Abundances of Elements Lighter than Scandium in the Atmosphere of the Standard Giant Star α Sex (HD 87887, A0 III)

The abundances of elements lighter than scandium in the atmosphere of the A0 III giant, α Sex, are derived using archival ultraviolet and optical spectra. Most of the strongest lines present in the far-ultraviolet spectrum of α Sex can be attributed to chemical elements lighter than calcium. The synthesis of selected lines in the optical and ultraviolet yields new abundances, in particular for elements which have few lines in the optical range. Helium, oxygen, silicon and sulfur are found to have solar abundances, most other elements are underabundant. Aluminium may be marginally overabundant. The lines of chlorine are probably present in the FUV but they are too blended to derive the abundance of chlorine.

Eta Carinae: A Tale of Two Periastron Passages

Since 2002, the far-ultraviolet (FUV) flux (1150–1680 Å) of Eta Carinae, monitored by the Hubble Space Telescope/Space Telescope Imaging Spectrograph, has increased by an order of magnitude. This increase is attributed to partial dissipation of a line-of-sight (LOS) occulter that blocks the central core of the system. Across the 2020 February periastron passage, changes in the FUV emission show a stronger wavelength dependence than occurred across the 2003 July periastron passage. Across both periastron passages, most of the FUV spectrum dropped in flux then recovered a few months later. The 2020 periastron passage included enhancements of FUV flux in narrow spectral intervals near periastron followed by a transient absorption and recovery to pre-periastron flux levels. The drop in flux is due to increased absorption by singly ionized species as the secondary star plunges deep into the wind of the primary star, which blocks the companion’s ionizing radiation. The enhanced FUV emission is caused by the companion’s wind-blown cavity briefly opening a window to deeper layers of the primary star. This is the first time transient brightening has been seen in the FUV comparable to transients previously seen at longer wavelengths. Changes in resonance line-velocity profiles hint that the dissipating occulter is associated with material in LOS moving at −100 to −300 km s−1, similar in velocity of structures previously associated with the 1890s lesser eruption.

DIISC-II: Unveiling the Connections between Star Formation and Interstellar Medium in the Extended Ultraviolet Disk of NGC 3344

We present our investigation of the extended ultraviolet (XUV) disk galaxy, NGC 3344, conducted as part of Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium survey. We use surface and aperture photometry of individual young stellar complexes to study star formation and its effect on the physical properties of the interstellar medium. We measure the specific star formation rate (sSFR) and find it to increase from 10−10 yr−1 in the inner disk to >10−8 yr−1 in the extended disk. This provides evidence for inside-out disk growth. If these sSFRs are maintained, the XUV disk stellar mass can double in ∼0.5 Gyr, suggesting a burst of star formation. The XUV disk will continue forming stars for a long time due to the high gas depletion times (τ dep). The stellar complexes in the XUV disk have high-ΣH I and low-ΣSFR with τ dep ∼ 10 Gyr, marking the onset of a deviation from the traditional Kennicutt–Schmidt law. We find that both far-ultraviolet (FUV) and a combination of FUV and 24 μm effectively trace star formation in the XUV disk. Hα is weaker in general and prone to stochasticities in the formation of massive stars. Investigation of the circumgalactic medium at 29.5 kpc resulted in the detection of two absorbing systems with metal-line species: the stronger absorption component is consistent with gas flows around the disk, most likely tracing inflow, while the weaker component is likely tracing corotating circumgalactic gas.

The H i Column Density Distribution of the Galactic Disk and Halo

We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm−2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude ∣b∣ ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ–Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = − 1.01 − 0.14 + 0.15 and β HVC = − 1.05 − 0.06 + 0.07 . Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with −0.88 ± 0.40 M ⊙ yr−1 of HVC inflow versus ≈0.20 ± 0.10 M ⊙ yr−1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.

Evolution of the Ultraviolet Upturn at 0.3 < z < 1: Exploring Helium-rich Stellar Populations

We measure the near-UV (rest-frame ∼2400 Å) to optical color for early-type galaxies in 12 clusters at 0.3 < z < 1.0. We show that this is a suitable proxy for the more common far-ultraviolet bandpass used to measure the ultraviolet upturn and find that the upturn is detected to z = 0.6 in these data, in agreement with previous work. We find evidence that the strength of the upturn starts to wane beyond this redshift and largely disappears at z = 1. Our data are most consistent with models where early-type galaxies contain minority stellar populations with non-cosmological helium abundances, up to around 46%, formed at z ≥ 3, resembling globular clusters with multiple stellar populations in our Galaxy. This suggests that elliptical galaxies and globular clusters share similar chemical evolution and star formation histories. The vast majority of the stellar mass in these galaxies also must have been in place at z > 3.

Molecular Gas within the Milky Way's Nuclear Wind

We report the first direct detection of molecular hydrogen associated with the Galactic nuclear wind. The Far-Ultraviolet Spectroscopic Explorer spectrum of LS 4825, a B1 Ib–II star at l, b = 1.67°,−6.63° lying d = 9.9 − 0.8 + 1.4 kpc from the Sun, ∼1 kpc below the Galactic plane near the Galactic center, shows two high-velocity H2 components at v LSR = −79 and −108 km s−1. In contrast, the FUSE spectrum of the nearby (∼0.6° away) foreground star HD 167402 at d = 4.9 − 0.7 + 0.8 kpc reveals no H2 absorption at these velocities. Over 60 lines of H2 from rotational levels J = 0 to 5 are identified in the high-velocity clouds. For the v LSR = −79 km s−1 cloud we measure total log N(H2) ≥ 16.75 cm−2, molecular fraction f H 2 ≥ 0.8%, and T 01 ≥ 97 and T 25 ≤ 439 K for the ground- and excited-state rotational excitation temperatures. At v LSR = −108 km s−1, we measure log N(H2) = 16.13 ± 0.10 cm−2, f H 2 ≥ 0.5%, and T 01 = 77 − 18 + 34 and T 25 = 1092 − 117 + 149 K, for which the excited-state ortho- to para-H2 is 1.0 − 0.1 + 0.3 , much less than the equilibrium value of 3 expected for gas at this temperature. This nonequilibrium ratio suggests that the −108 km s−1 cloud has been recently excited and has not yet had time to equilibrate. As the LS 4825 sight line passes close by a tilted section of the Galactic disk, we propose that we are probing a boundary region where the nuclear wind is removing gas from the disk.

Far-ultraviolet Spectra of Main-sequence O Stars at Extremely Low Metallicity

Metal-poor massive stars dominate the light we observe from star-forming dwarf galaxies and may have produced the bulk of energetic photons that reionized the universe at high redshift. Yet, the rarity of observations of individual O stars below the 20% solar metallicity (Z ⊙) of the Small Magellanic Cloud (SMC) hampers our ability to model the ionizing fluxes of metal-poor stellar populations. We present new Hubble Space Telescope far-ultraviolet (FUV) spectra of three O-dwarf stars in the galaxies Leo P (3% Z ⊙), Sextans A (6% Z ⊙), and WLM (14% Z ⊙). We quantify equivalent widths of photospheric metal lines and strengths of wind-sensitive features, confirming that both correlate with metallicity. We infer the stars’ fundamental properties by modeling their FUV through near-infrared spectral energy distributions and identify stars in the SMC with similar properties to each of our targets. Comparing to the FUV spectra of the SMC analogs suggests that (1) the star in WLM has an SMC-like metallicity, and (2) the most metal-poor star in Leo P is driving a much weaker stellar wind than its SMC counterparts. We measure projected rotation speeds and find that the two most metal-poor stars have high v sin ( i ) ≥ 290 km s−1, and estimate just a 3%–6% probability of finding two fast rotators if the metal-poor stars are drawn from the same v sin ( i ) distribution observed for O dwarfs in the SMC. These observations suggest that models should include the impact of rotation and weak winds on ionizing flux to accurately interpret observations of metal-poor galaxies in both the near and distant universe.

The Distribution of Oxygen, Calcium, and Strontium Over the Surface of 21 Com

Comparison of optical high resolution high signal-to-noise ELODIE spectra of 21 Com taken in 2004 April and separated by about half of the 2 days rotational period, reveals variations of the oxygen, calcium and strontium lines between rotational phases 0.94 and 0.48. Whereas the lines of oxygen and calcium are stronger at phase 0.94, those of strontium strengthen at phase 0.48. The synthesis of strong Sr ii lines yields disk-averaged estimates of the abundances of strontium: about 200 times solar at phase 0.94 and 450 solar at phase 0.48 for strontium. However the O i and Ca ii lines are too blended to derive unambiguous quantitative information on the abundance and distribution of oxygen and calcium over the surface of 21 Com. The likely underabundance of oxygen might be quantified using stronger far-ultraviolet transitions.