arXiv subjects

S. R. Federman

Publications and source records attributed to S. R. Federman.

At least 19 recordsLinked to original sources

arXiv2026

A Definitive Determination of the Interstellar Carbon Abundance toward rho Ophiuchi A and B

We present the results of an effort to derive interstellar gas-phase C II abundances along the lines of sight toward rho Oph A and B. Our analysis is based on high-resolution NUV and FUV archival spectra acquired with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Column densities of C II are derived both from fits to the weak C II] 2325 intersystem transition and from fits to the damping wings of the strong C II 1334 line. We find that the results from the weak-line and strong-line determinations agree with each other remarkably well for both sight lines, demonstrating the reliability of the f-values of the C II transitions. Furthermore, the gas-phase C abundance that we obtain for rho Oph A is in very good agreement with previous determinations of interstellar C abundances from measurements of the weak C II] 2325 line. By demonstrating the reliability of the damping wing fitting technique for the C II 1334 line, our analysis opens the door to future surveys of interstellar C abundances using the same methodology.

astro-ph.GA↗

arXiv2025

Rotational equilibrium of C$_2$ in diffuse interstellar clouds

Context. Recent spectroscopic measurements have revealed absorption from higher rotational levels in C$_2$ than previous observations. These improvements are accompanied by the availability of updated radiative and collisional data. Aims. We revisit the density and radiation field intensity diagnostics provided by the observations of many rotational levels of inter- stellar C$_2$ and extensive molecular information. Methods. We built an excitation model of C2 without spatial structure, including levels up to J= 34 where updated radiative and collisional excitation data are introduced as well as excitation by chemical formation. Results. We confirm the importance of the recent collisional excitation rate coefficients of C$_2$ by molecular H$_2$. We show that the new higher level observations cannot be explained by the standard balance between collisional excitation and radiative transitions. We propose that chemical excitation at formation provides a plausible mechanism to explain the observed high excitation of C$_2$. In addition, it allows us to lift the degeneracy of the density over radiation field strength parameter in the excitation model. Conclusions. A 0D model remains limited and it is highly desirable to use a full Photon Dominated Region (PDR) model, which includes all excitation processes introduced here and full chemical and thermal balance.

astro-ph.GA↗

arXiv2024

A refined search for high-velocity gas in the Cygnus Loop supernova remnant

We present the results of a sensitive search for high-velocity gas in interstellar absorption lines associated with the Cygnus Loop supernova remnant (SNR). We examine high-resolution, high signal-to-noise ratio optical spectra of six stars in the Cygnus Loop region with distances greater than ~700 pc. All stars show low-velocity Na I and Ca II absorption. However, only one star, HD 198301, exhibits high-velocity Ca II absorption components, at velocities of +62, +82, and +96 km/s. The distance to this star of ~870 pc helps to constrain the distance to the receding edge of the Cygnus Loop's expanding shock front. One of our targets, HD 335334, was previously thought to exhibit high positive and high negative velocity interstellar Na I and Ca II absorption. This was one factor leading Fesen et al. to derive a distance to the Cygnus Loop of 725 pc. However, we find that HD 335334 is in fact a double-line spectroscopic binary and shows no evidence of high-velocity interstellar absorption. As such, the distance to HD 335334 cannot be used to constrain the distance to the Cygnus Loop. Our detection of Ca II absorption approaching 100 km/s toward HD 198301 is the first conclusive detection of high-velocity absorption from a low ionization species associated with the Cygnus Loop SNR. A large jump in the Na I column density toward BD+31 4218, a star located beyond the northwestern boundary of the Cygnus Loop, helps to constrain the distance to a large molecular cloud complex with which the Cygnus Loop is evidently interacting.

astro-ph.GA↗

arXiv2023

The Distribution of Metallicities in the Local Galactic Interstellar Medium

In this investigation, we present an analysis of the metallicity distribution that pertains to neutral gas in the local Galactic interstellar medium (ISM). We derive relative ISM metallicities for a sample of 84 sight lines probing diffuse atomic and molecular gas within 4 kpc of the Sun. Our analysis is based, in large part, on column density measurements reported in the literature for 22 different elements that are commonly studied in interstellar clouds. We supplement the literature data with new column density determinations for certain key elements and for several individual sight lines important to our analysis. Our methodology involves comparing the relative gas-phase abundances of many different elements for a given sight line to simultaneously determine the strength of dust depletion in that direction and the overall metallicity offset. We find that many sight lines probe multiple distinct gas regions with different depletion properties, which complicates the metallicity analysis. Nevertheless, our results provide clear evidence that the dispersion in the metallicities of neutral interstellar clouds in the solar neighborhood is small ($\sim$0.10 dex) and only slightly larger than the typical measurement uncertainties. We find no evidence for the existence of very low metallicity gas (as has recently been reported by De Cia et al.) along any of the 84 sight lines in our sample. Our results are consistent with a local Galactic ISM that is well mixed and chemically homogeneous.

astro-ph.GA↗

arXiv2023

A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium

We present a comprehensive examination of interstellar P and Cl abundances based on an analysis of archival spectra acquired with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. Column densities of P II, Cl I, and Cl II are determined for a combined sample of 107 sight lines probing diffuse atomic and molecular gas in the local Galactic interstellar medium (ISM). We reevaluate the nearly linear relationship between the column densities of Cl I and H$_2$, which arises from the rapid conversion of Cl$^+$ to Cl$^0$ in regions where H$_2$ is abundant. Using the observed total gas-phase P and Cl abundances, we derive depletion parameters for these elements, adopting the methodology of Jenkins. We find that both P and Cl are essentially undepleted along sight lines showing the lowest overall depletions. Increasingly severe depletions of P are seen along molecule-rich sight lines. In contrast, gas-phase Cl abundances show no systematic variation with molecular hydrogen fraction. However, enhanced Cl (and P) depletion rates are found for a subset of sight lines showing elevated levels of Cl ionization. An analysis of neutral chlorine fractions yields estimates for the amount of atomic hydrogen associated with the H$_2$-bearing gas in each direction. These results indicate that the molecular fraction in the H$_2$-bearing gas is at least 10% for all sight lines with $\log N({\rm H}_2)\gtrsim18$ and that the gas is essentially fully molecular at $\log N({\rm H}_2)\approx21$.

astro-ph.GA↗

arXiv2022

Multireference configuration interaction study of the predissociation of C$_{2}$ via its $F\,^1Π_u$ state

Photodissociation is one of the main destruction pathways for dicarbon (C$_{2}$) in astronomical environments such as diffuse interstellar clouds, yet the accuracy of modern astrochemical models is limited by a lack of accurate photodissociation cross sections in the vacuum ultraviolet range. C$_{2}$ features a strong predissociative $F\,^1Π_u - X\,^1Σ_g^+$ electronic transition near 130 nm originally measured in 1969; however, no experimental studies of this transition have been carried out since, and theoretical studies of the $F\,^1Π_u$ state are limited. In this work, potential energy curves of excited electronic states of C$_{2}$ are calculated with the aim of describing the predissociative nature of the $F\,^1Π_u$ state and providing new ab initio photodissociation cross sections for astrochemical applications. Accurate electronic calculations of 56 singlet, triplet, and quintet states are carried out at the DW-SA-CASSCF/MRCI+Q level of theory with a CAS(8,12) active space and the aug-cc-pV5Z basis set augmented with additional diffuse functions. Photodissociation cross sections arising from the vibronic ground state to the $F\,^1Π_u$ state are calculated by a coupled-channel model. The total integrated cross section through the $F\,^1Π_u$ $v=0$ and $v=1$ bands is 1.198$\times$10$^{-13} $cm$^2$cm$^{-1}$, giving rise to a photodissociation rate of 5.02$\times$10$^{-10}$ s$^{-1}$ under the standard interstellar radiation field, much larger than the rate in the Leiden photodissociation database. In addition, we report a new $2\,^1Σ_u^+$ state that should be detectable via a strong $2\,^1Σ_u^+-X\,^1Σ_g^+$ band around 116 nm.

astro-ph.GA↗

arXiv2022

Fourier-transform spectroscopy of $^{13}$C$^{17}$O and deperturbation analysis of the A$^1Π$ ($ν$ = 0 - 3) levels

The high-resolution B$^1Σ^+$ - A$^1Π$ (0, 0) and (0, 3) emission bands of the less-abundant $^{13}$C$^{17}$O isotopologue have been investigated by Fourier-transform spectroscopy in the visible region using a Bruker IFS 125HR spectrometer at an accuracy 0.003 cm$^{-1}$. These spectra are combined with high-resolution photoabsorption measurements of the $^{13}$C$^{17}$O B$^1Σ^+$ $\leftarrow$ X$^1Σ^+$ (0, 0), B$^1Σ^+$ $\leftarrow$ X$^1Σ^+$ (1, 0) and C$^1Σ^+$ $\leftarrow$ X$^1Σ^+$ (0, 0) bands recorded with an accuracy of 0.01 cm$^{-1}$ using the vacuum ultraviolet Fourier-transform spectrometer, installed on the DESIRS beamline at the SOLEIL synchrotron. In the studied 17,950 - 22,500 cm$^{-1}$ and 86,800 - 92,100 cm$^{-1}$ regions, 480 transitions have been measured. These new experimental data were combined with data from the C $\to$ A and B $\to$ A systems, previously analyzed in 13C17O. The frequencies of 1003 transitions derived from 12 bands were used to analyze the perturbations between the A$^1Π$ ($ν$ = 0 - 3) levels and rovibrational levels of the d$^3Δ_i$, e$^3Σ^-$, a$'{}^3Σ^+$, I$^1Σ^-$ and D$^1Δ$ states as well as to a preliminary investigation of weak irregularities that appear in the B$^1Σ^+$ ($ν$ = 0) level. Deperturbed molecular constants and term values of the A$^1Π$ state were obtained. The spin-orbit and L-uncoupling interaction parameters as well as isotopologue-independent spin-orbit and rotation-electronic perturbation parameters were derived.

physics.chem-ph↗

arXiv2021

The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus

We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions toward stars behind the Taurus Molecular Cloud (TMC). New high-resolution spectra at visible and near infrared wavelengths of interstellar Ca II, Ca I, K I, CH, CH^+, C2, CN, and CO toward HD28975 and HD29647 are combined with data at visible wavelengths and published CO results from ultraviolet measurements for HD27778 and HD30122. Gas densities and temperatures are inferred from C2, CN, and CO excitation and CN chemistry. Our results for HD29647 are noteworthy because the CO column density is 10^{18} cm^{-2} while C2 and CO excitation reveals a temperature of 10 K and density about 1000 cm^{-3}, more like conditions found in dark molecular clouds. Similar results arise from our chemical analysis for CN through reactions involving observations of CH, C2, and NH. Enhanced potassium depletion and a reduced CH/H2 column density ratio also suggest the presence of a dark cloud. The directions toward HD27778 and HD30122 probe molecule-rich diffuse clouds, which can be considered CO-dark gas, while the sight line toward HD28975 represents an intermediate case. Maps of dust temperature help refine the description of the material along the four sight lines and provide an estimate of the distance between HD29647 and a clump in the TMC. An Appendix provides results for the direction toward HD26571; this star also probes diffuse molecular gas.

astro-ph.GA↗

arXiv2019

Lifetimes and Oscillator Strengths for Ultraviolet Transitions in Neutral Chlorine

We present lifetime measurements using beam-foil techniques for radiative transitions from the 3$p^4$($^1S$)4$s$ $^2S_{1/2}$, 3$p^4$($^3P$)5$s$ $^2P_{1/2,3/2}$, and 3$p^4$($^3P$)3$d$ $^2F_{5/2}$ levels in Cl I and the corresponding results of the oscillator strengths for transitions at 1004.68, 1079.88, 1090.73, and 1094.77 Å, respectively. We compare our experimental results with available theoretical calculations and astronomical observations in an effort to resolve discrepancies among them.

physics.atom-ph↗

arXiv2019

Runaway O-star Bow Shocks as Particle Accelerators? The Case of AE Aur revisited

We present results of our Chandra/ACIS observations of the field centered on the fast, runaway O star AE Aur and its bow shock. Previous XMM-Newton observations revealed an X-ray "blob" near the IR arc tracing the bow shock, possibly a nonthermal source consistent with models of Inverse Compton scattering of dust IR photons by electrons accelerated at the shock. The new, subarcsecond resolution Chandra data, while confirming the presence of the XMM-Newton source, clearly indicate that the latter is neither extended nor coincident with the IR arc and strongly suggest it is a background AGN. Motivated by results published for the bow shock of BD+43 3654, we extended our study to the radio domain, by analyzing archival EVLA data. We find no radio emission from the AE Aur bow shock either. The corresponding upper limits for the absorbed (unabsorbed) X-ray flux of 5.9(7.8)x10^-15 erg/cm^2/s (3 sigma) and, in the radio range, of 2 mJy (1.4 GHz), and 0.4 mJy (5.0 GHz), are used to put constraints on model predictions for particle acceleration within the bow shock. In the "classical" framework of Diffusive Shock Acceleration, we find that the predicted X-ray and radio emission by the bow shock is at least two orders of magnitude below the current upper limits, consistent with the systematic non-detections of up to 60 stellar bow shocks. The only exception so far remains that of BD+43 3654, probably the result of its very large mass-loss rate among runaway O stars.

astro-ph.HE↗

arXiv2019

Ab initio Study of Ground-State CS Photodissociation Via Highly Excited Electronic States

Photodissociation by ultraviolet radiation is the key destruction pathway for CS in photon-dominated regions, such as diffuse clouds. However, the large uncertainties of photodissociation cross sections and rates of CS, resulting from a lack of both laboratory experiments and theoretical calculations, limit the accuracy of calculated abundances of S-bearing molecules by modern astrochemical models. Here we show a detailed \textit{ab initio} study of CS photodissociation. Accurate potential energy curves of CS electronic states were obtained by choosing an active space CAS(8,10) in MRCI+Q/aug-cc-pV(5+d)Z calculation with additional diffuse functions, with a focus on the $B$ and $C\,^1Σ^+$ states. Cross sections for both direct photodissociation and predissociation from the vibronic ground state were calculated by applying the coupled-channel method. We found that the $C-X$ $(0-0)$ transition has extremely strong absorption due to a large transition dipole moment in the Franck-Condon region and the upper state is resonant with several triplet states via spin-orbit couplings, resulting in predissociation to the main atomic products C $(^3P)$ and S $(^1D)$. Our new calculations show the photodissociation rate under the standard interstellar radiation field is $2.9\ee{-9}$\,s$^{-1}$, with a 57\% contribution from $C-X$ $(0-0)$ transition. This value is larger than that adopted by the Leiden photodissociation and photoionization database by a factor of 3.0. Our accurate \textit{ab initio} calculations will allow more secure determination of S-bearing molecules in astrochemical models.

astro-ph.GA↗

arXiv2018

Oscillator Strengths for Ultraviolet Transitions in P II -The Multiplet at 1308 Å

We report lifetimes, branching fractions, and the resulting oscillator strengths for transitions within the P II multiplet (3s23p2 3P - 3s3p3 3Po) at 1308 Å. These comprehensive beam-foil measurements, which are the most precise set currently available experimentally, resolve discrepancies involving earlier experimental and theoretical results. Interstellar phosphorus abundances derived from λ1308 can now be interpreted with greater confidence. In the course of our measurements, we also obtained an experimental lifetime for the 3p4s 3P0o level of P IV. This lifetime agrees well with the available theoretical calculation.

physics.atom-ph↗

arXiv2018

The Connection Between Different Tracers Of The Diffuse Interstellar Medium: Kinematics

Using visible, radio, microwave, and sub-mm data, we study several lines of sight toward stars generally closer than 1 kpc on a component-by-component basis. We derive the component structure seen in absorption at visible wavelengths from Ca II, Ca I, K I, CH, CH$^{+}\!,$ and CN and compare it to emission from H I, CO and its isotopologues, and C$^{+}$ from the GOT C+ survey. The correspondence between components in emission and absorption help create a more unified picture of diffuse atomic and molecular gas in the interstellar medium. We also discuss how these tracers are related to the CO-dark H$_{2}$ gas probed by C$^{+}$ emission and discuss the kinematic connections among the species observed.

astro-ph.GA↗

arXiv2017

Parsec-scale Variations in the 7Li/6Li Isotope Ratio Toward IC 348 and the Per OB 2 Association

Measurements of the lithium isotopic ratio in the diffuse interstellar medium from high-resolution spectra of the LiI λ6708 resonance doublet have now been reported for a number of lines of sight. The majority of the results for the 7Li/6Li ratio are similar to the Solar System ratio of 12.2 but the line of sight toward o Per, a star near the star-forming region IC 348, gave a ratio of about 2, the expected value for gas exposed to spallation and fusion reactions driven by cosmic rays. To examine the association of IC 348 with cosmic rays more closely, we measured the lithium isotopic ratio for lines of sight to three stars within a few parsecs of o Per. One star, HD 281159, has 7Li/6Li ~ 2 confirming production by cosmic rays. The lithium isotopic ratio toward o Per and HD 281159 together with published analyses of the chemistry of interstellar diatomic molecules suggest that the superbubble surrounding IC 348 is the source of the cosmic rays.

astro-ph.GA↗

arXiv2013

OH+ in Diffuse Molecular Clouds

Near ultraviolet observations of OH+ and OH in diffuse molecular clouds reveal a preference for different environments. The dominant absorption feature in OH+ arises from a main component seen in CH+ (that with the highest CH+/CH column density ratio), while OH follows CN absorption. This distinction provides new constraints on OH chemistry in these clouds. Since CH+ detections favor low-density gas with small fractions of molecular hydrogen, this must be true for OH+ as well, confirming OH+ and H2O+ observations with the Herschel Space Telescope. Our observed correspondence indicates that the cosmic ray ionization rate derived from these measurements pertains to mainly atomic gas. The association of OH absorption with gas rich in CN is attributed to the need for high enough density and molecular fraction before detectable amounts are seen. Thus, while OH+ leads to OH production, chemical arguments suggest that their abundances are controlled by different sets of conditions and that they coexist with different sets of observed species. Of particular note is that non-thermal chemistry appears to play a limited role in the synthesis of OH in diffuse molecular clouds.

astro-ph.GA↗

arXiv2012

Ultraviolet Measurements of Interstellar C2

We analyzed archival spectra acquired with the Hubble Space Telescope for a study of interstellar C2. Absorption from the electronic transitions, D ^1Sigma^+_u -- X ^1Sigma^+_g (0,0) as well as F ^1Pi_u -- X ^1Sigma^+_g (0,0) and (1,0), was the focus of the study. Our profile syntheses revealed that the lines of the F-X bands were broadened as a result of a perturbation involving the upper levels. Further evidence for the perturbation came from anomalies in line strength and position for the F-X (0,0) band. The perturbation likely arises from a combination of triplet-singlet interactions involving spin-orbit mixing between ^3Pi_u states and F ^1Pi_u and an avoided crossing between the ^3Pi_u states. Tunneling through a potential barrier caused by the 3 and 4 ^1Pi_u states and spin-orbit mixing with other close-lying triplet states of ungerade symmetry are less likely. Except for the broadening, lines in the F-X (1,0) band appear free from anomalies and can be used to study interstellar C2; new results for 10 sight lines are presented.

astro-ph.GA↗

arXiv2012

The 7Li/6Li Isotope Ratio Near the Supernova Remnant IC 443

We present an analysis of 7Li/6Li isotope ratios along four sight lines that probe diffuse molecular gas near the supernova remnant IC 443. Recent gamma-ray observations have revealed the presence of shock-accelerated cosmic rays interacting with the molecular cloud surrounding the remnant. Our results indicate that the 7Li/6Li ratio is lower in regions more strongly affected by these interactions, a sign of recent Li production by cosmic rays. We find that 7Li/6Li ~ 7 toward HD 254755, which is located just outside the visible edge of IC 443, while 7Li/6Li ~ 3 along the line of sight to HD 43582, which probes the interior region of the supernova remnant. No evidence of 7Li synthesis by neutrino-induced spallation is found in material presumably contaminated by the ejecta of a core-collapse supernova. The lack of a neutrino signature in the 7Li/6Li ratios near IC 443 is consistent with recent models of Galactic chemical evolution, which suggest that the nu-process plays only a minor role in Li production.

astro-ph.GA↗

arXiv2011

The Impact of Recent Advances in Laboratory Astrophysics on our Understanding of the Cosmos

An emerging theme in modern astrophysics is the connection between astronomical observations and the underlying physical phenomena that drive our cosmos. Both the mechanisms responsible for the observed astrophysical phenomena and the tools used to probe such phenomena - the radiation and particle spectra we observe - have their roots in atomic, molecular, condensed matter, plasma, nuclear and particle physics. Chemistry is implicitly included in both molecular and condensed matter physics. This connection is the theme of the present report, which provides a broad, though non-exhaustive, overview of progress in our understanding of the cosmos resulting from recent theoretical and experimental advances in what is commonly called laboratory astrophysics. This work, carried out by a diverse community of laboratory astrophysicists, is increasingly important as astrophysics transitions into an era of precise measurement and high fidelity modeling.

astro-ph.IM↗