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1. Cool stars in the Galactic center as seen by APOGEE: M giants, AGB stars, and supergiant stars and candidates

   https://doi.org/10.1051/0004-6361/202038327  

   Schultheis, M. ; Rojas-Arriagada, A. ; Cunha, K. ; Zoccali, M. ; Chiappini, C. ; Zasowski, G. ; Queiroz, A. B. ; Minniti, D. ; Fritz, T. ; García-Hernández, D. A. ; et al ( October 2020 , Astronomy & Astrophysics)

   The Galactic center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE), allow the measurement of metallicities in the inner region of our Galaxy. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool Asymptotic Giant branch (AGB) stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the red giant branch. We studied metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ∼ +0.3 dex and a metal-poor peak around {Fe/H] = −0.5 dex, which is 0.2 dex poorer than Baade’s Window. The α -elements Mg, Si, Ca, and O show a similar trend to the Galactic bulge. The metal-poor component is enhanced in the α -elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high-velocity stars with v gal  >  300 km s −1 ; the metal-rich stars show a much higher rotation velocity (∼200 km s −1 ) with respect to the metal-poor stars (∼140 km s −1 ). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disk and the nuclear star cluster show distinct chemical signatures and might be formed differently. 

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2. Investigation of mid-infrared long-term variability of dusty AGB stars using multi-epoch scan data of AKARI and WISE

   https://doi.org/10.1093/pasj/psac088  

   Tachibana, Kengo ; Miyata, Takashi ; Kamizuka, Takafumi ; Ohsawa, Ryou ; Takita, Satoshi ; Nakagawa, Akiharu ; Ita, Yoshifusa ; Uchiyama, Mizuho ( April 2023 , Publications of the Astronomical Society of Japan)

   Asymptotic giant branch (AGB) stars are one of the main sources of dust in the Universe. They form and supply dust triggered by stellar pulsations, but the details of the mechanism are still unknown. Among all AGB stars, dusty AGB stars are the most important in terms of dust supply because they contain the star with a high mass-loss rate. To investigate the relationship between the pulsation and the dust supply for such dusty AGB stars, long-term mid-infrared monitoring is necessary. In this study, we combine data from a infrared astronomical satellites AKARI and WISE to generate mid-infrared long-term observation data. This collected data enables us to investigate the variability of dusty AGB stars with a variability period of more than several hundred days (>250 d). Furthermore, we determine the mid-infrared variability amplitudes of 169 O-rich AGB stars and 28 OH/IR stars in our Galaxy, as well as data at other wavelengths. This study is the largest study on mid-infrared variability. Additionally, we discover a positive correlation between the variability amplitude A18μm in the 18μm band and the $\mathit {W3}-\mathit {W4}$ infrared color which is the measure of the dust supply. Also, we find that this relationship is independent of the variability period and mode. Finally, we calculate the radiative transfer of circumstellar dust from AGB stars. The result of our calculations showed that $A_{18\mu \rm {m}}$ is strongly affected by the luminosity change ratio of the central star. These experimental results imply that the luminosity change ratio has a strong influence on the dust supply of AGB star.

    

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3. Pavo: Discovery of a Star-forming Dwarf Galaxy Just Outside the Local Group*

   https://doi.org/10.3847/2041-8213/ad0130  

   Jones, Michael G. ; Mutlu-Pakdil, Burçin ; Sand, David J. ; Donnerstein, Richard ; Crnojević, Denija ; Bennet, Paul ; Fielder, Catherine E. ; Karunakaran, Ananthan ; Spekkens, Kristine ; Strader, Jay ; et al ( October 2023 , The Astrophysical Journal Letters)

   We report the discovery of Pavo, a faint (M_V= −10.0), star-forming, irregular, and extremely isolated dwarf galaxy atD≈ 2 Mpc. Pavo was identified in Dark Energy Camera Legacy Survey imaging via a novel approach that combines low surface brightness galaxy search algorithms and machine-learning candidate classifications. Follow-up imaging with the Inamori-Magellan Areal Camera and Spectrograph on the 6.5 m Magellan Baade telescope revealed a color–magnitude diagram (CMD) with an old stellar population, in addition to the young population that dominates the integrated light, and a tip of the red giant branch distance estimate of${1.99}_{-0.22}^{+0.20}$Mpc. The blue population of stars in the CMD is consistent with the youngest stars having formed no later than 150 Myr ago. We also detected no Hαemission with SOAR telescope imaging, suggesting that we may be witnessing a temporary low in Pavo’s star formation. We estimate the total stellar mass of Pavo to be$\log M_{*}/M_{\odot }=5.6\pm 0.2$and measure an upper limit on its Higas mass of 1.0 × 10⁶M_⊙based on the HIPASS survey. Given these properties, Pavo’s closest analog is Leo P (D= 1.6 Mpc), previously the only known isolated, star-forming, Local Volume dwarf galaxy in this mass range. However, Pavo appears to be even more isolated, with no other known galaxy residing within over 600 kpc. As surveys and search techniques continue to improve, we anticipate an entire population of analogous objects being detected just outside the Local Group.

    

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4. Blanco DECam Bulge Survey (BDBS): V. Cleaning the foreground populations from Galactic bulge colour-magnitude diagrams using Gaia EDR3

   https://doi.org/10.1051/0004-6361/202243921  

   Marchetti, Tommaso ; Johnson, Christian I. ; Joyce, Meridith ; Rich, R.Michael ; Simion, Iulia T. ; Young, Michael D. ; Clarkson, William ; Pilachowski, Catherine A. ; Michael, Scott ; Kunder, Andrea ; et al ( August 2022 , Astronomy & Astrophysics)

   Aims. The Blanco DECam Bulge Survey (BDBS) has imaged more than 200 square degrees of the southern Galactic bulge, providing photometry in the ugrizy filters for ∼250 million unique stars. The presence of a strong foreground disk population, along with complex reddening and extreme image crowding, has made it difficult to constrain the presence of young and intermediate age stars in the bulge population. Methods. We employed an accurate cross-match of BDBS with the latest data release (EDR3) from the Gaia mission, matching more than 140 million sources with BDBS photometry and Gaia EDR3 photometry and astrometry. We relied on Gaia EDR3 astrometry, without any photometric selection, to produce clean BDBS bulge colour-magnitude diagrams (CMDs). Gaia parallaxes were used to filter out bright foreground sources, and a Gaussian mixture model fit to Galactic proper motions could identify stars kinematically consistent with bulge membership. We applied this method to 127 different bulge fields of 1 deg 2 each, with | ℓ | ≤ 9.5° and −9.5° ≤ b  ≤ −2.5°. Results. The astrometric cleaning procedure removes the majority of blue stars in each field, especially near the Galactic plane, where the ratio of blue to red stars is ≲10%, increasing to values ∼20% at higher Galactic latitudes. We rule out the presence of a widespread population of stars younger than 2 Gyr. The vast majority of blue stars brighter than the turnoff belong to the foreground population, according to their measured astrometry. We introduce the distance between the observed red giant branch bump and the red clump as a simple age proxy for the dominant population in the field, and we confirm the picture of a predominantly old bulge. Further work is needed to apply the method to estimate ages to fields at higher latitudes, and to model the complex morphology of the Galactic bulge. We also produce transverse kinematic maps, recovering expected patterns related to the presence of the bar and of the X-shaped nature of the bulge. 

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5. Low-density star cluster formation: discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247

   https://doi.org/10.1093/mnras/stac2898  

   Romanowsky, Aaron J. ; Larsen, Søren S. ; Villaume, Alexa ; Carlin, Jeffrey L. ; Janz, Joachim ; Sand, David J. ; Strader, Jay ; Brodie, Jean P. ; Chakrabarti, Sukanya ; Cheng, Chloe M. ; et al ( October 2022 , Monthly Notices of the Royal Astronomical Society)

   The classical globular clusters found in all galaxy types have half-light radii of rh ∼ 2–4 pc, which have been tied to formation in the dense cores of giant molecular clouds. Some old star clusters have larger sizes, and it is unclear if these represent a fundamentally different mode of low-density star cluster formation. We report the discovery of a rare, young ‘faint fuzzy’ star cluster, NGC 247-SC1, on the outskirts of the low-mass spiral galaxy NGC 247 in the nearby Sculptor group, and measure its radial velocity using Keck spectroscopy. We use Hubble Space Telescope imaging to measure the cluster half-light radius of rh ≃ 12 pc and a luminosity of LV ≃ 4 × 105L⊙. We produce a colour–magnitude diagram of cluster stars and compare to theoretical isochrones, finding an age of ≃300 Myr, a metallicity of [Z/H] ∼ −0.6 and an inferred mass of M⋆ ≃ 9 × 104M⊙. The narrow width of blue-loop star magnitudes implies an age spread of ≲50 Myr, while no old red-giant branch stars are found, so SC1 is consistent with hosting a single stellar population, modulo several unexplained bright ‘red straggler’ stars. SC1 appears to be surrounded by tidal debris, at the end of an ∼2 kpc long stellar filament that also hosts two low-mass, low-density clusters of a similar age. We explore a link between the formation of these unusual clusters and an external perturbation of their host galaxy, illuminating a possible channel by which some clusters are born with large sizes.

    

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