A Corona Australis cloud filament seen in NIR scattered light

More than 110 dense condensations of the dark clouds in Lupus, Corona Australis, Norma, Vela, and Scorpius were observed in the 13CO and C18O (J = 1-0) transitions. The condensations of dark clouds with high star formation activity like the Ophiuchus, Taurus, and Cepheus have average C18O and H2 column densities of 1.8 × 1015 and 1.1 × 1022 cm-2. If we take the average size of the condensations to be 0.2 pc, a condensation must have average H2 volumetric densities ≥2 × 104 cm-3 in order to be a good candidate to form stars. The four Lupus filaments have similar radial velocities and velocity dispersions, suggesting that they originated from the same parental cloud. Among these filaments, Lupus 1 is unique in having recent star formation activity, despite the high number of T Tauri stars observed toward the others. Lupus 1 also shows a complex velocity gradient along its main axis. The distribution of radial velocities of the condensations observed toward Scorpius are in good agreement with the hypothesis that they are in a region with expansion velocity smaller than or equal to 18 km s-1. The Corona Australis cloud has velocity gradients ranging from -0.5 km s-1 pc-1 at one extreme to 0.1 km s-1 pc-1 at the other.

We present a near-infrared extinction map of a large region ($\sim$870 deg$^2$) covering the isolated Corona Australis complex of molecular clouds. We reach a 1-$\sigma$ error of 0.02 mag in the K-band extinction with a resolution of 3 arcmin over the entire map. We find that the Corona Australis cloud is about three times as large as revealed by previous CO and dust emission surveys. The cloud consists of a 45 pc long complex of filamentary structure from the well known star forming Western-end (the head, $N \geq10^{23}$ cm$^{-2}$) to the diffuse Eastern-end the tail, ($N \leq10^{21}$ cm$^{-2}$). Remarkably, about two thirds of the complex both in size and mass lie beneath A$_V\sim1$ mag. We find that the PDF of the cloud cannot be described by a single log-normal function. Similar to prior studies, we found a significant excess at high column densities, but a log-normal + power-law tail fit does not work well at low column densities. We show that at low column densities near the peak of the observed PDF, both the amplitude and shape of the PDF are dominated by noise in the extinction measurements making it impractical to derive the intrinsic cloud PDF below A$_K <$ 0.15 mag. Above A$_K \sim 0.15$ mag, essentially the molecular component of the cloud, the PDF appears to be best described by a power-law with index $-3$, but could also described as the tail of a broad and relatively low amplitude, log-normal PDF that peaks at very low column densities.

We uncover the H2 flows in the Corona Australis molecular cloud and in\nparticular identify the flows from the Coronet cluster. Near-infrared H2 v=1--0\nS(1), 2.12micron-line, narrow-band imaging survey of the R CrA cloud core was\ncarried out. We identify the best candidate-driving source for each outflow by\ncomparing the flow properties, available proper motions, and the\nknown/estimated properties of the driving sources. We also adopted the\nthumbrule of outflow power as proportional to source luminosity and inversely\nproportional to the source age to reach a consensus.\n Results: Continuum-subtracted, narrow-band images reveal several new\nMolecular Hydrogen emission-line Objects (MHOs). Together with previously known\nMHOs and Herbig-Haro objects we catalog at least 14 individual flow components\nof which 11 appear to be driven by the RCrA aggregate members. The flows\noriginating in the Coronet cluster have lengths of ~0.1-0.2 pc. Eight out of\nnine submillimeter cores mapped in the Coronet cluster region display embedded\nstars driving an outflow component. Roughly 80% of the youngest objects in the\nCoronet are associated with outflows. The MHO flows to the west of the Coronet\ndisplay lobes moving to the west and vice-versa, resulting in nondetections of\nthe counter lobe in our deep imaging. We speculate that these counterflows may\nbe experiencing a stunting effect in penetrating the dense central core.\n Conclusions:Although this work has reduced the ambiguities for many flows in\nthe Coronet region, one of the brightest H2 feature (MHO2014) and a few fainter\nfeatures in the region remain unassociated with a clear driving source. The\nflows from Coronet, therefore, continue to be interesting targets for future\nstudies.\n

With recent Herschel observations, the northern filament of the Corona Australis cloud has now been mapped in a number of bands from 1.2um to 870um. The data set provides a good starting point for the study of the cloud over several orders of magnitude in density. We wish to examine the differences of the column density distributions derived from dust extinction, scattering, and emission, and to determine to what extent the observations are consistent with the standard dust models. From Herschel data, we calculate the column density distribution that is compared to the corresponding data derived in the near-infrared regime from the reddening of the background stars, and from the surface brightness attributed to light scattering. We construct three-dimensional radiative transfer models to describe the emission and the scattering. The scattered light traces low column densities of A_V~1mag better than the dust emission, remaining useful to A_V ~ 10-15 mag. Based on the models, the extinction and the level of dust emission are surprisingly consistent with a sub-millimetre dust emissivity typical of diffuse medium. However, the intensity of the scattered light is very low at the centre of the densest clump and this cannot be explained without a very low grain albedo. Both the scattered light and dust emission indicate an anisotropic radiation field. The modelling of the dust emission suggests that the radiation field intensity is at least three times the value of the normal interstellar radiation field. The inter-comparison between the extinction, light scattering, and dust emission provides very stringent constraints on the cloud structure, the illuminating radiation field, and the grain properties.

New CCD images show additional emission line objects close to R CrA in the Corona Australis cloud. Two faint knots, strong in [S II], are seen against the receding CO flow associated with R CrA. They are closely aligned with HH 104 A, B and R CrA and have thus been labelled HH 104 C,D. The position angle corresponds to that of the transient spike described by Hartigan and Grhaam 1987. HH 104 C,D lie on the edge of a diffuse patch of nebulosity, strongest in H-alpha. I suggest that this is illuminated by light scattered from R CrA in the direction of the receding CO flow. The emission lien star 1-100 obeserved by Hartigan and Graham has a variable spectrum. There is a [S II] knot 17"W, 9"S of this star. A faint 19th magnitude emission line star is identified. A change in the morphology of the HH 100 reflection nebula between 1973 and 1983 is noted.

Dense molecular cloud cores are studied statistically in nearby ( pc) star-forming regions (SFRs) that show various modes of star formation. As a result of the C18O survey of NANTEN and the 4 m radio telescopes of Nagoya University, 179 cores have been collected in the SFRs of Taurus, the ρ Oph cloud, the Ophiuchus north region, the Lupus clouds, L1333, the Corona Australis cloud, Southern Coalsack, and the Pipe nebula, and their physical properties investigated. According to their star-formation activities, the cores are divided into 3 categories as 136 starless, 36 star-forming, and 7 cluster-forming cores. It is found that cores with active star formation tend to have larger , , and M. The mass function of the cores does not appear to follow a single power-law function, but the power-law index is subject to change with the mass range. The average star-formation efficiency (SFE) of the cores is roughly ~10%, and the expected stellar mass function from the SFE approximates the stellar initial-mass function (IMF). Virial analysis shows that the star-forming cores are gravitationally more bound, with smaller virial ratios than the starless cores, while cluster-forming cores are marginally bound with moderate virial ratios. We found that turbulent decay is indicated by diminishing from the starless to the star-forming cores. It is suggested that the turbulent decay is necessary for star formation, while formed star clusters provide the turbulence and make the cores unbound. Molecular clouds associated with the clusters tend to have head-tail structures and the cluster formation takes place at the head. This implies that the clouds are affected by external shocks, which have triggered cluster formation. We suggest that star and cluster formation are strongly controlled by the initial amount of internal turbulence and the interaction with the external shocks.

Profiles of the 3 μm ice band with moderate spectral resolution (λ/Δλ = 1300) are presented for four young stellar objects (YSOs). Of special interest is a comparison between those for the embedded HH 100 IR source and the FU Orionis star V346 Normae (V346 Nor) near HH 57, whose most recent outburst was in 1983. In the new spectra, there is no sign of the absorption feature at 2.97 μm attributed by Graham & Chen in 1991 to ammonia ice. We now believe that this identification was spurious. The ice band in V346 Nor has a weaker long wavelength wing than that in HH 100 IR. It matches well a profile observed in the star Elias 13 that lies behind the Taurus dark cloud, and leads to the conclusion that the line of sight to V346 Nor passes through quiescent intracloud material rather than through the dense dust observed in emission at longer wavelengths. Fine structure in the ice-band wing, probably due to C—H stretch absorption, is detected at 3.47 μm in the embedded objects HH 100 IR and [TS84] 13.1 in the Corona Australis cloud but not in V346 Nor. A second dip at 3.55 μm, which is plausibly linked to CH3OH, is observed in HH 100 IR.

Strong and flaring X-ray emission, 2–5 orders of magnitude above main sequence levels, is well-established for low-mass T Tauri stars (ages 106–107 yr). This has been attributed to enhanced solar-type magnetic activity. We summarize here recent reports for X-ray emission from earlier phases of star formation, corresponding to Class I/II (ages ∼105 yr) and Class I (ages ∼104–105 yr) infrared sources. These include XZ Tau in the Taurus L1551 cloud, the Coronet Cluster of infrared sources in the R Corona Australis cloud, and IRS 43 in the Ophiuchi cloud core. Data are obtained from the PSPC and HRI detectors on ROSAT, and the SIS and GIS detectors on ASCA. Two of the sources, R1 in CrA and IRS 43 in Oph, exhibited powerful flares during the observations. The presence of X-rays in protostars may involve non-solar-type magnetic flaring events involving the circumstellar disk. Protostellar X-rays should have a significant effect on circumstellar material, such as producing ionization for magnetic coupling of disks and outflows, and promoting disk accretion through photoionization.

We study a northern part of the Corona Australis molecular cloud that consists of a filament and a dense sub-millimetre core inside the filament. Our aim is to measure dust temperature and sub-mm emissivity within the region. We also look for confirmation that near-infrared (NIR) surface brightness can be used to study the structure of even very dense clouds. We extend our previous NIR mapping south of the filament. The dust colour temperatures are estimated using Spitzer 160um and APEX/Laboca 870um maps. The column densities derived based on the reddening of background stars, NIR surface brightness, and thermal sub-mm dust emission are compared. A three dimensional toy model of the filament is used to study the effect of anisotropic illumination on near-infrared surface brightness and the reliability of dust temperature determination. Relative to visual extinction, the estimated emissivity at 870um is kappa(870) = (1.3 +- 0.4) x 10^{-5} 1/mag. This is similar to the values found in diffuse medium. A significant increase in the sub-millimetre emissivity seems to be excluded. In spite of saturation, NIR surface brightness was able to accurately pinpoint, and better than measurements of the colour excesses of background stars, the exact location of the column density maximum. Both near- and far-infrared data show that the intensity of the radiation field is higher south of the filament.

Preliminary analysis is presented for J, H, and K′ images of the Corona Australis cloud core. Color-color and color-magnitude diagrams reveal relatively few new cluster members. We conclude that the R Cr A infrared cluster consists of only about 30 members and its size reflects the available reservoir of molecular gas.

We report Atacama Large Millimeter/submillimeter Array/Atacama Compact Array observations of a high-density region of the Corona Australis cloud forming a young star cluster, and the results of resolving internal structures. In addition to embedded Class 0/I protostars in the continuum, a number of complex dense filamentary structures are detected in the C18O and SO lines by the 7 m array. These are substructures of the molecular clump that are detected by the total power array as extended emission. We identify 101 and 37 filamentary structures with widths of a few thousand astronomical units in C18O and SO, respectively, which are called feathers. The typical column density of the feathers in C18O is about 1022 cm−2, and the volume density and line mass are ∼105 cm−3 and a few M ☉ pc−1, respectively. This line mass is significantly smaller than the critical line mass expected for cold and dense gas. These structures have complex velocity fields, indicating a turbulent interior. The number of feathers associated with Class 0/I protostars is only ∼10, indicating that most of them do not form stars but rather are transient structures. The formation of feathers can be interpreted as a result of colliding gas flow because the morphology is well reproduced by MHD simulations, and this is supported by the presence of H i shells in the vicinity. The colliding gas flows may accumulate gas and form filaments and feathers, and trigger the active star formation of the R CrA cluster.

We present new isochrone fits to the colour–magnitude diagrams of the Galactic globular clusters NGC 6362 and NGC 6723. We utilize 22 and 26 photometric filters for NGC 6362 and NGC 6723, respectively, from the ultraviolet to mid-infrared using data sets from Hubble Space Telescope, Gaia, unWISE, and other photometric sources. We use models and isochrones from the Dartmouth Stellar Evolution Database (DSED) and Bag of Stellar Tracks and Isochrones (BaSTI) for α-enhanced [α/Fe] = +0.4 and different helium abundances. The metallicities [Fe/H] = −1.04 ± 0.07 and −1.09 ± 0.06 are derived from the red giant branch slopes in our fitting for NGC 6362 and NGC 6723, respectively. They agree with spectroscopic estimates from the literature. We find a differential reddening up to ΔE(B − V) = 0.13 mag in the NGC 6723 field due to the adjacent Corona Australis cloud complex. We derive the following for NGC 6362 and NGC 6723, respectively: distances 7.75 ± 0.03 ± 0.15 (statistic and systematic error) and 8.15 ± 0.04 ± 0.15 kpc; ages 12.0 ± 0.1 ± 0.8 and 12.4 ± 0.1 ± 0.8 Gyr; extinctions AV = 0.19 ± 0.04 ± 0.06 and 0.24 ± 0.03 ± 0.06 mag; reddenings E(B − V) = 0.056 ± 0.01 ± 0.02 and 0.068 ± 0.01 ± 0.02 mag. DSED provides systematically lower [Fe/H] and higher reddenings than BaSTI. However, the models agree in their relative estimates: NGC 6723 is 0.44 ± 0.04 kpc further, 0.5 ± 0.1 Gyr older, ΔE(B − V) = 0.007 ± 0.002 more reddened, and with 0.05 ± 0.01 dex lower [Fe/H] than NGC 6362. The lower metallicity and greater age of NGC 6723 with respect to NGC 6362 explain their horizontal branch morphology difference. This confirms age as the second parameter for these clusters. We provide lists of the cluster members from the Gaia Data Release 3.