Keywords

Observations with the Wisconsin ${\rm{H}}\alpha$ Mapper reveal a large, diffuse ionized halo that surrounds the Small Magellanic Cloud (SMC). We present the first kinematic ${\rm{H}}\alpha$ survey of an extended region around the galaxy, from $({\ell },b)=(289\buildrel{\circ}\over{.} 5,-35\buildrel{\circ}\over{.} 0)$ to $(315\buildrel{\circ}\over{.} 1,-5\buildrel{\circ}\over{.} 3)$ and covering $+90\leqslant {v}_{\mathrm{LSR}}\leqslant +210\ \mathrm{km}\,{{\rm{s}}}^{-1}$. The ionized gas emission extends far beyond the central stellar component of the galaxy, reaching similar distances to that of the diffuse neutral halo traced by 21 cm observations. ${\rm{H}}\alpha$ emission extends several degrees beyond the sensitivity of current H i surveys toward smaller galactic longitudes and more negative galactic latitudes. The velocity field of the ionized gas near the SMC appears similar to the neutral halo of the galaxy. Using the observed emission measure as a guide, we estimate the mass of this newly revealed ionized component to be roughly $(0.8\mbox{--}1.0)\times {10}^{9}\,{M}_{\odot }$, which is comparable to the total neutral mass in the same region of $(0.9\mbox{--}1.1)\times {10}^{9}\,{M}_{\odot }$. We find ratios of the total ionized gas mass divided by the total neutral plus ionized gas mass in three distinct subregions to be: (1) 46%–54% throughout the SMC and its extended halo, (2) 12%–32% in the SMC Tail that extends toward the Magellanic Bridge, and (3) 65%–79% in a filament that extends away from the SMC toward the Magellanic Stream. This newly discovered, coherent ${\rm{H}}\alpha$ filament does not appear to have a well-structured neutral component and is also not coincident with two previously identified filaments traced by 21 cm emission within the Stream.

We present a comparative study of the size–line width relation for substructures within six molecular clouds in the Large Magellanic Cloud (LMC) mapped with the Atacama Large Millimeter/submillimeter Array. Our sample extends our previous study, which compared a Planck detected cold cloud in the outskirts of the LMC with the 30 Doradus molecular cloud and found the typical line width for 1 pc radius structures to be five times larger in 30 Doradus. By observing clouds with intermediate levels of star formation activity, we find evidence that the line width at a given size increases with increasing local and cloud-scale 8 μm intensity. At the same time, the line width at a given size appears to independently correlate with measures of the mass surface density. Our results suggest that both virial-like motions due to gravity and local energy injection by star formation feedback play important roles in determining intracloud dynamics.

We present the first results from the Small Magellanic Cloud portion of a new Australia Telescope Compact Array H i absorption survey of both of the Magellanic Clouds, comprising over 800 hr of observations. Our new H i absorption line data allow us to measure the temperature and fraction of cold neutral gas in a low-metallicity environment. We observed 22 separate fields, targeting a total of 55 continuum sources, against 37 of which we detected H i absorption; from this we measure a column-density-weighted mean average spin temperature of $\langle {T}_{{\rm{s}}}\rangle$ = 150 K. Splitting the spectra into individual absorption line features, we estimate the temperatures of different gas components and find an average cold gas temperature of ∼30 K for this sample, lower than the average of ∼40 K in the Milky Way. The H i appears to be evenly distributed throughout the SMC, and we detect absorption in 67% of the lines of sight in our sample, including some outside the main body of the galaxy (N_{H i} $\gt 2\times {10}^{21}$ cm⁻²). The optical depth and temperature of the cold neutral atomic gas show no strong trend with location spatially or in velocity. Despite the low-metallicity environment, we find an average cold gas fraction of ∼20%, not dissimilar from that of the Milky Way.

RX J0046.5−7308 is a shell-type supernova remnant (SNR) in the Small Magellanic Cloud (SMC). We carried out new ¹²CO(J = 1–0, 3–2) observations toward the SNR using Mopra and the Atacama Submillimeter Telescope Experiment. We found eight molecular clouds (A–H) along the X-ray shell of the SNR. The typical cloud size and mass are ∼10–15 pc and ∼1000–3000 M_☉, respectively. The X-ray shell is slightly deformed and has the brightest peak in the southwestern shell where two molecular clouds A and B are located. The four molecular clouds A, B, F, and G have high intensity ratios of ¹²CO(J = 3–2)/¹²CO(J = 1–0) > 1.2, which are not attributable to any identified internal infrared sources or high-mass stars. The H i cavity and its expanding motion are found toward the SNR, which are likely created by strong stellar winds from a massive progenitor. We suggest that the molecular clouds A–D, F, and G and H i clouds within the wind-blown cavity at V_LSR = 117.1–122.5 km s⁻¹ are associated with the SNR. The X-ray spectroscopy reveals the dynamical age of ${26000}_{-2000}^{+1000}$ yr and the progenitor mass of ≳30 M_☉, which is also consistent with the proposed scenario. We determine physical conditions of the giant molecular cloud LIRS 36A using the large velocity gradient analysis with archival data sets of the Atacama Large Millimeter/submillimeter Array; the kinematic temperature is ${72}_{-37}^{+50}$ K and the number density of molecular hydrogen is ${1500}_{-300}^{+600}$ cm⁻³. The next generation of γ-ray observations will allow us to study the pion-decay γ-rays from the molecular clouds in the SMC SNR.

The supersoft X-ray and optical transient ASASSN-16oh has been interpreted by Maccarone et al. as having been induced by an accretion event on a massive white dwarf (WD), resembling a dwarf nova super-outburst. These authors argued that the supersoft X-ray spectrum had a different origin than in an atmosphere heated by shell nuclear burning, because no mass was ejected. We find instead that the event's timescale and other characteristics are typical of non-mass-ejecting thermonuclear runaways, as already predicted by Shara et al. and the extensive grid of nova models by Yaron et al. We suggest that the low X-ray and bolometric luminosity in comparison to the predictions of the models of nuclear burning are due to an optically thick accretion disk, hiding most of the WD surface. If this is the case, we calculated that the optical transient can be explained as a non-ejective thermonuclear event on a WD of ≃1.1 M_⊙ accreting at the rate of ≃3.5–5 × 10⁻⁷ M_⊙ yr⁻¹. We make predictions that should prove whether the nature of the transient event was due to thermonuclear burning or to accretion; observational proof should be obtained in the next few years, because a new outburst should occur within ≃10–15 yr of the event.

Protoplanetary disks orbiting intermediate-mass stars, Herbig Ae/Be stars, that have formed in a metal-poor environment may evolve differently than their Galactic cousins. A study of the planet-formation process in such an environment requires identification and characterization of a sample of candidates. We have observed several stars in the Small Magellanic Cloud, a nearby metal-poor dwarf galaxy, that have optical spectral properties of Herbig Ae/Be stars, including strong Hα emission, blue continuum excess, and spectral types ranging from early G to B. Infrared spectra of these sources from the Spitzer Space Telescope show strong excess emission indicating the presence of silicate dust, molecular and atomic gas, and polycyclic aromatic hydrocarbons. We present an analysis of the likelihood that these candidates are Herbig Ae/Be stars. This identification is the necessary first step to future investigations that will examine the role of metallicity in the evolution of protoplanetary disks.

Recent investigations of the extinction law in 30 Dor and the Tarantula Nebula, at optical and near-infrared wavelengths, have revealed a ratio of total to selective extinction R_V = A_V/E(B − V) of about 4.5. This indicates a larger fraction of big grains than in the Galactic diffuse interstellar medium (ISM). Possible origins include coalescence of small grains, small grain growth, selective destruction of small grains, and fresh injection of big grains. From a study of the ultraviolet extinction properties of three massive stars in the 30 Dor nebula (R139, R140, R145), observed with the International Ultraviolet Explorer, we show that the excess of big grains does not come at the expense of small grains, which are still present and possibly even more abundant. Fresh injection of large grains appears to be the dominant mechanism. A process able to naturally account for this in environments such as the Tarantula nebula, where formation of massive stars has been ongoing for over ∼20 Myr, is the explosion of massive stars as SNe II. The ensuing change in the conditions of the ISM is only temporary, lasting less than ∼100 Myr, because shattering and shocks will eventually break and destroy the bigger grains. However, this is the only time when star-forming regions are detectable as such in starburst and high-redshift galaxies, and we highlight the complexity inherent in interpreting observations of star-forming regions in these environments. If the extinction characteristics are not known properly, any attempts to derive quantitative physical parameters are bound to fail.

To place meaningful constraints on Big Bang Nucleosynthesis models, the primordial helium abundance determination is crucial. Low-metallicity H ii regions have been used to estimate it because their statistical uncertainties are relatively small. We present a new determination of the primordial helium abundance, based on long-slit spectra of the H ii region NGC 346 in the small Magellanic cloud. We obtained spectra using three 409'' × 0.51'' slits divided into 97 subsets. They cover the range λλ3600–7400 of the electromagnetic spectrum. We used PyNeb and standard reduction procedures to determine the physical conditions and chemical composition. We found that for NGC 346 X = 0.7465, Y = 0.2505, and Z = 0.0030. By assuming ΔY/ΔO = 3.3 ± 0.7 we found that the primordial helium abundance is Y_P = 0.2451 ± 0.0026 (1σ). Our Y_P value is in agreement with the value of neutrino families, N_ν, and with the neutron half-life time, τ_n, obtained in the laboratory.

We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the Draine & Li dust model. We derive the abundance of the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction (${q}_{{\rm{PAH}}}$, the fraction of the dust mass in the form of PAHs) is smaller in the SMC (${1.0}_{-0.3}^{+0.3}$ %) than in the LMC (${3.3}_{-1.3}^{+1.4}$ %). We measure the PAH fraction in different gas phases (H ii regions, ionized gas outside of H ii regions, molecular gas, and diffuse neutral gas). H ii regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way (∼4.6%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.