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Register a new userDetecting hot stars in the Galactic centre with combined near- and mid-infrared photometry
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Added by
Miguel Cano Gonz\\'alez
Publication date
2021
fields
Physics
and research's language is
English
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{The Galactic centre (GC) is a unique astrophysical laboratory to study the stellar population of galactic nuclei because it is the only galactic nucleus whose stars can be resolved down to milliparsec scales. However, the extreme and spatially highly variable interstellar extinction towards the GC poses a serious obstacle to photometric stellar classification.} {Our goal is to identify hot, massive stars in the nuclear stellar disc (NSD) region through combining near-infrared (NIR) and mid-infrared (MIR) photometry, and thus to demonstrate the feasibility of this technique, which may gain great importance with the arrival of the James Webb Space Telescope (JWST).} {We combined the GALACTICNUCLEUS NIR survey with the IRAC/Spitzer MIR survey of the GC. We applied the so-called Rayleigh-Jeans colour excess (RJCE) de-reddening method to our combined NIR-MIR data to identify potential hot stars in colour-magnitude diagrams (CMDs).} {Despite the very low angular resolution of IRAC we find 12 clear candidates for young massive stars among the $1,065$ sources that meet our selection criteria. Seven out of these 12 stars are previously known hot, massive stars belonging to the Arches and Quintuplet clusters, as well as sources detected by the Hubble Space Telescope/NICMOS Paschen-$alpha$ survey. Five of our massive star candidates have not been previously reported in the literature.} {We show that the RJCE method is a valuable tool to identify hot stars in the GC using photometry alone. Upcoming instruments with high angular resolution MIR imaging capabilities such as the JWST could surely make more substantial use of this de-reddening method and help establish a far more complete census of hot, young stars in the GC area than what is possible at the moment.}
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Mass-loss from evolved stars chemically enriches the ISM. Stellar winds from massive stars and their explosions as SNs shape the ISM and trigger star formation. Studying evolved stars is fundamental for understanding galaxy formation and evolution, at any redshift. We aim to establish a photometric classification scheme for Galactic mass-losing evolved stars (e.g., WR, RSG, and AGB stars) with the goal of identifying new ones, and subsequently to use the sample as tracers of Galactic structure. We searched for counterparts of known Galactic WR, LBV, RSG, and O-rich AGBs in the 2MASS, GLIMPSE, and MSX catalogs, and we analyzed their properties with near- and mid-infrared color-color diagrams. We used the Q1 parameter, which measures the deviation from the interstellar reddening vector in the J-H versus H-Ks diagram, and we defined a new parameter, Q2, that measures the deviation from the interstellar reddening vector in the J-Ks versus Ks-[8.0] diagram. The latter plane enables to distinguish between interstellar and circumstellar reddening, and to identify stars with envelopes. WR stars and late-type mass-losing stars are distributed in two different regions of the Q1 versus Ks-[8.0] diagram. A sequence of increasing [3.6]-[4.5] and [3.6]-[8.0] colors with increasing pulsation amplitudes (SRs, Miras, and OH/IRs) is found. Spectra of Miras and OH/IRs have stronger H2O absorption at 3.0um than SRs or most of the RSGs. Masing Miras have H2O, but stronger SiO (~ 4 um) and CO2 absorption (~4.25 um), as suggested by their bluer [3.6]-[4.5] colors. A fraction of RSGs (22%) have the bluest [3.6]-[4.5] colors, but small Q2 values. We propose a new set of photometric criteria to distinguish among IR bright Galactic stars. The GLIMPSE catalog is a powerful tool for photometric classification of mass-losing evolved stars. Our new criteria will yield many new RSGs and WRs.
Planetary nebulae (PNe) are powerful tracers of evolved stellar populations. Among the 3000 known PNe in the Galaxy, about 600 are located within the 520 square-degree area covered by the VVV survey. The VVV photometric catalogue provides an important new dataset for the study of PNe, with high-resolution imaging in five near-infrared bands. Aperture photometry of known PNe in the VVV area was retrieved from source catalogues. Care was taken to minimise any confusion with field stars. The colours of the PNe we are determined for H-Ks, J-H, Z-Y, and Y-J, and compared to stars and to other types of emission line objects. Cloudy photo-ionisation models were used to predict colours for typical PNe. We present near-infrared photometry for 353 known PNe. The best separation from other objects is obtained in the H-Ks vs. J-H diagram. We calculated the emission-line contribution to the in-band flux based on a model for NGC 6720: we find that this is highest in the Z and Y bands at over 50%, lower in the J band at 40%, and lowest in the H and Ks bands at 20%. A new view of PNe in the wavelength domain of the Z and Y bands is shown. Photo-ionisation models are used to explore the observed colours in these bands. The Y band is shown to be dominated by HeI 1.083 mu and HeII 1.012 mu, and colours involving this band are very sensitive to the temperature of the ionizing star. The VVV survey represents a unique dataset for studing crowded and obscured regions in the Galactic plane. The diagnostic diagrams presented here allow one to study the properties of known PNe and to uncover objects not previously classified.
Two groups of astronomers used large telescopes Keck and VLT for decades to observe trajectories of bright stars near the Galactic Centre. Based on results of their observations astronomers concluded that trajectories of the stars are roughly elliptical and foci of the orbits are approximately coincide with the Galactic Centre position. It gives an opportunity to claim that the Newtonian potential of point like mass around $4.3times 10^6 M_odot$ is a good initial approximation for the gravitational potential near the Galactic Centre. In the last years, the astronomers found that gravitational redshift of S2 star near pericenter passage in May 2018 is in accordance with general relativity predictions. In 2020 the GRAVITY team found that the observed relativistic precession of S2 star orbit is also consistent with theoretical estimates calculated for a weak gravitational field approximation in a Schwarzschild black hole. In last years a a self-gravitating dark matter core--halo distribution suggested by Ruffini, Arguelles and Rueda (MNRAS, 2015) (RAR model) was proposed and recently Becerra-Vergara et al. (MNRAS, 2021) claimed that this model provides a better fit of trajectories of bright stars in comparison with the conventional model with the supermassive black hole. We confirm that in the case of this dark matter distribution model for a dense core trajectories of test bodies are elliptical but in this case centers (not foci) of these ellipses should coincide with the Galactic Centre and orbital periods do not depend on semi-major axis and it contradicts observational data and therefore, we concluded supermassive black hole is a preferable model in comparison with the a dense core--diluted halo density profile for the Galactic Centre.
We present an extinction map of the inner $sim$SI{15}{arcminute} by {16}{arcminute} of the Galactic Center (GC) with map `pixels measuring SI{5}{arcsecond} $times$ SI{5}{arcsecond} using integrated light color measurements in the near- and mid-infrared. We use a variant of the Rayleigh-Jeans Color Excess (RJCE) method first described by Majewski et al. (2011) as the basis of our work, although we have approached our problem with a Bayesian mindset and dispensed with point-source photometry in favor of surface photometry, turning the challenge of the extremely crowded field at the GC into an advantage. Our results show that extinction at the GC is not inconsistent with a single power law coefficient, $beta=2.03pm0.06$, and compare our results with those using the Red Clump (RC) point-source photometry method of extinction estimation. We find that our measurement of $beta$ and its apparent lack of spatial variation are in agreement with prior studies, despite the bimodal distribution of values in our extinction map at the GC with peaks at um{5} and SI{7.5}{mag}. This bimodal nature of extinction is likely due to the InfraRed Dark Clouds that obscure portions of the inner GC field. We present our extinction law and map and de-reddened NIR CMDs and color-color diagram of the GC region using the point-source catalog of IR sources compiled by DeWitt et al. (2010). The de-reddening is limited by the error in the extinction measurement (typically SI{0.6}{mag}), which is affected by the size of our map pixels and is not fine-grained enough to separate out the multiple stellar populations present toward the GC.
The new multi-epoch near-infrared VVV survey (VISTA Variables in the Via Lactea) is sampling 562 sq. deg of the Galactic bulge and adjacent regions of the disk. Accurate astrometry established for the region surveyed allows the VVV data to be merged with overlapping surveys (e.g., GLIMPSE, WISE, 2MASS, etc.), thereby enabling the construction of longer baseline spectral energy distributions for astronomical targets. However, in order to maximize use of the VVV data, a set of transformation equations are required to place the VVV JHKs photometry onto the 2MASS system. The impetus for this work is to develop those transformations via a comparison of 2MASS targets in 152 VVV fields sampling the Galactic disk. The transformation coefficients derived exhibit a reliance on variables such as extinction. The transformed data were subsequently employed to establish a mean reddening law of E_{J-H}/E_{H-Ks}=2.13 +/- 0.04, which is the most precise determination to date and merely emphasizes the pertinence of the VVV data for determining such important parameters.
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