Examinando por Autor "Matsunaga, Noriyuki"

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    Mira variables in the Milky Way's nuclear stellar disc: Discovery and classification
    (Oxford University Press, 2022-11) Sanders, Jason L.; Matsunaga, Noriyuki; Kawata, Daisuke; Smith, Leigh C.; Minniti, Dante; Lucas, Philip W.
    The properties of the Milky Way's nuclear stellar disc give crucial information on the epoch of bar formation. Mira variables are promising bright candidates to study the nuclear stellar disc, and through their period-age relation dissect its star formation history. We report on a sample of 1782 Mira variable candidates across the central $3\times 3\, \mathrm{deg}2$ of the Galaxy using the multi-epoch infrared VISTA Variables in Via Lactea (VVV) survey. We describe the algorithms employed to select candidate variable stars and then model their light curves using periodogram and Gaussian process methods. By combining with WISE, 2MASS, and other archival photometry, we model the multiband light curves to refine the periods and inspect the amplitude variation between different photometric bands. The infrared brightness of the Mira variables means many are too bright and missed by VVV. However, our sample follows a well-defined selection function as expected from artificial star tests. The multiband photometry is modelled using stellar models with circumstellar dust that characterize the mass-loss rates. We demonstrate how 90 per cent of our sample is consistent with O-rich chemistry. Comparison to period-luminosity relations demonstrates that the bulk of the short period stars are situated at the Galactic Centre distance. Many of the longer period variables are very dusty, falling significantly under the O-rich Magellanic Cloud and solar neighbourhood period-luminosity relations and exhibit high mass-loss rates of. The period distribution appears consistent with the nuclear stellar disc forming ago, although it is not possible to disentangle the relative contributions of the nuclear stellar disc and the contaminating bulge. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
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    The globular cluster VVV CL002 falling down to the hazardous Galactic centre
    (EDP Sciences, 2024-03-01) Minniti, Dante; Matsunaga, Noriyuki; Fernández-Trincado, José G.; Otsubo, Shogo; Sarugaku, Yuki; Takeuchi, Tomomi; Katoh, Haruki; Hamano, Satoshi; Ikeda, Yuji; Kawakita, Hideyo; Lucas, Philip W.; Smith, Leigh C.
    Context. The Galactic centre is hazardous for stellar clusters because of the strong tidal force in action there. It is believed that many clusters were destroyed there and contributed stars to the crowded stellar field of the bulge and the nuclear stellar cluster. However, the development of a realistic model to predict the long-term evolution of the complex inner Galaxy has proven difficult, and observations of surviving clusters in the central region would provide crucial insights into destruction processes. Aims. Among the known Galactic globular clusters, VVV CL002 is the closest to the centre, at 0.4 kpc, but has a very high transverse velocity of 400 km s-1. The nature of this cluster and its impact on Galactic astronomy need to be addressed with spectroscopic follow up. Methods. Here we report the first measurements of its radial velocity and chemical abundance based on near-infrared high-resolution spectroscopy. Results. We find that this cluster has a counter-rotating orbit constrained within 1.0 kpc of the centre, and as close as 0.2 kpc at the perigalacticon, confirming that the cluster is not a passerby from the halo but a genuine survivor enduring the harsh conditions of the tidal forces of the Galactic mill. In addition, its metallicity and α abundance ([α/Fe] ≃ +0.4 and [Fe/H] = -0.54) are similar to those of some globular clusters in the bulge. Recent studies suggest that stars with such α-enhanced stars were more common at 3- 6 kpc from the centre around 10 Gyr ago. Conclusions. We infer that VVV CL002 was formed outside but is currently falling down to the centre, showcasing a real-time event that must have occurred to many clusters a long time ago.