J/ApJ/964/164 Binary fraction of young stars in ∼0.4pc of the GC (Gautam+, 2024)
An estimate of the binary star fraction among young stars at the Galactic
Center: possible evidence of a radial dependence.
Gautam A.K., Do T., Ghez A.M., Chu D.S., Hosek M.W.J., Sakai S., Naoz S.,
Morris M.R., Ciurlo A., Haggard Z., Lu J.R.
<Astrophys. J., 964, 164 (2024)>
=2024ApJ...964..164G 2024ApJ...964..164G
ADC_Keywords: Galactic center; Stars, double and multiple;
Binaries, eclipsing; Photometry, infrared;
Stars, ages; Spectral types; Radial velocities
Keywords: Galactic center ; Binary stars ; Eclipsing binary stars ;
Star formation ; Dynamical evolution
Abstract:
We present the first estimate of the intrinsic binary fraction of
young stars across the central ∼0.4pc surrounding the supermassive
black hole (SMBH) at the Milky Way Galactic center (GC). This
experiment searched for photometric variability in
102 spectroscopically confirmed young stars, using 119 nights of 10"
wide adaptive optics imaging observations taken at W. M. Keck
Observatory over 16yr in the K'-[2.1um] and H-[1.6um] bands. We
photometrically detected three binary stars, all of which are situated
more than 1" (0.04pc) from the SMBH and one of which, S2-36, is newly
reported here with spectroscopic confirmation. All are contact
binaries or have photometric variability originating from stellar
irradiation. To convert the observed binary fraction into an estimate
of the underlying binary fraction, we determined the experimental
sensitivity through detailed light-curve simulations, incorporating
photometric effects of eclipses, irradiation, and tidal distortion in
binaries. The simulations assumed a population of young binaries, with
stellar ages (4Myr) and masses matched to the most probable values
measured for the GC young star population, and underlying binary
system parameters (periods, mass ratios, and eccentricities) similar
to those of local massive stars. As might be expected, our
experimental sensitivity decreases for eclipses narrower in phase. The
detections and simulations imply that the young, massive stars in the
GC have a stellar binary fraction ≥71% (68% confidence), or ≥42%
(95% confidence). This inferred GC young star binary fraction is
consistent with that typically seen in young stellar populations in
the solar neighborhood. Furthermore, our measured binary fraction is
significantly higher than that recently reported by Chu et al. based
on radial velocity measurements for stars ≳1" of the SMBH.
Constrained with these two studies, the probability that the same
underlying young star binary fraction extends across the entire region
is <1.4%. This tension provides support for a radial dependence of the
binary star fraction, and therefore, for the dynamical predictions of
binary merger and evaporation events close to the SMBH.
Description:
Our binary fraction experiment employed Galactic Center Orbits
Initiative (GCOI; PI: A. M. Ghez) AO imaging observations of the
Galactic center. The experiment's images were obtained at the 10m
Keck II telescope with the NIRC2 NIR facility imager (PI: K. Matthews)
and were used to measure the stellar fluxes used in this work. The
photometric analysis used data taken between 2006 and 2022 in the K'
(λ0=2.124um, Δλ=0.351um) and H bandpasses
(λ0=1.633um, Δλ=0.296um). This work built off of
the 2006-2017 K'-band data set presented by G19
(Gautam+ 2019, J/ApJ/871/103), with the following additions:
1. Newer K'-band observations were performed since the analysis
presented in G19. These observations were conducted from 2018-2022.
2. Additional observations were taken in the H bandpass, conducted in
2009 and from 2017-2022.
3. Several nights of K'-band observations were conducted in 2006,
2009, and 2014-2017 that were not previously included in G19. These
nightly combined images consisted of fewer individual frames, and
therefore, are not as sensitive to fainter stars and typically have
larger photometric flux uncertainties. Despite these challenges,
including these nights in our experiment's data set allowed greater
sensitivity to periodic variability for the bright stars detected in
the shallow data.
We performed two laser-guide-star AO NIR spectroscopic observations of
S2-36 at the 10m W. M. Keck I telescope using the OSIRIS integral
field spectrograph (IFS): on 2020-07-29 and on 2021-08-18. Both
observations were performed in the Kbb bandpass (wavelength range:
1.965-2.381um). The 2020 observation had a pixel scale of 35mas/pix,
while the 2021 observation was performed with a higher spatial
resolution of 20mas/pix and used Keck I's NIR tip-tilt sensor, TRICK
allowing better AO correction and separation of S2-36's flux from the
brighter nearby star IRS 16CC.
The RV measurements that we derived from the two observations of S2-36
are listed in Table 5, while Table 6 lists RV measurements of the
nearby star IRS 16CC. See Appendix C.
Objects:
----------------------------------------------------------------
RA (2000) DE Designation(s)
----------------------------------------------------------------
17 45 39.99 -29 00 28.0 YNC = NAME Gal Center IRS 16
17 45 40.0359 -29 00 28.169 SgrA* = NAME Sgr A *
17 45 40.19 -29 00 27.5 IRS 16CC = GCIRS 16CC
17 45 40.19 -29 00 27.6 S2-36 = [GKM98] S2-36
----------------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 43 119 Observations used in this experiment
table2.dat 50 102 Known young star sample
table4.dat 85 12 Photometric calibrator star bandpass corrected
reference fluxes
table5.dat 62 2 OSIRIS spectroscopic observations and RV
measurements of S2-36
table6.dat 36 2 RV measurements of IRS 16CC
table7.dat 89 1129 Full stellar sample and injected binary recovery
fraction (Appendix F)
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See also:
J/ApJ/523/248 : Variable stars in the Galactic center (Ott+, 1999)
J/ApJ/597/323 : Cool stars at the Galactic Center (Blum+, 2003)
J/ApJS/161/154 : Near-IR spectral atlas of OB stars (Hanson+, 2005)
J/ApJ/643/1011 : Early-type stars in center of the Galaxy (Paumard+, 2006)
J/ApJ/659/1241 : Stellar variability in Galactic Center (Rafelski+, 2007)
J/ApJ/697/1741 : Warped disks of YSOs in Galactic center (Bartko+, 2009)
J/A+A/499/483 : NIR observations of sources in the GC (Buchholz+, 2009)
J/ApJ/703/1323 : Spectroscopy of stars:Galaxy's nuclear cluster (Do+ 2009)
J/A+A/502/91 : Proper motions of stars near SgrA* (Schoedel+, 2009)
J/A+A/511/A18 : NIR observations of stars near SgrA* (Schoedel+, 2010)
J/A+A/537/A146 : Star models with rot. 0.8<M<120,Z=0.014 (Ekstrom+, 2012)
J/other/Sci/337.444 : RV curves of Galactic massive O stars (Sana+, 2012)
J/ApJ/764/154 : Stellar populations in the central 0.5pc. I. (Do+, 2013)
J/ApJS/208/9 : Intrinsic colors & temp of PMS stars (Pecaut+, 2013)
J/A+A/570/A2 : Spectra and RVs of nuclear stars (Feldmeier+, 2014)
J/ApJ/783/131 : Kinematic of stars in Galactic center (Yelda+, 2014)
J/A+A/584/A2 : KMOS view of Galactic centre.I. (Feldmeier-Krause+, 2015)
J/ApJ/810/61 : Early-type EBs: intermediate orbit periods (Moe+, 2015)
J/ApJ/808/106 : NIR spectroscopic obs. in the GC (Stostad+, 2015)
J/ApJ/830/17 : GC secondary IR astrometric standards (Boehle+, 2016)
J/ApJ/829/34 : Kepler heartbeat star radial velocities (Shporer+, 2016)
J/ApJ/837/30 : 25yrs monitoring star orbits in the GC (Gillessen+, 2017)
J/A+A/606/A92 : Gaia LMC EBs & multiple systems (Mowlavi+, 2017)
J/A+A/610/A83 : GALACTICNUCLEUS: JHKs survey (Nogueras-Lara+, 2018)
J/ApJ/871/103 : Catalog of 563 stars in the GC (Gautam+, 2019)
J/ApJ/873/65 : Keck/NIRC2 obs. of the Galactic Center (Sakai+, 2019)
J/AJ/168/166 : GC: high-PM stars magnitude analysis (Haggard+, 2024)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "Y/M/D" Date UT Date
12 A1 --- f_Date [n] Flag on UT date (1)
14- 22 F9.3 d MJD Modified Julian Date of observation
24- 25 A2 --- Filt Bandpass (H or K')
27- 30 I4 --- Ns [186/1038] Number of sample stars detected
32- 34 I3 --- Nbright [185/490] Number of stars detected with
magK'≤16
36- 40 F5.3 mag MedErr [0.02/0.12] Median photometric uncertainty
of bright stars
42- 43 I2 --- Nyoung [55/99] Number of known young stars detected
--------------------------------------------------------------------------------
Note (1): Flag as follows:
n = Denotes a photometric observation not previously reported in
Gautam+ 2019, J/ApJ/871/103
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Name of star
12- 16 F5.2 mag Kpmag [10/17.2] Mean Vega magnitude of star in K' band
18- 22 F5.2 mag Hmag [12/19.2]? Mean Vega magnitude of star in
H band (1)
24- 26 I3 --- n_Kpmag [33/100] Number of K' band observations of star
28- 29 I2 --- n_Hmag [0/19] Number of H band observations of star (1)
31- 35 F5.2 arcsec oRA [-4.4/5.6] Offset East from Sgr A* position
(17:45:40.0359 -29:00:28.169)
37- 41 F5.2 arcsec oDE [-6.4/3.7] Offset North from Sgr A* position
(17:45:40.0359 -29:00:28.169)
43- 50 F8.3 yr t0 [2006.3/2012.1] Reference time for oRA, oDE in
Julian years
--------------------------------------------------------------------------------
Note (1): Stars not detected or identified in H-band have n_Hmag=0 and
null (blank) values for Hmag.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Name Name of star
10- 19 A10 --- SpType Spectral type
21- 25 F5.2 mag KsS10 [10.1/15.0] Ks band Vega magnitude from S10 (1)
27- 30 F4.2 mag e_KsS10 [0.06/0.09] Uncertainty in KsS10
32- 36 F5.2 mag dKmag [-0.07/-0.04] Magnitude difference,
KsS10-K'NIRC2
38- 41 F4.2 mag e_dKmag [0.01]? Uncertainty in dKmag
43- 47 F5.2 mag KpNIRC2 [10.1/15.1] K' band NIRC2 Vega magnitude,
this article
49- 52 F4.2 mag e_KpNIRC2 [0.06/0.09] Uncertainty in K'NIRC2
54- 58 F5.2 mag HS10 [12.0/16.9] H band Vega magnitude from S10 (1)
60- 63 F4.2 mag e_HS10 [0.06/0.09] Uncertainty in HS10
65- 69 F5.2 mag dHmag [-0.20/-0.13] Magnitude difference,
HS10-HNIRC2
71- 74 F4.2 mag e_dHmag [0.01]? Uncertainty in dHmag
76- 80 F5.2 mag HNIRC2 [12.1/16.7] H band NIRC2 Vega magnitude,
this article
82- 85 F4.2 mag e_HNIRC2 [0.07/0.10] Uncertainty in HNIRC2
--------------------------------------------------------------------------------
Note (1): S10 = Schodel+ 2010, J/A+A/511/A18
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table5.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "Y/M/D" Date UT Date
12 A1 --- f_Date [T] Flag on UT Date (1)
14- 22 F9.3 d MJD Modified Julian Date of observation
24- 28 F5.3 um lambdaMin [1.965] Wavelength range, minimum
30- 34 F5.3 um lambdaMax [2.381] Wavelength range, maximum
36- 38 A3 --- Filt Filter name
40 I1 --- Frames [4/6] Frames
42- 44 I3 s ExpTime [900] Integration time
46- 47 I2 mas/pix Scale [20/35] Scale, milliarcseconds/pixel
49- 51 I3 --- SNR [73/118] Signal-to-noise
53- 57 F5.1 km/s RVel [152/219] Radial velocity
59- 62 F4.1 km/s e_RVel [8.1/13.4] Uncertainty in radial velocity
--------------------------------------------------------------------------------
Note (1): T = denotes observation conducted with TRICK, the NIR
tip-tilt sensor on Keck I, allowing for improved AO correction.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "Y/M/D" Date UT Date
12 A1 --- f_Date [T] T= observation conducted with TRICK
14- 22 F9.3 d MJD Modified Julian Date of observation
24- 26 I3 --- SNR [165/182] Signal-to-noise
28- 32 F5.1 km/s RVel [175/180] Radial velocity
34- 36 F3.1 km/s e_RVel [2.8/3.7] Uncertainty in radial velocity
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table7.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Name of star
12- 16 F5.2 mag Kpmag [10.0/19.4] Mean Vega magnitude of star
in K' band
18- 22 F5.2 mag Hmag [12.0/21.5]? Mean Vega magnitude of star
in H band (1)
24- 34 A11 --- Age Spectroscopically confirmed age of
population (2)
36- 38 I3 --- n_Kpmag [30/100] Number of K' band observations of
star
40- 41 I2 --- n_Hmag [0/19] Number of H band observations of
star (1)
43- 48 F6.2 --- KprChi2 [0.1/274] Reduced χ2 from no
variability model in K' band
50- 55 F6.2 --- HrChi2 [0.03/399]? Reduced χ2 from no
variability model in H band (1)
57- 59 A3 --- KpVar Variable in K' band? Yes/No
61- 63 A3 --- HVar Variable in H band? Yes/No (1)
65- 68 F4.2 --- Frac [0.0/0.2] Fraction of injected binaries
recovered from stellar light curve
70- 74 F5.2 arcsec oRA [-5.4/5.8] Offset East from Sgr A*
position (17:45:40.0359 -29:00:28.169)
76- 80 F5.2 arcsec oDE [-6.5/4.2] Offset North from Sgr A*
position (17:45:40.0359 -29:00:28.169)
82- 89 F8.3 yr t0 Reference time for oRA, oDE in Julian
years
--------------------------------------------------------------------------------
Note (1): Stars not detected or identified in H-band have n_Hmag=0 and
null (blank) values for Hmag and HVar.
Note (2): The three categories of population age are as follows:
Known Old = 187 spectroscopically confirmed "old" stars in sample;
Known Young = 101 spectroscopically known young objects from the
literature plus the star S2-36 spectroscopically confirmed
as young in this work;
Unknown Age = 840 stars of unknown age.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 27-May-2025