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NASA NTRS2023

The First Extragalactic Detection of Higher-Order Hydrogen Recombination Lines

The Large Magellanic Cloud (LMC) is the nearest (~50 kpc) star-forming galaxy characterized by a low metallicity (Z~0.3-0.5 Z⨀) similar to galaxies during the early phases of their assembly. As a result, star formation studies in the LMC provide a stepping stone to understanding star formation at earlier epochs of the universe where these processes cannot be directly observed. N113 is one of the most prominent star-forming regions in the LMC hosting one of the most massive giant molecular clouds. N113 is small enough to be imaged in its entirety, but large enough to showcase many important phenomena such as multiple generations of stars, stellar feedback, and different environments. We present our findings from an investigation of the early stages of star formation in the N113 region using the Atacama Large Millimeter/submillimeter Array (ALMA) molecular line data probing a wide density range: 12CO, 13CO, and C18O (2-1), 13CO and C18O (1-0), HCN (1-0), HCO+ (1-0), H13CN (1-0) and (3-2), H13CO+ (1-0) and (3-2), CS (2-1) and (5-4), as well as 1.3 mm and 3 mm continuum. We used the Python package quickclump to identify molecular clumps. We utilized the multiline non-LTE fitting tool based on models from RADEX developed by Finn et al. (2021, ApJ, 917, 106) to construct the CO, HCN, HCO+, and CS temperature and column density, and the H2 density maps of N113. We constructed a catalog of molecular clumps including their physical properties, chemical abundances, sizes, velocities, and velocity dispersions. To establish the evolutionary status of the clumps, their positions were compared with previously identified young stellar objects (YSOs) from the Spitzer/SAGE and Herschel/HERITAGE surveys, as well as water and OH masers. We compared the properties of the clumps in N113 to those in the Galaxy and other regions in the LMC to assess the impact of the environment (e.g., metallicity, stellar feedback) on the star formation process.

Marta M Sewilo↗

NASA NTRS2022

The Polstar High Resolution Spectropolarimetry MIDEX Mission

The Polstar mission will provide a space-borne 60 cm spectropolarimeter operating at ultraviolet (UV) wavelengths, capturing all four Stokes parameters (intensity, two linear polarization components, and circular polarization). Polstar’s capabilities are designed to meet its goal of determining how circumstellar gas flows alter and inform massive star evolution, affect the stellar remnant population, and stir and enrich the interstellar medium (ISM). These will be achieved by investigating the dynamical geometries in the winds and disks of hot stars, the composition and magnetic alignment of interstellar dust, and the star-forming accretion disks of UV-bright stars at an important transition boundary. Together these areas map out a kind of two-way interface between massive stars and their effect on our galaxy, wherein the stellar winds enrich the ISM with metals and kinetic energy, preconditioning their environment and the stellar endpoints prior to undergoing supernova. The ISM dust in turn reveals the composition and magnetic environment leading to new star formation, and the accretion disks of Herbig Ae/Be stars reveal how the ISM gas returns to make new massive stars. Polstar will combine high-resolution spectroscopy in the time domain with high-precision UV polarimetry. Doppler-shifted UV resonance line opacity will provide information about circumstellar kinematics, while polarization gives complementary geometric information about unseen structures. The composition and magnetic alignment of the smallest interstellar dust grains provides a probe of the ISM utilizing radiative alignment theory (RAT). Polstar will operate in the far-UV (FUV) at 122–200 nm at high spectral resolution of around R ∼ 30k, and at FUV and near-UV (NUV) wavelengths of 122–320 nm at lower spectral resolutions of 0.1 - 1k. Detection of polarization levels as weak as 0.1% are expected, with a temporal cadence ranging from 5–10 minutes for most wind variability studies, to hours or days for sampling rotation, to days or weeks for sampling binary orbits, to months to a year for sampling substructure in the inner regions of protoplanetary disks. Sub-meter-class aperture is well suited to access this wide array of time domain science, made possible by restricting to a few hundred bright, massive stars, necessarily extincted by a small to moderate column of interstellar dust, informing both the attributes of the stars and the ISM through which they are seen. As such, the focus is on our own galaxy and its evolutionary drivers, but a few targets in the Magellanic clouds offer the potential to extend this understanding to low-metallicity environments.

Paul A Scowen↗

NASA NTRS2023

Far-Infrared Line Emission from the Outer Galaxy Cluster Gy 3–7 With SOFIA/FIFI-LS: Physical Conditions and UV Fields

Context. Far-infrared (FIR) line emission provides key information about the gas cooling and heating due to shocks and UV radiation associated with the early stages of star formation. Gas cooling via FIR lines might, however, depend on metallicity. Aims. We aim to quantify the FIR line emission and determine the spatial distribution of the CO rotational temperature, ultraviolet (UV) radiation field, and H2 number density toward the embedded cluster Gy 3–7 in the CMa–l224 star-forming region, whose metallicity is expected to be intermediate between that of the Large Magellanic Cloud and the Solar neighborhood. By comparing the total luminosities of CO and [O I] toward Gy 3–7 with values found for low- and high-mass protostars extending over a broad range of metallicities, we also aim to identify the possible effects of metallicity on the FIR line cooling within our Galaxy. Methods. We studied SOFIA/FIFI-LS spectra of Gy 3–7, covering several CO transitions from J = 14–13 to 31-30, the OH doublet at 79 µm, the [O I] 63.2 and 145.5 µm, and the [C II] 158 µm lines. The field of view covers a 2 0 × 1 0 region with a resolution of ∼7 00–1800 . Results. The spatial extent of CO high-J (Jup ≥14) emission resembles that of the elongated 160 µm continuum emission detected with Herschel, but its peaks are offset from the positions of the dense cores. The [O I] lines at 63.2 µm and 145.5 µm follow a similar pattern, but their peaks are found closer to the positions of the cores. The CO transitions from J = 14–13 to J = 16–15 are detected throughout the cluster and show a median rotational temperature of 170 ± 30 K on Boltzmann diagrams. Comparisons to other protostars observed with Herschel show a good agreement with intermediate-mass sources in the inner Galaxy. Assuming an origin of the [O I] and high-J CO emission in UV-irradiated C−shocks, we obtained pre-shock H2 number densities of 104–105 cm−3 and UV radiation field strengths of 0.1–10 Habing fields (G0). Conclusions. Far-IR line observations reveal ongoing star formation in Gy 3–7, dominated by intermediate-mass Class 0/I young stellar objects. The ratio of molecular-to-atomic far-IR line emission shows a decreasing trend with bolometric luminosities of the protostars. However, it does not indicate that the low-metallicity has an impact on the line cooling in Gy 3–7.

stars: formation↗

NASA NTRSDate not supplied

JWST Photometry and Spectra Reveal Dust and Ice in Super Star Cluster Located in N79

Super star clusters (SSCs) are the most extreme mode of star formation; however they are rare in the local universe. With the serendipitous discovery of SSC candidate H72.97-69.39 in the N79 Region of the Large Magellanic Cloud, we have a unique opportunity to study the early phases of an SSC formation. H72.97-69.39 has an uncertain luminosity with a lower limit of 1,500,000 L⊙, based on spectral energy distribution fit, with a potential to reach the luminosity of the R136 SSC in 30 Doradus (10,000,000 L⊙). We will present James Webb Space Telescope (JWST) near- and mid-infrared (NIRCam and MIRI) observations of H72.97-69.39, including mid-infrared spectra of six young stellar objects taken with the MIRI instrument. The emission and absorption lines seen in the high-resolution spectra include fine-structure, molecular hydrogen, polyaromatic hydrocarbon, and ice features. Hydrogen recombination lines indicate these protostars have accretion rates as high as 0.02 M⊙ per year. Photometry is accomplished using the software package StarbugII, which was developed for JWST imaging of crowded fields and complex backgrounds. Near- and mid-infrared photometry from JWST reveal over 100 young stellar objects within a 30-pc x 40 pc (120” x 160”) region with spectral energy distribution fits indicating the masses of these objects areas low as 0.5 M⊙ and as high as 40 M⊙. We use color magnitude diagrams to separate the young stellar objects from the red giant branch and asymptotic giant branch stellar populations. We compare our JWST observations to ALMA, HST, and Chandra observations to present a clearer picture of how star formation takes place within an SSC.

Isha Nayak↗

DOE OSTI2020

Sensitivity of future liquid argon dark matter search experiments to core-collapse supernova neutrinos

Future liquid-argon DarkSide-20k and ARGO detectors, designed for direct dark matter search, will be sensitive also to core-collapse supernova neutrinos, via coherent elastic neutrino-nucleus scattering. This interaction channel is flavor-insensitive with a high-cross section, enabling for a high-statistics neutrino detection with target masses of $\sim$50~t and $\sim$360~t for DarkSide-20k and ARGO, respectively. Thanks to the low-energy threshold of $\sim$0.5~keV$_{nr}$ achievable by exploiting the ionization channel, DarkSide-20k and ARGO have the potential to discover supernova bursts throughout our galaxy and up to the Small Magellanic Cloud, respectively, assuming a 11-M$_{\odot}$ progenitor star. We report also on the sensitivity to the neutronization burst, whose electron neutrino flux is suppressed by oscillations when detected via charged current and elastic scattering. Finally, the accuracies in the reconstruction of the average and total neutrino energy in the different phases of the supernova burst, as well as its time profile, are also discussed, taking into account the expected background and the detector response.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2022

Neutrino Interaction Identification for the DUNE Trigger

The DUNE will be a long baseline neutrino oscillation experiment using a high purity muon neutrino beam and near detector, both located at the FNAL, and a far detector hosted 1300 km downstream at the SURF. The 10 kt fiducial mass of LAr will allow DUNE to have a rich off-beam neutrino physics programme, including the study of neutrino signals from core collapse supernovae. The SP DUNE FD module will read out ionisation data at a rate of 1.2 TB\textsuperscript{-1} whilst only a total data volume of 30 PB per year can be permanently stored. DUNE will make use of FPGA resources in the front-end of the DAQ as part of the necessitated triggering system. This thesis presents a validation study of the FPGA-based TPG in the front-end DAQ using data collected by the ProtoDUNE experiment hosted at the CERN. The FPGA-based TPG was utilised as the first stage of a baseline SNB trigger whose performance was evaluated using simulated neutrino interactions for a $11.2 m_\odot$ progenitor star. The efficiency of the baseline SNB trigger was determined to have a lower limit of $97.7\substack{+ 0.2\\ -0.3}\%$ for supernovae at a distance of 20kpc, achieving the technical requirements set out for DUNE. To improve the performance of the SNB trigger at greater SNB distances, the use of a bounding box proposal network, YOLOv3, was explored. This was found to improve the efficiency of the SNB trigger to 100\% up to the far side of the Milky Way galaxy and to $92.5\substack{+ 0.5\\ -0.5}\%$ at the Large Magellanic Cloud.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

$S^5$: Tidal Disruption in Crater 2 and Formation of Diffuse Dwarf Galaxies in the Local Group

We present results of a spectroscopic campaign around the diffuse dwarf galaxy Crater 2 (Cra2) and its tidal tails as part of the Southern Stellar Stream Spectroscopic Survey ($S^5$). Cra2 is a Milky Way dwarf spheroidal satellite with extremely cold kinematics, but a huge size similar to the Small Magellanic Cloud, which may be difficult to explain within collisionless cold dark matter. We identify 143 Cra2 members, of which 114 belong to the galaxy's main body and 29 are deemed part of its stellar stream. We confirm that Cra2 is dynamically cold (central velocity dispersion $2.51^{+0.33}_{-0.30}\,{\rm km\,s^{-1}}$) and also discover a $\approx$7$Ï$ velocity gradient consistent with its tidal debris track. We separately estimate the stream velocity dispersion to be $5.74^{+0.98}_{-0.83}\,{\rm km\,s^{-1}}$. We develop a suite of $N$-body simulations with both cuspy and cored density profiles on a realistic Cra2 orbit to compare with $S^5$ observations. We find that the velocity dispersion ratio between Cra2 stream and galaxy ($2.30^{+0.41}_{-0.35}$) is difficult to reconcile with a cuspy halo with fiducial concentration and an initial mass predicted by standard stellar mass$-$halo mass relationships. Instead, either a cored halo with relatively small core radius or a low-concentration cuspy model can reproduce this ratio. Despite tidal mass loss, Cra2 is metal-poor ($\langle \rm[Fe/H]\rangle=-2.16\pm0.04$) compared to the stellar mass$-$metallicity relation for its luminosity. Other diffuse dwarf galaxies similar to Cra2 in the Local Group (Antlia 2 and Andromeda 19) also challenge galaxy formation models. Finally, we discuss possible formation scenarios for Cra2, including ram-pressure stripping of a gas-rich progenitor combined with tides.

Limberg, Guilherme [Chicago U., KICP; Chicago U.] ↗

NASA NTRS1994

Probing the galactic disk and halo: Metal abundances in the Magellanic Stream

We derive metal abundance limits for two clouds in the Magellanic Stream using Goddard High Resolution Spectrograph (GHRS) medium resolution spectra of the Seyfert galaxy Fairall 9. We find Si/H is greater than or equal to 0.2 solar and S/H is less than or equal to 0.9 solar for the +170 km/s cloud with a N(H I) = 2 x 10(exp 19)/sq cm and Si/H is greater than or equal to 0.07 solar and S/H is less than or equal to 0.3 solar for the +210 km/s cloud with a N(H I) = 6 x 10(exp 19)/sq cm. These abundance limits rule out the Magellanic Stream as primordial gas. The metal abundance limits are consistent with the Magellanic Cloud abundances. If the two Magellanic Stream clouds have the same metal abundances, then Si/H is greater than or equal to 0.2 solar and S/H is less than or equal to 0.3 solar for the gas. The ratio Si/S is then greater than or equal to 0.6 of the solar ratio, suggesting that dust depletion is probably not very significant in the Magellanic Stream.

Lu, Limin↗

DOE OSTI2021

Conversions between gas-phase metallicities in MaNGA

ABSTRACT We present polynomial conversions between each of 11 different strong line gas-phase metallicity calibrations, each based on ∼1.1 million star-forming spaxels in the public Sloan Digital Sky Survey (SDSS) Data Release 15 (DR15) Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. For this sample, which is ∼20 times larger than previous works, we present fifth-order polynomial fits for each of 110 possible calibration conversions, for both Small Magellanic Cloud-type and Milky Way-type dust corrections. The typical 2σ scatter around our polynomial fits is 0.1 dex; we present the range over which the metallicities are valid. Conversions between metallicities which rely on the same set of line ratios, or a heavily shared set of emission lines, have reduced scatter in their conversions relative to those conversions with little overlap in required emission lines. Calibration conversions with less consistent sets of emission lines also have increased galaxy-to-galaxy variability, and this variability can account for up to 35 per cent of the total scatter. We also compare our conversions to previous work with the single fibre SDSS DR7 spectra along with higher spatial resolution data from the TYPHOON Integral Field Spectroscopy survey, resulting in comparison samples with spatial resolutions from several kpc down to ∼100 pc. Our metallicity conversions, obtained with the large sample of MaNGA, are robust against the influence of diffuse ionized gas, redshift, effective radius, and spatial blurring, and are therefore consistent across both integrated spectra and the high-resolution integral field spectroscopy data.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

A comparative analysis of the chemical compositions of Gaia-Enceladus/Sausage and Milky Way satellites using APOGEE

ABSTRACT We use data from the 17th data release of the Apache Point Observatory Galactic Evolution Experiment (APOGEE 2) to contrast the chemical composition of the recently discovered Gaia Enceladus/Sausage system (GE/S) to those of 10 Milky Way (MW) dwarf satellite galaxies: LMC, SMC, Boötes I, Carina, Draco, Fornax, Sagittarius, Sculptor, Sextans, and Ursa Minor. Our main focus is on the distributions of the stellar populations of those systems in the [Mg/Fe]–[Fe/H] and [Mg/Mn]–[Al/Fe] planes, which are commonly employed in the literature for chemical diagnosis and where dwarf galaxies can be distinguished from in situ populations. We show that, unlike MW satellites, a GE/S sample defined purely on the basis of orbital parameters falls almost entirely within the locus of ‘accreted’ stellar populations in chemical space, which is likely caused by an early quenching of star formation in GE/S. Due to a more protracted history of star formation, stars in the metal-rich end of the MW satellite populations are characterized by lower [Mg/Mn] than those of their GE/S counterparts. The chemical compositions of GE/S stars are consistent with a higher early star formation rate (SFR) than MW satellites of comparable and even higher mass, suggesting that star formation in the early universe was strongly influenced by other parameters in addition to mass. We find that the direction of the metallicity gradient in the [Mg/Mn]–[Al/Fe] plane of dwarf galaxies is an indicator of the early SFR of the system.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

MCG+07-20-052: Interacting Dwarf Pair in a Group Environment

We present an observational study of the interacting pair of dwarf galaxies, MCG+07-20-052, in the vicinity of Milky Way mass spiral galaxy NGC 2998. MCG+07-20-052 is located at a sky-projected distance of 105 kpc from NGC 2998 and the two have a relative line-of-sight velocity of 60 km s{sup −1}. We observed tidal tail-like extensions on both members (D1 and D2) of the interacting pair MCG+07-20-052. The interacting dwarf galaxies, D1 and D2, have B-band absolute magnitudes of −17.17 and −17.14 mag, respectively, and D2 is significantly bluer than D1. We obtained H i 21 cm line data of the NGC 2998 system using the Giant Metrewave Radio Telescope to get a more detailed view of the neutral hydrogen (H i) emission in the interacting dwarf galaxies and in the galaxy members of the NGC 2998 group. Evidence of a merger between the dwarf galaxies in the MCG+07-20-052 is also present in the H i kinematics and morphology where we find that H i is mostly concentrated around D2, which also shows a higher level of star-forming activity and a bluer g–r color index compared to D1. In addition, we detect extended tenuous H i emission around another member galaxy, NGC 3006, located close to the MCG+07-20-052 pair at a sky-projected distance of 41 kpc. We compare here our results from the MCG+07-20-052 pair NGC 2998 system with other known Large Magellanic Cloud/Small Magellanic Cloud/Milky Way-type systems and discuss the possible origin of the dwarf–dwarf interaction.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

The Most Metal-poor Stars in Omega Centauri (NGC 5139)

The most massive and complex globular clusters in the Galaxy are thought to have originated as the nuclear cores of now tidally disrupted dwarf galaxies, but the connection between globular clusters and dwarf galaxies is tenuous with the M54/Sagittarius system representing the only unambiguous link. The globular cluster Omega Centauri (ω Cen) is more massive and chemically diverse than M 54, and is thought to have been the nuclear star cluster of either the Sequoia or Gaia-Enceladus galaxy. Local Group dwarf galaxies with masses equivalent to these systems often host significant populations of very metal-poor stars ([Fe/H] < −2.5), and one might expect to find such objects in ω Cen. Using high-resolution spectra from Magellan-M2FS, we detected 11 stars in a targeted sample of 395 that have [Fe/H] ranging from −2.30 to −2.52. These are the most metal-poor stars discovered in the cluster, and are five times more metal-poor than ω Cen’s dominant population. However, these stars are not so metal-poor as to be unambiguously linked to a dwarf galaxy origin. The cluster’s metal-poor tail appears to contain two populations near [Fe/H] ∼ −2.1 and −2.4, which are very centrally concentrated but do not exhibit any peculiar kinematic signatures. Several possible origins for these stars are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

The Most Metal-poor Stars in the Inner Bulge

The bulge is the oldest component of the Milky Way. Since numerous simulations of Milky Way formation have predicted that the oldest stars at a given metallicity are found on tightly bound orbits, the Galaxy’s oldest stars are likely metal-poor stars in the inner bulge with small apocenters (i.e., R {sub apo} ≲ 4 kpc). In the past, stars with these properties have been impossible to find due to extreme reddening and extinction along the line of sight to the inner bulge. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey (2014) on Spitzer/Galactic Legacy Infrared Mid-Plane Survey Extraordinaire data to overcome these problems and target candidate inner bulge metal-poor giants for moderate-resolution spectroscopy with Anglo-Australian Telescope/AAOmega. We used those data to select three confirmed metal-poor giants ([Fe/H] = −3.15, −2.56, −2.03) for follow-up high-resolution Magellan/Magellan Inamori Kyocera Echelle spectroscopy. A comprehensive orbit analysis using Gaia DR2 astrometry and our measured radial velocities confirms that these stars are tightly bound inner bulge stars. We determine the elemental abundances of each star and find high titanium and iron-peak abundances relative to iron in our most metal-poor star. We propose that the distinct abundance signature we detect is a product of nucleosynthesis in the Chandrasekhar-mass thermonuclear supernova of a CO white dwarf accreting from a helium star with a delay time of about 10 Myr. Even though chemical evolution is expected to occur quickly in the bulge, the intense star formation in the core of the nascent Milky Way was apparently able to produce at least one Chandrasekhar-mass thermonuclear supernova progenitor before chemical evolution advanced beyond [Fe/H]∼−3.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Symphony: Cosmological Zoom-in Simulation Suites over Four Decades of Host Halo Mass

Abstract We present Symphony, a compilation of 262 cosmological, cold-dark-matter-only zoom-in simulations spanning four decades of host halo mass, from 10 11 –10 15 M ⊙ . This compilation includes three existing simulation suites at the cluster and Milky Way–mass scales, and two new suites: 39 Large Magellanic Cloud-mass (10 11 M ⊙ ) and 49 strong-lens-analog (10 13 M ⊙ ) group-mass hosts. Across the entire host halo mass range, the highest-resolution regions in these simulations are resolved with a dark matter particle mass of ≈3 × 10 −7 times the host virial mass and a Plummer-equivalent gravitational softening length of ≈9 × 10 −4 times the host virial radius, on average. We measure correlations between subhalo abundance and host concentration, formation time, and maximum subhalo mass, all of which peak at the Milky Way host halo mass scale. Subhalo abundances are ≈50% higher in clusters than in lower-mass hosts at fixed sub-to-host halo mass ratios. Subhalo radial distributions are approximately self-similar as a function of host mass and are less concentrated than hosts’ underlying dark matter distributions. We compare our results to the semianalytic model Galacticus , which predicts subhalo mass functions with a higher normalization at the low-mass end and radial distributions that are slightly more concentrated than Symphony. We use UniverseMachine to model halo and subhalo star formation histories in Symphony, and we demonstrate that these predictions resolve the formation histories of the halos that host nearly all currently observable satellite galaxies in the universe. To promote open use of Symphony, data products are publicly available at http://web.stanford.edu/group/gfc/symphony .

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback

We report the discovery of three faint and ultra-faint dwarf galaxies -- Sculptor A, Sculptor B and Sculptor C -- in the direction of NGC 300 (D=2.0 Mpc), a Large Magellanic Cloud-mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or HI gas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C (D=2.04$^{+0.10}_{-0.13}$ Mpc; $M_V=-9.1\pm 0.1$ mag or $L_V=(3.7^{+0.4}_{-0.3})\times 10^5L_{\odot})$ is consistent with being a satellite of NGC 300. Sculptor A (D=1.35$^{+0.22}_{-0.08}$ Mpc; $M_V=-6.9\pm 0.3$ mag or $L_V=(5^{+1}_{-1})\times 10^4 L_{\odot})$ is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultra-faint Tucana B in terms of its physical properties and environment. Sculptor B (D=2.48$^{+0.21}_{-0.24}$ Mpc; $M_V=-8.1\pm 0.3$ mag or $L_V=(1.5^{+0.5}_{-0.4})\times 10^5L_{\odot})$ is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are $\gtrsim 2-4 r_{vir}$ from NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram pressure stripping, reionization and internal feedback in influencing the star formation history of the faintest stellar systems.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1997

The Extragalactic X-ray Background in the 0.2 - 2 keV Range

We made the first measurement of the extragalactic 0.7 keV background. We detected the X-ray shadow of a neutral gas cloud in the Magellanic Bridge. We further constrained the point-like source contribution based on the mean spectrum of detected sources and on our early autocorrelation function analysis of the background. We find that our measurement extragalactic background intensity is significantly greater than the total point-like source contribution expected if sources are responsible for all the observed background intensity in the 1-2 keV range. For a further confirmation of the theoretical prediction of the hot intergalactic medium, we have conducted a pilot project to search for enhanced X-ray-emitting features near rich clusters of galaxies. We have reported the discovery of an elongated complex of extended X-ray-emitting objects in and around the galaxy cluster A2125, based on an archival deep ROSAT/PSPC observation. Using multicolor optical imaging of galaxies in the field, we find that this complex represents a hierarchical superstructure spanning approx. 11 Mpc at the redshift approx. 0.247. The multiple peak X-ray morphology and large blue galaxy fraction of A2125 indicate that the cluster is undergoing a coalescence of subunits. The superstructure contains two additional clusters, projected at distances of only 3 and 4.3 Mpc from A2125. The most interesting feature is, however, the low-surface-brightness X-ray emission from a moderate galaxy concentration away from individual clusters. The emission likely arises in a hot (approx. 10(exp 7) K) intergalactic medium, as predicted in N-body/hydro simulations of structure formation. These results demonstrate the potential of X-ray observations as a powerful tool to study the large-scale structure of the universe.

Wang, Q. Daniel↗

DOE OSTI2020

ACCESS: Confirmation of No Potassium in the Atmosphere of WASP-31b

We present a new optical (400–950 nm) transmission spectrum of the hot Jupiter WASP-31b (M = 0.48 M {sub J}; R = 1.54 R {sub J}; P = 3.41 days), obtained by combining four transit observations. These transits were observed with IMACS on the Magellan Baade Telescope at Las Campanas Observatory as part of the ACCESS project. We investigate the presence of clouds/hazes in the upper atmosphere of this planet, as well as the contribution of stellar activity on the observed features. In addition, we search for absorption features of the alkali elements Na i and K i, with particular focus on K i, for which there have been two previously published disagreeing results. Observations with Hubble Space Telescope (HST)/STIS detected K i, whereas ground-based low- and high-resolution observations did not. We use equilibrium and nonequilibrium chemistry retrievals to explore the planetary and stellar parameter space of the system with our optical data combined with existing near-IR observations. Our best-fit model is that with a scattering slope consistent with a Rayleigh slope (α=5.3{sub −3.1}{sup +2.9}), high-altitude clouds at a log cloud top pressure of −3.6{sub −2.1}{sup +2.7} bars, and possible muted H{sub 2}O features. We find that our observations support other ground-based claims of no K i. Clouds are likely why signals like H{sub 2}O are extremely muted and Na or K cannot be detected. We then juxtapose our Magellan/IMACS transmission spectrum with existing VLT/FORS2, HST/WFC3, HST/STIS, and Spitzer observations to further constrain the optical-to-infrared atmospheric features of the planet. We find that a steeper scattering slope (α = 8.3 ± 1.5) is anchored by STIS wavelengths blueward of 400 nm and only the original STIS observations show significant potassium signal.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1994

H2O-H2SO4 system in Venus' clouds and OCS, CO, and H2SO4 profiles in Venus' troposphere

A coupled problem of diffusion and condensation is solved for the H2SO4-H2O system in Venus' cloud layer. The position of the lower cloud boundary and profiles of the H2O and H2SO4 vapor mixing ratios and of the H2O/H2SO4 ratio of sulfuric acid aerosol and its flux are calculated as functions of the column photochemical production rate of sulfuric acid, Phi(sub H2SO4). Variations of the lower cloud boundary are considered. Our basic model, which is constrained to yield f(sub H2O) (30 km) = 30 ppm (Pollack et al. 1993), predicts the position of the lower cloud boundary at 48.4 km coinciding with the mean Pioneer Venus value, the peak H2SO4 mixing ratio of 5.4 ppm, and the H2SO4 production rate Phi(sub H2SO4) = 2.2 x 10(exp 12)/sq cm/s. The sulfur to sulfuric acid mass flux ratio in the clouds is 1:27 in this model, and the mass loading ratio may be larger than this value if sulfur particles are smaller than those of sulfuric acid. The model suggests that the extinction coefficient of sulfuric acid particles with radius 3.7 micrometers (mode 3) is equal to 0.3/km in the middle cloud layer. The downward flux of CO is equal to 1.7 x 10(exp 12)/sq cm/s in this model. Our second model, which is constrained to yield f(sub H2O) = 10 ppm at the lower cloud boundary, close to the value measured by the Magellan radiooccultations, predicts the position of this boundary to be at 46.5 km, which agrees with the Magellan data; f(sub H2O) (30 km) = 90 ppm, close to the data of Moroz et al. (1983) at this altitude; Phi(sub H2SO4) = 6.4 x 10(exp 12)/sq cm/s; and Phi(sub co) = 4.2 x 10(exp 12)/sq cm/s. The S/H2SO4 flux mass ratio is 1:18, and the extinction coefficient of the mode 3 sulfuric acid particles is equal to 0.9 km in the middle cloud layer. A strong gradient of the H2SO4 vapor mixing ratio near the bottom of the cloud layer drives a large upward flux of H2SO4, which condenses and forms the excessive downward flux of liquid sulfuric acid, which is larger by a factor of 4-7 than the flux in the middle cloud layer. This is the mechanism of formation of the lower cloud layer. Variations of the lower cloud layer are discussed. Our modeling of the OCS and CO profiles in the lower atmosphere measured by Pollack et al. (1993) provides a reasonable explanation of these data and shows that the rate coefficient of the reaction SO3 + CO yields CO2 + SO2 is equal to 10(exp -11) exp(-(13,100 +/- 1000)/T) cc/s. The main channel of the reaction between SO3 and OCS is CO2 + (SO)2, and its rate coefficient is equal to 10(exp -11) exp(-(8900 +/- 500)/T) cc/s. In the conditions of Venus' lower atmosphere, (SO)2 is removed by the reaction (SO)2 + OCS yields CO + S2 + SO2. The model predicts an OCS mixing ratio of 28 ppm near the surface.

Krasnopolsky, V. A.↗