ can be used to infer the properties of the gas in which both species reside. We present observations of both molecules along the diffuse cloud sight line toward Cyg OB2 No. 12. Using our computed column densities and upper limits, we put constraints on the f_H2 vs. T parameter space in which CH+ and CH3+ form. We find that average, static, diffuse molecular cloud conditions (i.e. f_H2>0.2, T~60 K) are excluded by our analysis. However, current theory suggests that non-equilibrium effects drive the reaction C+ + H_2 --> CH+ + H, endothermic by 4640 K. If we consider a higher effective temperature due to collisions between neutrals and accelerated ions, the CH3+ partition function predicts that the overall population will be spread out into several excited rotational levels. As a result, observations of more CH3+ transitions with higher signal-to-noise ratios are necessary to place any constraints on models where magnetic acceleration of ions drives the formation of CH+.

astro-ph.GA↗

arXiv2009

Interstellar Metastable Helium Absorption as a Probe of the Cosmic-Ray Ionization Rate

The ionization rate of interstellar material by cosmic rays has been a major source of controversy, with different estimates varying by three orders of magnitude. Observational constraints of this rate have all depended on analyzing the chemistry of various molecules that are produced following cosmic-ray ionization, and in many cases these analyses contain significant uncertainties. Even in the simplest case (H3+) the derived ionization rate depends on an (uncertain) estimate of the absorption path length. In this paper we examine the feasibility of inferring the cosmic-ray ionization rate using the 10830 A absorption line of metastable helium. Observations through the diffuse clouds toward HD 183143 are presented, but yield only an upper limit on the metastable helium column density. A thorough investigation of He+ chemistry reveals that only a small fraction of He+ will recombine into the triplet state and populate the metastable level. In addition, excitation to the triplet manifold of helium by secondary electrons must be accounted for as it is the dominant mechanism which produces He* in some environments. Incorporating these various formation and destruction pathways, we derive new equations for the steady state abundance of metastable helium. Using these equations in concert with our observations, we find zeta_He < 1.2*10^-15 s^-1, an upper limit about 5 times larger than the ionization rate previously inferred for this sight line using H3+. While observations of interstellar He* are extremely difficult at present...

astro-ph.SR↗

arXiv2009

The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds

Diffuse interstellar clouds show large abundances of H_3^+ which can be maintained only by a high ionization rate of H_2. Cosmic rays are the dominant ionization mechanism in this environment, so the large ionization rate implies a high cosmic-ray flux, and a large amount of energy residing in cosmic rays. In this paper we find that the standard propagated cosmic-ray spectrum predicts an ionization rate much lower than that inferred from H_3^+. Low-energy (~10 MeV) cosmic rays are the most efficient at ionizing hydrogen, but cannot be directly detected; consequently, an otherwise unobservable enhancement of the low-energy cosmic-ray flux offers a plausible explanation for the H_3^+ results. Beyond ionization, cosmic rays also interact with the interstellar medium by spalling atomic nuclei and exciting atomic nuclear states. These processes produce the light elements Li, Be, and B, as well as gamma-ray lines. To test the consequences of an enhanced low-energy cosmic-ray flux, we adopt two physically-motivated cosmic-ray spectra which by construction reproduce the ionization rate inferred in diffuse clouds, and investigate the implications of these spectra on dense cloud ionization rates, light element abundances, gamma-ray fluxes, and energetics. One spectrum proposed here provides an explanation for the high ionization rate seen in diffuse clouds while still appearing to be broadly consistent with other observables, but the shape of this spectrum suggests that supernovae remnants may not be the predominant accelerators of low-energy cosmic rays.

astro-ph.HE↗

arXiv2007

H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate

Using high resolution infrared spectroscopy we have surveyed twenty sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14 cm^-2. This brings to fourteen the total number of diffuse cloud sightlines where H3+ has been detected. These detections are mostly along sightlines concentrated in the Galactic plane, but well dispersed in Galactic longitude. The results imply that abundant H3+ is common in the diffuse interstellar medium. Because of the simple chemistry associated with H3+ production and destruction, these column density measurements can be used in concert with various other data to infer the primary cosmic-ray ionization rate, zeta_p. Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16 s^-1. Where H3+ is not detected the upper limits on the ionization rate are consistent with this range. The average value of zeta_p is about an order of magnitude larger than both the canonical rate and rates previously reported by other groups using measurements of OH and HD. The discrepancy is most likely due to inaccurate measurements of rate constants and the omission of effects which were unknown when those studies were performed. We believe that the observed column density of H3+ is the most direct tracer for the cosmic-ray ionization rate due to its simple chemistry. Recent models of diffuse cloud chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to reproduce observed abundances of various atomic and molecular species, in rough accord with our observational findings.

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