J/ApJ/816/L4 Candidate BHB stars in Ophiuchus stream (Sesar+, 2016)
Evidence of fanning in the Ophiuchus stream.
Sesar B., Price-Whelan A.M., Cohen J.G., Rix H.-W., Pearson S.,
Johnston K.V., Bernard E.J., Ferguson A.M.N., Martin N.F., Slater C.T.,
Chambers K.C., Flewelling H., Wainscoat R.J., Waters C.
<Astrophys. J., 816, L4 (2016)>
=2016ApJ...816L...4S 2016ApJ...816L...4S (SIMBAD/NED BibCode)
ADC_Keywords: Stars, horizontal branch ; Radial velocities ; Photometry, ugriz
Keywords: Galaxy: halo; Galaxy: kinematics and dynamics; Galaxy: structure;
globular clusters: general
Abstract:
The Ophiuchus stellar stream presents a dynamical puzzle: its old
stellar populations (∼12Gyr) cannot be reconciled with (1) its orbit
in a simple model for the Milky Way potential and (2) its short
angular extent, both of which imply that the observed stream formed
within the last <1Gyr. Recent theoretical work has shown that streams
on chaotic orbits may abruptly fan out near their apparent ends; stars
in these fans are dispersed in both position and velocity and may be
difficult to associate with the stream. Here we present the first
evidence of such stream-fanning in the Ophiuchus stream, traced by
four blue horizontal branch stars beyond the apparent end of the
stream. These stars stand out from the background by their high
velocities (vlos>230km/s) against ∼40 other stars: their velocities
are comparable to those of the stream, but would be exceptional if
they were unrelated halo stars. Their positions and velocities are,
however, inconsistent with simple extrapolation of the observed cold,
high-density portion of the stream. These observations suggest that
stream-fanning may be a real, observable effect and, therefore, that
Ophiuchus may be on a chaotic orbit. They also show that the Ophiuchus
stream is more extended and hence dynamically older than previously
thought, easing the stellar population versus dynamical age tension.
Description:
In Figure 1, we show the spatial distribution of candidate blue
horizontal branch (BHB) stars selected for spectroscopic follow-up,
and in Table 1 we list their positions.
A total of 16 stars were observed using the DEIMOS spectrograph on the
WMKO Keck II 10-m telescope (project ID 2015A-C252D, PI: J. Cohen).
The remaining 27 stars were observed using the TWIN spectrograph on
the Calar Alto 3.5-m telescope (project ID H15-3.5-011, PI: B. Sesar).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 81 43 Positions and velocities of observed candidate
BHB stars
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See also:
J/ApJ/813/83 : Multiple star formation in Ophiuchus (Cheetham+, 2015)
J/ApJ/809/59 : Ophiuchus stellar stream with PS1 data (Sesar+, 2015)
J/ApJ/775/63 : Gould's Belt VLA Survey. I. Ophiuchus complex (Dzib+, 2013)
J/ApJ/750/99 : The Pan-STARRS1 photometric system (Tonry+, 2012)
J/ApJ/731/119 : BHB candidates in Sagittarius stream (Ruhland+, 2011)
J/ApJ/703/1964 : Spectra of three nearby star-forming regions (Furlan+, 2009)
J/ApJ/684/1143 : BHB candidates in the Milky Way (Xue+, 2008)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 6 A6 --- ID Source identifier ("candNN" among 8 to 59)
8- 16 F9.5 deg RAdeg Right Ascension in decimal degrees (J2000)
18- 25 F8.5 deg DEdeg Declination in decimal degrees (J2000)
27- 32 F6.1 km/s Vlos [-268/319] Heliocentric velocity νlos
34- 37 F4.1 km/s e_Vlos [1/12] Uncertainty in Vlos
39- 43 F5.2 mag gmag [15.2/16.7] PS1 g band magnitude;
not corrected for extinction
45- 49 F5.2 mag imag [14.9/16.7] PS1 i band magnitude
not corrected for extinction
51- 53 F3.1 --- cHg [0.1/4]? Best-fit c value of Sersic profile
to the Hγ line
55- 58 F4.1 --- e_cHg [0.1/10.5]? The 1σ uncertainty in cHg
60- 63 F4.1 --- bHg [0.2/11]? Best-fit b value of Sersic profile
to the Hγ line
65- 68 F4.2 --- e_bHg [0.2/0.8]? The 1σ uncertainty in bHg
70- 76 A7 --- Inst Instrument used in the observation (1)
78 A1 --- BHB [ NY] BHB star flag (2)
81 A1 --- Mm [NY] Member code
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Note (1): Instrument as follows:
DEI = Keck-II 10m telescope with DEIMOS spectrograph;
TWN = Calar Alto 3.5m telescope with TWIN spectrograph.
Note (2): A flag indicating whether the best-fit Sersic profile of the Hγ
line is consistent with those of BHB stars (within measurement
uncertainties). DEIMOS spectra do not include the Hγ line, and
thus stars observed exclusively by DEIMOS lack cγ and
bγ measurements, as well as the BHB classification.
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History:
From electronic version of the journal
Acknowledgements:
Branimir Sesar [Max Planck Institute for Astronomy] for the corrected value
of Cand50's imag value (was "0.9" in the published MRT table).
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 24-Mar-2016