A Nascent Tidal Dwarf Galaxy Forming within the Northern H i Streamer of M82

We identify a ∼600 pc wide region of active star formation located within a tidal streamer of M82 via Hα emission (F Hα ∼ 6.5 × 10−14 erg s−1 cm−2), using a pathfinder instrument based on the Dragonfly Telephoto Array. The object is kinematically decoupled from the disk of M82 as confirmed via Keck/LRIS spectroscopy and is spatially and kinematically coincident with an overdensity of H i and molecular hydrogen within the “northern H i streamer” induced by the passage of M81 several hundred Myr ago. From H i data, we estimate that ∼5 × 107 M ⊙ of gas is present in the specific overdensity coincident with the Hα source. The object’s derived metallicity (12+ log ( O / H ) ≃ 8.6 ), position within a gas-rich tidal feature, and morphology (600 pc diameter with multiple star-forming clumps), indicate that it is likely a tidal dwarf galaxy in the earliest stages of formation.

Star formation in ‘the Brick’: ALMA reveals an active protocluster in the Galactic centre cloud G0.253+0.016

ABSTRACT G0.253+0.016, aka ‘the Brick’, is one of the most massive (>105 M⊙) and dense (>104 cm−3) molecular clouds in the Milky Way’s Central Molecular Zone. Previous observations have detected tentative signs of active star formation, most notably a water maser that is associated with a dust continuum source. We present ALMA Band 6 observations with an angular resolution of 0.13 arcsec (1000 AU) towards this ‘maser core’ and report unambiguous evidence of active star formation within G0.253+0.016. We detect a population of eighteen continuum sources (median mass ∼2 M⊙), nine of which are driving bi-polar molecular outflows as seen via SiO (5–4) emission. At the location of the water maser, we find evidence for a protostellar binary/multiple with multidirectional outflow emission. Despite the high density of G0.253+0.016, we find no evidence for high-mass protostars in our ALMA field. The observed sources are instead consistent with a cluster of low-to-intermediate-mass protostars. However, the measured outflow properties are consistent with those expected for intermediate-to-high-mass star formation. We conclude that the sources are young and rapidly accreting, and may potentially form intermediate- and high-mass stars in the future. The masses and projected spatial distribution of the cores are generally consistent with thermal fragmentation, suggesting that the large-scale turbulence and strong magnetic field in the cloud do not dominate on these scales, and that star formation on the scale of individual protostars is similar to that in Galactic disc environments.

Keeping up with the cool stars: one TESS year in the life of AB Doradus

The long-term, high precision photometry delivered by the Transiting Exoplanet Survey Satellite (TESS) enables us to gain new insight into known and hitherto well-studied stars. In this paper, we present the result of our TESS study of the photospheric activity of the rapid rotator AB Doradus. Due to its favorable position near the southern ecliptic pole, the TESS satellite recorded almost 600 rotations of AB Doradus with high cadence, allowing us to study starspots and flares on this ultra-active star. The observed peak-to-peak variation of the rotational modulations reaches almost 11%, and we find that the starspots on AB Doradus show highly preferred longitudinal positions. Using spot modeling, we measured the positions of the active regions on AB Doradus and we find that preferred spot configurations should include large regions extending from low to high stellar latitudes. We interpret the apparent movement of spots as the result of both differential rotation and spot evolution and argue that the typical spot lifetimes should range between 10 and 20 days. We further find a connection between the flare occurrence on AB Doradus and the visibility of the active regions on its surface, and we finally recalculated the star’s rotation period using different methods and we compared it with previous determinations.