J/ApJS/278/62 Blue horizontal-branch stars from SkyMapper colors (Hu+, 2025)
Identification of blue horizontal-branch stars using synthetic SkyMapper colors
from Gaia XP spectra.
Hu G., Huang Y., Cui W., Wang T., Xiao K., Shi R., Ju J., Huang B., Li C.,
Liu Z., Wang X., Luo C.
<Astrophys. J. Suppl. Ser., 278, 62 (2025)>
=2025ApJS..278...62H 2025ApJS..278...62H
ADC_Keywords: Stars, horizontal branch; Stars, blue; Colors; Optical; Surveys;
Milky Way; Stars, distances; Proper motions; Photometry
Keywords: Milky Way stellar halo ; Horizontal branch stars
Abstract:
Blue horizontal-branch (BHB) stars are ideal tracers for mapping the
structure of the Galactic stellar halo. Traditionally, BHB sample
stars are built from large-scale spectroscopic surveys utilizing their
spectral features; however, the resulting sample sizes have been quite
limited. In this paper, we construct a catalog of BHB stars based on
synthetic colors (u-v)0 and (g-i)0 in SkyMapper photometric systems,
which are convolved from Gaia XP spectra. A total of 49,733 BHB stars
are selected from nearly the entire sky (excluding regions of low
Galactic latitudes |b|≤8° with heavy reddening), with a
completeness and purity exceeding 90%. Using member stars of globular
clusters with precise distance determinations, we carefully calibrate
the relationship between the g-band absolute magnitude and (g-i)0,
achieving a precision of 0.11mag, which corresponds to a 5%
uncertainty in distance. This relation is applied to derive distances
for all BHB stars in the constructed sample. Given the current
capabilities of Gaia XP observations, the constructed BHB sample is
primarily located within 20 kpc, enabling detailed mapping of the
inner stellar halo. To extend this depth to the outer halo or even the
edge of our Galaxy, we explore the potential of the Chinese Space
Station Telescope (CSST) and its broadband photometry for detecting
BHB stars. Using mock data from synthetic spectra, we find that it is
feasible to distinguish BHB stars from blue stragglers stars using
CSST near-ultraviolet band (u) photometry. Thanks to the deep limiting
magnitude of CSST, its data will provide a groundbreaking perspective
on our Galaxy, particularly regarding the outer halo, in an
unprecedented volume.
Description:
The synthetic magnitudes and their errors for approximately
200 million stars of the six bands in the SkyMapper Southern Sky
Survey (SMSS) system were initially compiled and were constructed
using an improved synthetic photometry method (Xiao+ 2023ApJS..269...58X 2023ApJS..269...58X)
based on the "corrected" Gaia XP spectra (Huang+ 2024ApJS..271...13H 2024ApJS..271...13H).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 134 49733 All newly identified BHB stars
table2.dat 58 22 List of all GCs with newly identified BHB stars
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See also:
VII/195 : Globular Clusters in the Milky Way (Harris, 1996)
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
I/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)
II/379 : SkyMapper Southern Sky Survey. DR4 (Onken+, 2024)
J/AJ/127/899 : Blue HB stars in SDSS (Sirko+, 2004)
J/AJ/135/564 : Photometry and spectroscopy of BHB candidates (Brown+, 2008)
J/ApJ/684/1143 : BHB candidates in the Milky Way (Xue+, 2008)
J/ApJ/738/79 : SDSS-DR8 BHB stars in the Milky Way's halo (Xue+, 2011)
J/PASP/124/140 : UBVRI Hp BT and VT photonic responses (Bessell+, 2012)
J/MNRAS/444/392 : Synthetic Stellar Photometry. I. (Casagrande+, 2014)
J/MNRAS/505/5978 : Gaia EDR3 view on Gal. globular clusters (Vasiliev+, 2021)
J/A+A/672/A81 : Blue straggler stars in open clusters (Li+, 2023)
J/ApJ/944/88 : Phot. metallicity & distances for Gaia RR Lyrae (Li+, 2023)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- Gaia Gaia DR3 source identifier
21- 26 F6.2 deg RAdeg Right Ascension (ICRS) at Epoch=2016.0
28- 33 F6.2 deg DEdeg Declination (ICRS) at Epoch=2016.0
35- 41 F7.2 mas/yr pmRA [-120.1/145]?=-999 Proper motion along RA
43- 49 F7.2 mas/yr e_pmRA [0/0.9]?=-999 Uncertainty in RA
51- 57 F7.2 mas/yr pmDE [-209.1/53.2]?=-999 Proper motion along DE
59- 65 F7.2 mas/yr e_pmDE [0.01/0.9]?=-999 Uncertainty in DE
67- 71 F5.2 mag umag [5.4/19] SkyMapper Southern Sky Survey u band
apparent magnitude (1)
73- 75 F3.2 mag e_umag [0.04/0.4] Uncertainty in umag
77- 81 F5.2 mag vmag [3.6/17.7] SkyMapper Southern Sky Survey v
band apparent magnitude (1)
83- 85 F3.2 mag e_vmag [0.01/0.4] Uncertainty in vmag
87- 91 F5.2 mag gmag [3.5/17.3] SkyMapper Southern Sky Survey g
band apparent magnitude (1)
93- 95 F3.2 mag e_gmag [0/0.2] Uncertainty in gmag
97- 101 F5.2 mag imag [3.9/17.8] SkyMapper Southern Sky Survey i
band apparent magnitude (1)
103- 105 F3.2 mag e_imag [0/0.2] Uncertainty in imag
107- 110 F4.2 mag E(B-V) [0/1.15] The (B-V) color excess (2)
112- 119 F8.2 pc Dist [39/20050] Distance
121- 125 F5.3 pc e_Dist [0.07/0.09] Uncertainty in Dist
127- 134 A8 --- Cluster Corresponding globular cluster identifier (3)
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Note (1): From Xiao et al. (2025, in preparation).
Note (2): Accepted from Schlegel+ 1998ApJ...500..525S 1998ApJ...500..525S
Note (3): NGC 6904 was a misprint for NGC 5904; corrected at CDS.
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- Cluster Name of the globular cluster (GC)
10- 11 I2 --- Nbhb [2/40] Number of BHB stars in the GC
13- 19 F7.3 deg RAdeg [13.18/325.1] Right ascension (J2000)
21- 27 F7.3 deg DEdeg [-72.7/43.2] Declination (J2000)
29- 34 F6.2 mas/yr pmRA [-12.51/8.35] Proper motion in RA (1)
36- 41 F6.2 mas/yr pmDE [-19.02/3.3] Proper motion in declination (1)
43- 46 F4.2 arcmin rh [0.5/5] Half-light radius (2)
48- 52 F5.2 [-] [Fe/H] [-2.4/-0.99] Metallicity (2)
54- 58 I5 pc Dist [1850/13080] Distance (2)
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Note (1): Vasiliev & Baumgardt 2021, J/MNRAS/505/5978
Note (2): Harris 1996AJ....112.1487H 1996AJ....112.1487H
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 20-Feb-2026