FUSE Measurements of Far-Ultraviolet Extinction. III. The Dependence on R(V) and Discrete Feature Limits from 75 Galactic Sightlines

We present a sample of 75 extinction curves derived from FUSE far-ultraviolet spectra supplemented by existing IUE spectra. The extinction curves were created using the standard pair method based on a new set of dereddened FUSE+IUE comparison stars. Molecular hydrogen absorption features were removed using individualized H_2 models for each sightline. The general shape of the FUSE extinction (8.4 micron^-1 < lambda^-1 < 11 micron^-1) was found to be broadly consistent with extrapolations from the IUE extinction (3.3 micron-1 < lambda^-1 < 8.6 micron^-1) curve. Significant differences were seen in the strength of the far-UV rise and the width of the 2175 A bump. All the FUSE+IUE extinction curves had positive far-UV slopes giving no indication that the far-UV rise was turning over at the shortest wavelengths. The dependence of A(lambda)/A(V) versus R(V)^-1 in the far-UV using the sightlines in our sample was found to be stronger than tentatively indicated by previous work. We present an updated R(V) dependent relationship for the full UV wavelength range (3.3 micron^-1 <= lambda^-1 <= 11 micron^-1). Finally, we searched for discrete absorption features in the far-ultraviolet. We found a 3 sigma upper limit of ~0.12 A(V) on features with a resolution of 250 (~4 A width) and 3 sigma upper limits of ~0.15 A(V) for lambda^-1 < 9.6 micron^-1 and ~0.68 A(V) for lambda^-1 > 9.6 micron^-1 on features with a resolution of 10^4 (~0.1 A width).Comment: 16 pages, 13 figures, ApJ, in pres

Declining Public Awareness of Heart Attack Warning Symptoms in the Years Following an Australian Public Awareness Campaign: A Cross-Sectional Study

Background: The National Heart Foundation of Australia's (NHFA) Warning Signs campaign ran between 2010 and 2013. This study examines trends in Australian adults’ ability to name heart attack symptoms during the campaign and in the years following. Methods: Using the NHFA's HeartWatch data (quarterly online surveys) for adults aged 30–59 years, we conducted an adjusted piecewise regression analysis comparing trends in the ability to name symptoms during the campaign period plus one year lag (2010–2014) to the post-campaign period (2015–2020) Results: Over the study period, there were 101,936 Australian adults surveyed. Symptom awareness was high or increased during the campaign period. However, there was a significant downward trend in each year following the campaign period for most symptoms (e.g., chest pain: adjusted odds ratio [AOR] =0.91, 95%CI: 0.56–0.80; arm pain: AOR=0.92, 95%CI: 0.90–0.94). Conversely, the inability to name any heart attack symptom increased in each year following the campaign (3.7% in 2010 to 19.9% in 2020; AOR=1.13, 95%CI: 1.10–1.15); these respondents were more likely to be younger, male, have less than 12 years of education, identify as Aboriginal and/or Torres Strait Islander Peoples, speak a language other than English at home and have no cardiovascular risk factors. Conclusion: Awareness of heart attack symptoms has decreased in the years since the Warning Signs campaign in Australia, with 1 in 5 adults currently unable to name a single heart attack symptom. New approaches are needed to promote and sustain this knowledge, and to ensure people act appropriately and promptly if symptoms occur

Interstellar Carbon in Translucent Sightlines

We report interstellar C II column densities or upper limits determined from weak absorption of the 2325.4029 A intersystem transition observed in six translucent sightlines with STIS. The sightlines sample a wide range of interstellar characteristics including total-to-selective extinction, R_{V} = 2.6 - 5.1; average hydrogen density along the sightline, = 3 - 14 cm^{-3}; and fraction of H in molecular form, 0 - 40%. Four of the sightlines, those toward HD 37021, HD 37061, HD 147888 and HD 207198, have interstellar gas-phase abundances that are consistent with the diffuse sightline ratio of 161 +/- 17 carbon atoms in the gas per million hydrogen nuclei. We note that while it has a gas-phase carbon abundance that is consistent with the other sightlines, a large fraction of the C II toward HD 37061 is in an excited state. The sightline toward HD 152590 has a measured interstellar gas-phase carbon abundance that is well above the diffuse sightline average; the column density of C in this sightline may be overestimated due to noise structure in the data. Toward HD 27778 we find a 3 sigma abundance upper limit of <108 C atoms in the gas per million H, a substantially enhanced depletion of C as compared to the diffuse sightline value. The interstellar characteristics toward HD 27778 are otherwise not extreme among the sample except for an unusually large abundance of CO molecules in the gas.Comment: Accepted for publication in the Astrophysical Journa

Far-ultraviolet Dust Extinction and Molecular Hydrogen in the Diffuse Milky Way Interstellar Medium

We aim to compare variations in the full-UV dust extinction curve (912-3000 Angstrom), with the HI/H$_2$/total H content along diffuse Milky Way sightlines, to investigate possible connections between ISM conditions and dust properties. We combine an existing sample of 75 UV extinction curves based on IUE and FUSE data, with atomic and molecular column densities measured through UV absorption. The H$_2$ column density data are based on existing Lyman-Werner absorption band models from earlier work on the extinction curves. Literature values for the HI column density were compiled, and improved for 23 stars by fitting a Ly$\alpha$ profile to archived spectra. We discover a strong correlation between the H$_2$ column and the far-UV extinction, and the underlying cause is a linear relationship between H$_2$ and the strength of the far-UV rise feature. This extinction does not scale with HI, and the total H column scales best with $A(V)$ instead. The carrier of the far-UV rise therefore coincides with molecular gas, and further connections are shown by comparing the UV extinction features to the molecular fraction. Variations in the gas-to-extinction ratio $N(\rm{H})/A(V)$ correlate with the UV-to-optical extinction ratio, and we speculate this could be due to coagulation or shattering effects. Based on the H$_2$ temperature, the strongest far-UV rise strengths are found to appear in colder and denser sightlines.Comment: 24 pages, 9 figures, accepted for publication in ApJ; fix missing text in generated pdf due to broken tex comman

The Homogeneity of Interstellar Krypton in the Galactic Disk

We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of Kr I 1236 Angstrom absorption in seven sight lines that probe a variety of interstellar environments. In combination with krypton and hydrogen column densities derived from current and archival STIS and Far-Ultraviolet Spectroscopic Explorer data, the number of sight lines with reliable Kr/H ISM abundance ratios has been increased by 50% to 26--including paths that sample a range of nearly 5 orders of magnitude in f(H_2), over 2 orders of magnitude in , and extending up to 4.8 kpc in length. For sight lines contained entirely within the local spiral arm (the Orion Spur), the spread of Kr/H ratios about the mean of log_10[N(Kr)/N(H)]_ISM = -9.02+/-0.02 is remarkably tight (0.06 dex), less than the typical datapoint uncertainty. Intriguingly, the only two sight lines that extend through neighboring structures, in particular gas associated with the Carina/Sagittarius Arm, exhibit relatively large, near-solar krypton abundances (log_10[N(Kr)/N(H)]_combined = -8.75+0.09_-0.11). Although these deviations are only measured at the 2 sigma level, they suggest the possibility that krypton abundances beyond the Orion Spur may differ from the local value.Comment: 15 page, including 3 figures and 2 tables, accepted for publication in Ap

The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions

We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using a steady state photodissociation region code that treats the freeze-out of gas species, grain surface chemistry, and desorption of ices from grains. The code includes PAHs, which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A_V<~1 and one at A_V~3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far ultraviolet flux on the cloud ({\chi}<~ 1000; {\chi}= 1= local interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic ray primary ionization rate {\zeta}(crp) divided by the hydrogen nucleus density n. The H3O+ column is dominated by the second peak, and we show that if PAHs are present, N(H3O+) ~ 4x10^{13} cm^{-2} independent of {\zeta}(crp) or n. If there are no PAHs or very small grains at the second peak, N(H3O+) can attain such columns only if low ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used to estimate {\zeta}(crp)/n, {\chi}/n and A_V in them. We compare our models to Herschel observations of these two ions, and estimate {\zeta}(crp) ~ 4-6 x 10^-16 (n/100 cm^-3) s^-1 and \chi/n = 0.03 cm^3 for diffuse foreground clouds towards W49N

The Homogeneity of Interstellar Oxygen in the Galactic Disk

We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f(H_2), over 2 orders of magnitude in mean sight line density, and that extend up to 6.5 kpc in length. Consequently, we have undertaken the study of gas-phase O/H abundance ratio homogeneity using the current sample and previously published Goddard High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance with increasing mean sight line density and a gap between the mean O/H ratio for sight lines shorter and longer than about 800 pc. The first effect is a smooth transition between two depletion levels associated with large mean density intervals; it is centered near a density of 1.5 cm^-3 and is similar to trends evident in gas-phase abundances of other elements. Paths less dense than the central value exhibit a mean O/H ratio of log_10 (O/H) = -3.41+/-0.01 (or 390+/-10 ppm), which is consistent with averages determined for several long, low-density paths observed by STIS (Andre et al. 2003) and short low-density paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density exhibit an average O/H value of log_10 (O/H) = -3.55+/-0.02 (284+/-12 ppm). The datapoints for low-density paths are scattered more widely than those for denser sight lines, due to O/H ratios for paths shorter than 800 pc that are generally about 0.10 dex lower than the values for longer ones.Comment: 33 pages, including 8 figures and 4 tables; accepted for publication in ApJ, tentatively in Oct 200

FUSE Measurements of Far Ultraviolet Extinction. I. Galactic Sight Lines

We present extinction curves that include data down to far ultraviolet wavelengths (FUV; 1050 - 1200 A) for nine Galactic sight lines. The FUV extinction was measured using data from the Far Ultraviolet Spectroscopic Explorer. The sight lines were chosen for their unusual extinction properties in the infrared through the ultraviolet; that they probe a wide range of dust environments is evidenced by the large spread in their measured ratios of total-to-selective extinction, R_V = 2.43 - 3.81. We find that extrapolation of the Fitzpatrick & Massa relationship from the ultraviolet appears to be a good predictor of the FUV extinction behavior. We find that predictions of the FUV extinction based upon the Cardelli, Clayton & Mathis (CCM) dependence on R_V give mixed results. For the seven extinction curves well represented by CCM in the infrared through ultraviolet, the FUV extinction is well predicted in three sight lines, over-predicted in two sight lines, and under-predicted in 2 sight lines. A Maximum Entropy Method analysis using a simple three component grain model shows that seven of the nine sight lines in the study require a larger fraction of grain materials to be in dust when FUV extinction is included in the models. Most of the added grain material is in the form of small (radii < 200 A) grains.Comment: Accepted for publication in the Astrophysical Journal. 31 pages with 7 figure

Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud

The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background. The predicted primordial 7Li abundance is four times that measured in the atmospheres of Galactic halo stars. This discrepancy could be caused by modification of surface lithium abundances during the stars' lifetimes or by physics beyond the Standard Model that affects early nucleosynthesis. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar 7Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's metallicity. The present-day 7Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.Comment: Published in Nature. Includes main text and Supplementary Information. Replaced with final title and abstrac