nuclear stellar disc
using the Milky Way's nuclear stellar disc as an indicator of the bar formation epoch
A significant structure at the centre of the Galaxy is the nuclear stellar disc. It is approximately 1 billions solar masses condensed into a region only a few 100 pc across. It is key to the Milky Way as its formation is potentially linked to the larger scale Milky Way bar.
We can trace this region using Mira variable stars so I have spent some time understanding more about this interesting tracer as well as building dynamical models of the NSD region.
References
2023
- MNRASThe period-luminosity relation for Mira variables in the Milky Way using Gaia DR3: a further distance anchor for H\(_0\)Jason L. SandersMNRAS, Aug 2023
Gaia DR3 parallaxes are used to calibrate preliminary period-luminosity relations of O-rich Mira variables in the 2MASS J, H, and K\(_s\) bands using a probabilistic model accounting for variations in the parallax zero-point and underestimation of the parallax uncertainties. The derived relations are compared to those measured for the Large and Small Magellanic Clouds, the Sagittarius dwarf spheroidal galaxy, globular cluster members, and the subset of Milky Way Mira variables with VLBI parallaxes. The Milky Way linear JHK\(_s\) relations are slightly steeper and thus fainter at short period than the corresponding LMC relations, suggesting population effects in the near-infrared are perhaps larger than previous observational works have claimed. Models of the Gaia astrometry for the Mira variables suggest that, despite the intrinsic photocentre wobble and use of mean photometry in the astrometric solution of the current data reduction, the recovered parallaxes should be on average unbiased but with underestimated uncertainties for the nearest stars. The recommended Gaia EDR3 parallax zero-point corrections evaluated at \(ν_\mathrmeff=1.25 μ\mathrmm^-1\) require minimal (\(≲5 μ\mathrmas\) ) corrections for redder five-parameter sources, but overcorrect the parallaxes for redder six-parameter sources, and the parallax uncertainties are underestimated at most by a factor \(∼\) 1.6 at \(G≈12.5 \mathrmmag\) . The derived period-luminosity relations are used as anchors for the Mira variables in the Type Ia host galaxy NGC 1559 to find \(H_0=(73.7\pm 4.4) \mathrmkm\,s^-1 \mathrmMpc^-1\) .
@article{2023MNRAS.523.2369S, author = {{Sanders}, Jason L.}, title = {{The period-luminosity relation for Mira variables in the Milky Way using Gaia DR3: a further distance anchor for H\(_{0}\)\ }}, journal = {\mnras}, keywords = {stars: AGB, stars: distances, stars: variables: general, cosmological parameters, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - Solar and Stellar Astrophysics}, year = {2023}, month = aug, volume = {523}, number = {2}, pages = {2369-2398}, doi = {10.1093/mnras/stad1431}, archiveprefix = {arXiv}, eprint = {2304.01671}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.523.2369S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} } - MNRASHunting for C-rich long-period variable stars in the Milky Way’s bar-bulge using unsupervised classification of Gaia BP/RP spectraJason L. Sanders, and Noriyuki MatsunagaMNRAS, May 2023
The separation of oxygen- and carbon-rich asymptotic giant branch sources is crucial for their accurate use as local and cosmological distance and age/metallicity indicators. We investigate the use of unsupervised learning algorithms for classifying the chemistry of long-period variables from Gaia DR3’s BP/RP spectra. Even in the presence of significant interstellar dust, the spectra separate into two groups attributable to O-rich and C-rich sources. Given these classifications, we utilize a supervised approach to separate O-rich and C-rich sources without blue and red photometers (BP/RP) spectra but instead given broadband optical and infrared photometry finding a purity of our C-rich classifications of around 95 per cent. We test and validate the classifications against other advocated colour-colour separations based on photometry. Furthermore, we demonstrate the potential of BP/RP spectra for finding S-type stars or those possibly symbiotic sources with strong emission lines. Although our classification suggests the Galactic bar-bulge is host to very few C-rich long- period variable stars, we do find a small fraction of C-rich stars with periods \( 250 \mathrmday\) that are spatially and kinematically consistent with bar-bulge membership. We argue the combination of the observed number, the spatial alignment, the kinematics, and the period distribution disfavour young metal- poor star formation scenarios either in situ or in an accreted host, and instead, these stars are highly likely to be the result of binary evolution and the evolved versions of blue straggler stars already observed in the bar-bulge.
@article{2023MNRAS.521.2745S, author = {{Sanders}, Jason L. and {Matsunaga}, Noriyuki}, title = {{Hunting for C-rich long-period variable stars in the Milky Way's bar-bulge using unsupervised classification of Gaia BP/RP spectra}}, journal = {\mnras}, keywords = {stars: AGB, stars: variables: general, Galaxy: bulge, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2023}, month = may, volume = {521}, number = {2}, pages = {2745-2764}, doi = {10.1093/mnras/stad574}, archiveprefix = {arXiv}, eprint = {2302.10022}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.521.2745S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} } - MNRASA kinematic calibration of the O-rich Mira variable period-age relation from GaiaHanyuan Zhang, and Jason L. SandersMNRAS, May 2023
Empirical and theoretical studies have demonstrated that the periods of Mira variable stars are related to their ages. This, together with their brightness in the infrared, makes them powerful probes of the formation and evolution of highly-extincted or distant parts of the Local Group. Here we utilize the Gaia DR3 catalogue of long-period variable candidates to calibrate the period-age relation of the Mira variables. Dynamical models are fitted to the O-rich Mira variable population across the extended solar neighbourhood and then the resulting solar neighbourhood period-kinematic relations are compared to external calibrations of the age-kinematic relations to derive a Mira variable period-age relation of \(τ≈(6.9\pm 0.3) \mathrmGyr(1+\tanh ((330 \mathrmd-P)/(400\pm 90)\mathrmd)\) . Our results compare well with previous calibrations using smaller data sets as well as the period-age properties of Local Group cluster members. This calibration opens the possibility of accurately characterizing the star formation and the impact of different evolutionary processes throughout the Local Group.
@article{2023MNRAS.521.1462Z, author = {{Zhang}, Hanyuan and {Sanders}, Jason L.}, title = {{A kinematic calibration of the O-rich Mira variable period-age relation from Gaia}}, journal = {\mnras}, keywords = {Galaxy: disc, Galaxy: evolution, Galaxy: kinematics and dynamics, stars: variables: general, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2023}, month = may, volume = {521}, number = {1}, pages = {1462-1478}, doi = {10.1093/mnras/stad575}, archiveprefix = {arXiv}, eprint = {2302.10024}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.521.1462Z}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
2022
- MNRASThe extinction law in the inner 3 \texttimes 3 deg\(^2\) of the Milky Way and the red clump absolute magnitude in the inner bar-bulgeJason L. Sanders, Leigh Smith, Carlos González-Fernández, and 2 more authorsMNRAS, Aug 2022
The extinction law from 0.9 to 8 microns in the inner \(3\times 3\deg ^2\) of the Milky Way is measured using data from VISTA Variables in the Via Lactea, GLIMPSE, and WISE. Absolute extinction ratios are found by requiring that the observed red clump density peaks at the GRAVITY collaboration distance to the Galactic centre. When combined with selective extinction ratios measured from the bulge giant colour-colour diagrams, we find an extinction law of \(A_Z:A_Y:A_J:A_H:A_K_s:A_W1:A_[3.6]:A_[4.5]:A_W2:A_[5 .8]:A_[8.0] =7.19(0.30):5.11(0.20):3.23(0.11):1.77(0.04):1:0. 54(0.02):0.46(0.03):0.34(0.03):0.32(0.03):0.24(0.04):0.28(0.03)\) valid for low extinctions where non-linearities are unimportant. These results imply an extinction law from the Rayleigh Jeans colour excess method of \(A_K_s=0.677(H-[4.5]-0.188)\) . We find little evidence for significant selective extinction ratio variation over the inspected region (around \(5 \mathrmper\,cent\) ). Assuming the absolute extinction ratios do not vary across the inspected region gives an independent measurement of the absolute K\(_s\) magnitude of the red clump at the Galactic Centre of \((-1.61\pm 0.07) \mathrmmag\) . This is very similar to the value measured for solar neighbourhood red clump stars giving confidence in the use of red clump stars as standard candles across the Galaxy. As part of our analysis, we inspect the completeness of PSF photometry from the VVV survey using artificial star tests, finding \(90 \mathrmper\,cent\) completeness at \(K_s≈16 (17)\) in high (low) density regions and good agreement with the number counts with respect to the GALACTICNUCLEUS and DECAPS catalogues over small regions of the survey.
@article{2022MNRAS.514.2407S, author = {{Sanders}, Jason L. and {Smith}, Leigh and {Gonz{\'a}lez-Fern{\'a}ndez}, Carlos and {Lucas}, Philip and {Minniti}, Dante}, title = {{The extinction law in the inner 3 {\texttimes} 3 deg\(^{2}\)\ of the Milky Way and the red clump absolute magnitude in the inner bar-bulge}}, journal = {\mnras}, keywords = {stars: distances, dust, extinction, Galaxy: bulge, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = aug, volume = {514}, number = {2}, pages = {2407-2424}, doi = {10.1093/mnras/stac1367}, archiveprefix = {arXiv}, eprint = {2205.10378}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.514.2407S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} } - MNRASMira variables in the Milky Way’s nuclear stellar disc: discovery and classificationJason L. Sanders, Noriyuki Matsunaga, Daisuke Kawata, and 3 more authorsMNRAS, Nov 2022
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 \(3\times 3 \mathrmdeg^2\) 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 \(∼2.5\times 10^-5M_⊙ \mathrmyr^-1\) . The period distribution appears consistent with the nuclear stellar disc forming \(rsim 8 \mathrmGyr\) ago, although it is not possible to disentangle the relative contributions of the nuclear stellar disc and the contaminating bulge.
@article{2022MNRAS.517..257S, author = {{Sanders}, Jason L. and {Matsunaga}, Noriyuki and {Kawata}, Daisuke and {Smith}, Leigh C. and {Minniti}, Dante and {Lucas}, Philip W.}, title = {{Mira variables in the Milky Way's nuclear stellar disc: discovery and classification}}, journal = {\mnras}, keywords = {stars: AGB and post-AGB, stars: variables: general, Galaxy: bulge, Galaxy: centre, Galaxy: stellar content, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2022}, month = nov, volume = {517}, number = {1}, pages = {257-280}, doi = {10.1093/mnras/stac2274}, archiveprefix = {arXiv}, eprint = {2208.04966}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.517..257S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} } - MNRASSelf-consistent modelling of the Milky Way’s nuclear stellar discMattia C. Sormani, Jason L. Sanders, Tobias K. Fritz, and 14 more authorsMNRAS, May 2022
The nuclear stellar disc (NSD) is a flattened high-density stellar structure that dominates the gravitational field of the Milky Way at Galactocentric radius \(30 \rm pc≲R≲300 \rm pc\) . We construct axisymmetric self-consistent equilibrium dynamical models of the NSD in which the distribution function is an analytic function of the action variables. We fit the models to the normalized kinematic distributions (line-of-sight velocities + VIRAC2 proper motions) of stars in the NSD survey of Fritz et al., taking the foreground contamination due to the Galactic Bar explicitly into account using an N-body model. The posterior marginalized probability distributions give a total mass of \(M_\rm NSD = 10.5^+1.1_-1.0 \times 10^8 \rm M_⊙\) , roughly exponential radial and vertical scale lengths of \(R_\rm disc = 88.6^+9.2_-6.9 \rm pc\) and \(H_\rm disc=28.4^+5.5_-5.5 \rm pc\) , respectively, and a velocity dispersion \(σ≃70 \rm km\,s^-1\) that decreases with radius. We find that the assumption that the NSD is axisymmetric provides a good representation of the data. We quantify contamination from the Galactic Bar in the sample, which is substantial in most observed fields. Our models provide the full 6D (position + velocity) distribution function of the NSD, which can be used to generate predictions for future surveys. We make the models publicly available as part of the software package AGAMA.
@article{2022MNRAS.512.1857S, author = {{Sormani}, Mattia C. and {Sanders}, Jason L. and {Fritz}, Tobias K. and {Smith}, Leigh C. and {Gerhard}, Ortwin and {Sch{\"o}del}, Rainer and {Magorrian}, John and {Neumayer}, Nadine and {Nogueras-Lara}, Francisco and {Feldmeier-Krause}, Anja and {Mastrobuono-Battisti}, Alessandra and {Schultheis}, Mathias and {Shahzamanian}, Banafsheh and {Vasiliev}, Eugene and {Klessen}, Ralf S. and {Lucas}, Philip and {Minniti}, Dante}, title = {{Self-consistent modelling of the Milky Way's nuclear stellar disc}}, journal = {\mnras}, keywords = {Galaxy: centre, Galaxy: kinematics and dynamics, Galaxy: structure, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = may, volume = {512}, number = {2}, pages = {1857-1884}, doi = {10.1093/mnras/stac639}, archiveprefix = {arXiv}, eprint = {2111.12713}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.512.1857S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} } - MNRASVariable star classification across the Galactic bulge and disc with the VISTA Variables in the Vı́a Láctea surveyThomas A. Molnar, Jason L. Sanders, Leigh C. Smith, and 3 more authorsMNRAS, Jan 2022
We present VIVACE, the VIrac VAriable Classification Ensemble, a catalogue of variable stars extracted from an automated classification pipeline for the Vista Variables in the Vı́a Láctea (VVV) infrared survey of the Galactic bar/bulge and southern disc. Our procedure utilizes a two-stage hierarchical classifier to first isolate likely variable sources using simple variability summary statistics and training sets of non-variable sources from the Gaia early third data release, and then classify candidate variables using more detailed light-curve statistics and training labels primarily from OGLE and VSX. The methodology is applied to point-spread-function photometry for \(∼\) 490 million light curves from the VIRAC v2 astrometric and photometric catalogue resulting in a catalogue of \(∼\) 1.4 million likely variable stars, of which \(∼\) 39 000 are high-confidence (classification probability >0.9) RR Lyrae ab stars, \(∼\) 8000 RR Lyrae c/d stars, \(∼\) 187, 000 detached/semi-detached eclipsing binaries, \(∼\) 18, 000 contact eclipsing binaries, \(∼\) 1400 classical Cepheid variables and \(∼\) 2200 Type II Cepheid variables. Comparison with OGLE-4 suggests a completeness of around \(90 \mathrmper\,cent\) for RRab and \(≲60 \mathrmper\,cent\) for RRc/d, and a misclassification rate for known RR Lyrae stars of around \(1 \mathrmper\,cent\) for the high confidence sample. We close with two science demonstrations of our new VIVACE catalogue: first, a brief investigation of the spatial and kinematic properties of the RR Lyrae stars within the disc/bulge, demonstrating the spatial elongation of bar- bulge RR Lyrae stars is in the same sense as the more metal-rich red giant population whilst having a slower rotation rate of \(∼\!40 \mathrmkm\,s^-1\mathrmkpc^-1\) ; and secondly, an investigation of the GaiaEDR3 parallax zero-point using contact eclipsing binaries across the Galactic disc plane and bulge.
@article{2022MNRAS.509.2566M, author = {{Molnar}, Thomas A. and {Sanders}, Jason L. and {Smith}, Leigh C. and {Belokurov}, Vasily and {Lucas}, Philip and {Minniti}, Dante}, title = {{Variable star classification across the Galactic bulge and disc with the VISTA Variables in the V{\'\i}a L{\'a}ctea survey}}, journal = {\mnras}, keywords = {catalogues, surveys, binaries: eclipsing, stars: variables: general, stars: variables: RR Lyrae, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = jan, volume = {509}, number = {2}, pages = {2566-2592}, doi = {10.1093/mnras/stab3116}, archiveprefix = {arXiv}, eprint = {2110.15371}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.509.2566M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }