arXiv subjects

Cecile Gry

Publications and source records attributed to Cecile Gry.

18 recordsLinked to original sources

arXiv2020

The Composition, Excitation, and Physical State of Atomic Gas in the Debris Disk Surrounding 51 Oph

We measured 304 absorption features in the ultraviolet and visible spectra of the star 51 Oph, which is known to have a debris disk with a high inclination. We analyzed the relative populations of atoms in excited fine-structure and metastable levels that are maintained by optical pumping and collisional excitation by electrons, and we found that most of the gas is situated at about 6AU from the star, has an electron volume density $10^5 < n(e) < 3\times 10^6{\rm cm}^{-3}$, and a temperature T=8000K. Our interpretations reveal that the gas is partly ionized, has a column density of neutral hydrogen equal to $10^{21}\,{\rm cm}^{-2}$, and has a composition similar to that of a mildly depleted interstellar medium or that of Jupiter-family comets. Compared to results for disks around some other stars, such as $\beta$ Pic and 49 Cet, we find surprisingly little neutral carbon. No molecular features were detected, which indicates that our line of sight misses the molecule-rich central plane of the disk. The tilt of the disk is also validated by our being able to detect resonant scattering of the starlight by oxygen atoms.

astro-ph.SR↗

arXiv2019

Precision measurement of magnetic field from near to far, from fine to large scales in ISM

Magnetic fields have important or dominant effects in many areas of astrophysics, but have been very difficult to quantify. Spectropolarimetry from Ground State Alignment (GSA) has been suggested as a direct tracer of magnetic field in interstellar diffuse medium. Owing to the long life of the atoms on ground states the Larmor precession in an external magnetic field imprints the direction of the field onto the polarization of absorbing species. This provides a unique tool for studies of sub-gauss magnetic fields using polarimetry of UV, optical and radio lines. Many spectral lines with strong signals from GSA are in the UV band. By discerning magnetic fields in gas with different dynamical properties, high spectral resolution measurement of spectral polarization will allow the study of 3D magnetic field distribution and interstellar turbulence. GSA provides also a unique chance to map 3D direction of magnetic field on small scales, e.g., disks, where grain alignment is unreliable. The range of objects suitable for studies is extremely wide and includes magnetic fields in the interplanetary medium, in the interstellar medium, and in circumstellar regions as well as diffuse media in extragalactic objects.

astro-ph.GA↗

arXiv2019

Far to near UV spectroscopy of the interstellar medium at very high resolution and very high signal-to-noise ratio

A comprehensive study of interstellar medium phases and the nature of their boundaries or connections requires comparing abundances and velocity profiles from tracers of the different phases. In the UV, studies of a wealth of absorption features appearing in the spectra of hot stars yield fundamental insights into the composition and physical characteristics of all phases of the ISM along with the processes that influence them. They also inform us on the nature of boundaries between them. However no single instrument has as yet given access to species in all ISM phases at the same high spectral resolution: from the molecular bands of CO and H2 in the far-UV, to the cold neutral medium tracers C I and S I and the warm medium tracers like C II, NI, OI, Mg II, Fe II, Si II etc, and to the high ions of the hot ionized medium C IV, Si IV in the UV, as well as O VI in the far UV. We have yet to design the spectrometer that will enable observing the full UV domain at resolving power R>200 000 and signal-to-noise ratio SNR>500. The line FWHM being governed by turbulence, temperature, and species mass, such a resolution is necessary to resolve lines from both the cold molecular hydrogen and the warm metal ions with a turbulence of ~1 km/s, and to differentiate distinct velocity components, typically separated by less than 2 km/s.

astro-ph.GA↗

arXiv2016

The nearby interstellar medium towards alpha Leo -- UV observations and modeling of a warm cloud within hot gas

We analyze interstellar absorption features in the full UV spectrum of the nearby (d = 24 pc) B8 IVn star alpha Leo (Regulus) obtained at high resolution and high S/N by the HST ASTRAL Treasury program. We derive column densities for many key atomic species and interpret their partial ionizations. The gas in front of alpha Leo exhibits two absorption components, one of which coincides in velocity with the local interstellar cloud (LIC) that surrounds the Sun. The second, smaller, component is shifted by +5.6 km/s relative to the main component, in agreement with results for other lines of sight in this region of the sky. The excitation of the C II fine-structure levels and the ratio of Mg I to Mg II reveal a temperature T = 6500 (+750,-600)K and electron density n(e) = 0.11 (+0.025,-0.03) cm^-3. Our investigation of the ionization balance of all the available species indicates that about 1/3 of the hydrogen atoms are ionized and that metals are significantly depleted onto grains. We infer that N(H I) = 1.9 (+0.9,-0.6) X 10^{18} cm^-2, which indicates that this partly neutral gas occupies only 2 to 8 pc (about 13%) of the space toward the star, with the remaining volume presumably being filled with a hot gas that emits soft X-rays. We do not detect any absorption features from the highly ionized species that could be produced in an interface between the warm medium and the surrounding hot gas. Finally, the radial velocity of the LIC agrees with that of the Local Leo Cold Cloud, indicating that they may be physically related.

astro-ph.GA↗

arXiv2015

Herschel observations of interstellar chloronium. II - Detections toward G29.96-0.02, W49N, W51, and W3(OH), and determinations of the ortho-to-para and $^{35}$Cl/$^{37}$Cl isotopic ratios

We report additional detections of the chloronium molecular ion, H$_2$Cl$^+$, toward four bright submillimeter continuum sources: G29.96, W49N, W51, and W3(OH). With the use of the HIFI instrument on the Herschel Space Observatory, we observed the $2_{12}-1_{01}$ transition of ortho-H$_2^{35}$Cl$^+$ at 781.627 GHz in absorption toward all four sources. Much of the detected absorption arises in diffuse foreground clouds that are unassociated with the background continuum sources and in which our best estimates of the $N({\rm H_2Cl^+})/N({\rm H})$ ratio lie in the range $(0.9 - 4.8) \times 10^{-9}$. These chloronium abundances relative to atomic hydrogen can exceed the predictions of current astrochemical models by up to a factor of 5. Toward W49N, we have also detected the $2_{12}-1_{01}$ transition of ortho-H$_2^{37}$Cl$^+$ at 780.053 GHz and the $1_{11}-0_{00}$ transition of para-H$_2^{35}$Cl$^+$ at 485.418 GHz. These observations imply $\rm H_2^{35}Cl^+/H_2^{37}Cl^+$ column density ratios that are consistent with the solar system $^{35}$Cl/$^{37}$Cl isotopic ratio of 3.1, and chloronium ortho-to-para ratios consistent with 3, the ratio of spin statistical weights.

astro-ph.GA↗

arXiv2014

A new perspective on the interstellar cloud surrounding the Sun from UV absorption line results

We offer a new, more inclusive, picture of the local interstellar medium, where it is composed of a single, monolithic cloud that surrounds the Sun in all directions. Our study of velocities based on Mg II and Fe II ultraviolet absorption lines indicates that the cloud has an average motion consistent with the velocity vector of gas impacting the heliosphere and does not behave like a rigid body: gas within the cloud is being differentially decelerated in the direction of motion, and the cloud is expanding in directions perpendicular to this flow, much like the squashing of a balloon. The outer boundary of the cloud is in average 10 pc away from us but is highly irregular, being only a few parsecs away in some directions, with possibly a few extensions up to 20 pc. Average H I volume densities vary between 0.03 and 0.1 cm-3 over different sight lines. Metals appear to be significantly depleted onto grains, and there is a steady increase in this effect from the rear of the cloud to the apex of motion. There is no evidence that changes in the ionizing radiation influence the apparent abundances. Additional, secondary velocity components are detected in 60 % of the sight lines. Almost all of them appear to be interior to the volume holding the gas that we identify with the main cloud. Half of the sight lines exhibit a secondary component moving at about - 7.2 km/s with respect to the main component, which may be the signature of an implosive shock propagating toward the cloud's interior.

astro-ph.GA↗

arXiv2014

The interstellar cloud surrounding the Sun: a new perspective

Aims: We offer a new, simpler picture of the local interstellar medium, made of a single continuous cloud enveloping the Sun. This new outlook enables the description of a diffuse cloud from within and brings to light some unexpected properties. Methods: We re-examine the kinematics and abundances of the local interstellar gas, as revealed by the published results for the ultraviolet absorption lines of MgII, FeII, and HI. Results: In contrast to previous representations, our new picture of the local interstellar medium consists of a single, monolithic cloud that surrounds the Sun in all directions and accounts for most of the matter present in the first 50 parsecs around the Sun. The cloud fills the space around us out to about 9 pc in most directions, although its boundary is very irregular with possibly a few extensions up to 20 pc. The cloud does not behave like a rigid body: gas within the cloud is being differentially decelerated in the direction of motion, and the cloud is expanding in directions perpendicular to this flow, much like a squashed balloon. Average HI volume densities inside the cloud vary between 0.03 and 0.1 cm-3 over different directions. Metals appear to be significantly depleted onto grains, and there is a steady increase in depletion from the rear of the cloud to the apex of motion. There is no evidence that changes in the ionizing radiation influence the apparent abundances. Secondary absorption components are detected in 60% of the sight lines. Almost all of them appear to be interior to the volume occupied by the main cloud. Half of the sight lines exhibit a secondary component moving at about -7.2 km/s with respect to the main component, which may be the signature of a shock propagating toward the cloud's interior.

astro-ph.GA↗

arXiv2013

Building galaxies, stars, planets and the ingredients for life between the stars. A scientific proposal for a European Ultraviolet-Visible Observatory (EUVO)

The growth of luminous structures and the building blocks of life in the Universe began as primordial gas was processed in stars and mixed at galactic scales. The mechanisms responsible for this development are not well understood and have changed over the intervening 13 billion years. To follow the evolution of matter over cosmic time, it is necessary to study the strongest (resonance) transitions of the most abundant species in the Universe. Most of them are in the ultraviolet (UV; 950A-3000A) spectral range that is unobservable from the ground. A versatile space observatory with UV sensitivity a factor of 50-100 greater than existing facilities will revolutionize our understanding of the Universe. Habitable planets grow in protostellar discs under ultraviolet irradiation, a by-product of the star-disk interaction that drives the physical and chemical evolution of discs and young planetary systems. The electronic transitions of the most abundant molecules are pumped by the UV field, providing unique diagnostics of the planet-forming environment that cannot be accessed from the ground. Earth's atmosphere is in constant interaction with the interplanetary medium and the solar UV radiation field. A 50-100 times improvement in sensitivity would enable the observation of the key atmospheric ingredients of Earth-like exoplanets (carbon, oxygen, ozone), provide crucial input for models of biologically active worlds outside the solar system, and provide the phenomenological baseline to understand the Earth atmosphere in context. In this white paper, we outline the key science that such a facility would make possible and outline the instrumentation to be implemented.

astro-ph.IM↗

arXiv2010

Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4

We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6$-$0.4 as well as in the molecular gas directly associated with that source. The OH+ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O+ lines remains lower than 1 in the same velocity range, and the H3O+ line shows only weak absorption. For LSR velocities between 7 and 50 kms$^{-1}$ we estimate total column densities of $N$(OH+) $> 2.5 \times 10^{14}$ cm$^{-2}$, $N$(H2O+) $\sim 6 \times 10^{13}$ cm$^{-2}$ and $N$(H3O+) $\sim 4.0 \times 10^{13}$ cm$^{-2}$. These detections confirm the role of O$^+$ and OH$^+$ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH+ by the H2O+ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.

astro-ph.GA↗

arXiv2008

Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample

We report total abundances and related parameters for the full sample of the FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results are presented for the "second half" of the survey involving 15 lines of sight to supplement data for the first 23 lines of sight already published. We assess the correlations between molecular hydrogen and various extinction parameters in the full sample, which covers a broader range of conditions than the initial sample. In particular, we are now able to confirm that many, but not all, lines of sight with shallow far-UV extinction curves and large values of the total-to-selective extinction ratio, $R_V$ = $A_V$ / $E(B-V)$ -- characteristic of larger than average dust grains -- are associated with particularly low hydrogen molecular fractions ($f_{\rm H2}$). In the lines of sight with large $R_V$, there is in fact a wide range in molecular fractions, despite the expectation that the larger grains should lead to less H$_2$ formation. However, we see specific evidence that the molecular fractions in this sub-sample are inversely related to the estimated strength of the UV radiation field and thus the latter factor is more important in this regime. We have provided an update to previous values of the gas-to-dust ratio, $N$(H$_{\rm tot}$)/$E(B-V)$, based on direct measurements of $N$(H$_2$) and $N$(H I). Although our value is nearly identical to that found with Copernicus data, it extends the relationship by a factor of 2 in reddening. Finally, as the new lines of sight generally show low to moderate molecular fractions, we still find little evidence for single monolithic "translucent clouds" with $f_{\rm H2}$ $\sim$ 1.

astro-ph↗

arXiv2008

Modeling of diffuse molecular gas applied to HD 102065 observations

Aims. We model a diffuse molecular cloud present along the line of sight to the star HD 102065. We compare our modeling with observations to test our understanding of physical conditions and chemistry in diffuse molecular clouds. Methods. We analyze an extensive set of spectroscopic observations which characterize the diffuse molecular cloud observed toward HD 102065. Absorption observations provide the extinction curve, H2, C I, CO, CH, and CH+ column densities and excitation. These data are complemented by observations of CII, CO and dust emission. Physical conditions are determined using the Meudon PDR model of UV illuminated gas. Results. We find that all observational results, except column densities of CH, CH+ and H2 in its excited (J > 2) levels, are consistent with a cloud model implying a Galactic radiation field (G~0.4 in Draine's unit), a density of 80 cm-3 and a temperature (60-80 K) set by the equilibrium between heating and cooling processes. To account for excited (J >2) H2 levels column densities, an additional component of warm (~ 250K) and dense (nH>10^4 cm-3) gas within 0.03 pc of the star would be required. This solution reproduces the observations only if the ortho-to-para H2 ratio at formation is 1. In view of the extreme physical conditions and the unsupported requirement on the ortho-to-para ratio, we conclude that H2 excitation is most likely to be accounted for by the presence of warm molecular gas within the diffuse cloud heated by the local dissipation of turbulent kinetic energy. This warm H2 is required to account for the CH+ column density. It could also contribute to the CH abundance and explain the inhomogeneity of the CO abundance indicated by the comparison of absorption and emission spectra.

astro-ph↗

arXiv2006

What is the Total Deuterium Abundance in the Local Galactic Disk?

Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and IMAPS instruments, reveal an unexplained very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of gas-phase D/H ratios observed in the intermediate regime (log N(H I} = 19.2-20.7), and the low gas-phase D/H ratios observed at larger hydrogen column densities. We consider empirical tests of the deuterium depletion hypothesis: (i) correlations of gas-phase D/H ratios with depletions of the refractory metals iron and silicon, and (ii) correlation with the molecular hydrogen rotational temperature. Both of these tests are consistent with deuterium depletion from the gas phase in cold, not recently shocked, regions of the ISM, and high gas-phase D/H ratios in gas that has been shocked or otherwise heated recently. We argue that the most representative value for the total (gas plus dust) D/H ratio within 1 kpc of the Sun is >=23.1 +/- 2.4 (1 sigma) parts per million (ppm). This ratio constrains Galactic chemical evolution models to have a very small deuterium astration factor, the ratio of primordial to total (D/H) ratio in the local region of the Galactic disk, which we estimate to be f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma) depending on the adopted light element nuclear reaction rates.

astro-ph↗

arXiv2002

H2 formation and excitation in the diffuse interstellar medium

We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and excitation of H2 in the diffuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photon-illuminated gas. The data constrain well the nR product between gas density and H2 formation rate on dust grains: nR = 1 to 2.2 e-15 s-1. For each line of sight the mean effective H2 density n, assumed uniform, is obtained by the best fit of the model to the observed N(J=1)/N(J=0) ratio, since the radiation field is known. Combining n with the nR values, we find similar H2 formation rates for the three stars of about R = 4 e-17 cm3/s. Because the target stars do not interact with the absorbing matter we can show that the H2 excitation in the J>2 levels cannot be accounted for by the UV pumping of the cold H2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H2 is corroborated by the fact that the star with the largest column density of CH+ has the largest amount of warm H2.

astro-ph↗

arXiv2002

A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds

We report the first ensemble results from the FUSE survey of molecular hydrogen in lines of sight with A_V $\gtrsim$ 1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J = 0 and J = 1, which contain $\gtrsim$99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of 10^21 cm^-2, measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175A bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like zeta Oph. We suggest a modification in terminology to distinguish between a "translucent line of sight" and a "translucent cloud."

astro-ph↗

arXiv2001

FUSE Observations towards HD 34078 Detection of highly excited H2 and HD

We present FUSE observations of the extincted O9.5 star, HD 34078. The 19 first levels of H2 are detected (i.e. from J=0 to v=1, J=5) as well as HD in its two first levels. The excitation of H2 up to J=7 can be explained using a combination of Photon Dominated Region (PDR) and MHD shock models. However, understanding the large amount of H2 found in higher excitation states seems to require more energetic processes that have yet to be identified.

astro-ph↗

arXiv2001

Small scale structure in diffuse molecular gas from repeated FUSE and visible spectra of HD 34078

We present preliminary results from an ongoing program devoted to a study of small scale structure in the spatial distribution of molecular gas. Our work is based on multi-epoch FUSE and visible observations of HD34078. A detailed comparison of H2, CH and CH+ absorption lines is performed. No short term variations are seen (except for highly excited H2) but long-term changes in N(CH) are clearly detected when comparing our data to spectra taken about 10 years ago.

astro-ph↗

arXiv2000

Local clouds: ionization, temperatures, electron densities and interfaces, from GHRS and IMAPS spectra of epsilon Canis Majoris

The composition and physical properties of several local clouds, including the Local Interstellar Cloud (LIC) in which the Sun is embedded, are derived from absorption features in the UV spectrum of the star epsilon CMa. We derive temperatures and densities for three components by combining our interpretations of the ionization balance of magnesium and the relative population of C II in an excited fine-structure level. We find that for the LIC n(e) = 0.12 +/-0.05 cm-3 and T = 7000 +/-1200 K. We derive the ionization fractions of hydrogen and discuss the ionizing processes. In particular the hydrogen and helium ionizations in the LIC are compatible with photoionization by the local EUV radiation fields from the hot stars and the cloud interface with the hot gas. We confirm the detection of high ionization species : Si III is detected in all clouds and C IV in two of them, including the LIC, suggesting the presence of ionized interfaces around the local clouds.

astro-ph↗

arXiv2000

Electron densities, temperatures and ionization rates in two interstellar clouds in front of beta Canis Majoris, as revealed by UV absorption lines observed with IMAPS

The spectrum of beta CMa (Mirzam) between 1000 and 1200 A was recorded at a wavelength resolving power ~ 60 000 by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) during its orbital flight on the ORFEUS-SPAS II mission in 1996. New information about interstellar absorption lines of CII, CII*, NI and OI from this spectrum are combined with the HST results reported by Dupin & Gry (1998) to arrive at new conclusions about the physical properties of the absorbing gas in front of beta CMa. For two prominent velocity components centered at heliocentric velocities of +20.0 and +30.5 km/s, designated by Dupin & Gry as Components C and D respectively, we use determinations of N(CII*)/N(CII) and N(MgI)/N(MgII) to solve for temperatures and electron densities. From our knowledge that oxygen and nitrogen have their ionizations coupled to that of hydrogen through charge exchange reactions, we can derive the hydrogen ionizations by comparing these elements to sulfur, which is likely not to be depleted onto dust grains. For Component C with an approximate column density of neutral and ionized hydrogen N(Htotal)=6 10^{18} cm^-2, we find that the neutral fraction n(HI)/n(Htotal)=0.25, 400<T<6500 K, and 0.08<n(e)<0.6 cm^-3, while for Component D with N(Htotal)=1.2 10^{19}cm^-2, we arrive at n(HI)/n(Htotal)=0.035, 8000<T<14000 K, and 0.09<n(e)<0.2cm^-3. The relatively large ionization fractions of H can arise if the clouds are about 130 pc away from us, so that they are exposed to the strong, ionizing radiation fields from epsilon and beta CMa. The presence of SiIII indicates the existence of additional gas with even higher levels of ionization.

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