J/ApJ/887/19 DECam phot. of Gaia stars in Price-Whelan 1 (Price-Whelan+, 2019)
Discovery of a disrupting open cluster far into the Milky Way halo: a recent
star formation event in the leading arm of the Magellanic Stream?
Price-Whelan A.M., Nidever D.L., Choi Y., Schlafly E.F., Morton T.,
Koposov S.E., Belokurov V.
<Astrophys. J., 887, 19 (2019)>
=2019ApJ...887...19P 2019ApJ...887...19P
ADC_Keywords: Associations, stellar; Photometry, ugriz; Magellanic Clouds;
Milky Way
Keywords: Galaxy: halo ; open clusters and associations: general ;
stars: formation ; surveys
Abstract:
We report the discovery of a young (τ∼117Myr), low-mass
(M∼1200M☉), metal-poor ([Fe/H]~-1.14) stellar association at a
heliocentric distance D∼28.7kpc, placing it far into the Milky Way
(MW) halo. At its present Galactocentric position (R,z)~(23,15)kpc,
the association is (on the sky) near the leading arm of the gas stream
emanating from the Magellanic Cloud system, but is located ∼60°
from the Large Magellanic Cloud center on the other side of the MW
disk. If the cluster is colocated with HI gas in the stream, we
directly measure the distance to the leading arm of the Magellanic
stream. The measured distance is inconsistent with Magellanic stream
model predictions that do not account for ram pressure and gas
interaction with the MW disk. The estimated age of the cluster is
consistent with the time of last passage of the leading arm gas
through the Galactic midplane; we therefore speculate that this star
formation event was triggered by its last disk midplane passage. Most
details of this idea remain a puzzle: the Magellanic stream has low
column density, the MW disk at large radii has low gas density, and
the relative velocity of the leading arm and MW gas is large. However
it formed, the discovery of a young stellar cluster in the MW halo
presents an interesting opportunity for study. This cluster was
discovered with Gaia astrometry and photometry alone, but follow-up
DECam photometry was crucial for measuring its properties.
Description:
We obtained DECam u-, g-, and i-band imaging of a single field
centered on the "a" spatial component of the cluster (see Figure 1)
discovered using the Gaia DR2 data. Observations were obtained with
the Dark Energy Camera (DECam) on the CTIO 4m Blanco telescope on UT
2018 May 20.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
decam.dat 83 12794 DECam photometry and sky positions for point sources
in the observed DECam field
gaia.dat 99 5983 Gaia DR2 data and membership probabilities computed
for all stars in the cluster region
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See also:
II/358 : SkyMapper Southern Sky Survey. DR1.1 (Wolf+, 2018)
VII/195 : Globular Clusters in the Milky Way (Harris, 1996)
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
J/AJ/144/4 : Dwarf galaxies in the Local Group (McConnachie+, 2012)
J/A+A/547/A12 : Leading arm Magellanic Cloud Catalog (Venzmer+, 2012)
J/A+A/564/A125 : AGN Torus model comparison of AGN in the CDFS (Buchner+, 2014)
J/MNRAS/442/1680 : Red giants in SMC. Abundances (Dobbie+, 2014)
J/ApJ/792/43 : Detected sources in the Magellanic Stream region (For+, 2014)
J/ApJ/795/64 : Exoplanet physical parameters (Foreman-Mackey+, 2014)
J/MNRAS/471/4571 : Magellanic Inter-Cloud Project. III (Carrera+, 2017)
J/AJ/154/199 : Survey of the MAgellanic Stellar History (Nidever+, 2017)
J/A+A/616/A10 : Open clusters GaiaDR2 HR diagrams (Gaia Collaboration, 2018)
J/A+A/619/A180 : GaiaDR2 photometric sensitivity curves (Maiz Apellaniz+, 2018)
J/AJ/156/18 : APOGEE DR14: companions of evolved stars (Price-Whelan+, 2018)
Byte-by-byte Description of file: decam.dat
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Bytes Format Units Label Explanations
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1- 19 A19 --- Name Name of the source (JHHMMSS.ss+DDMMSS.s)
21- 30 F10.6 deg RAdeg [177.5/180.1] Right ascension (J2000) (ra)
32- 41 F10.6 deg DEdeg [-30.5/-28.5] Declination (J2000) (dec)
43- 49 F7.4 mag gmag [13.14/22] DECam g-band magnitude (g)
51- 56 F6.4 mag e_gmag [0.0012/0.5] Uncertainty on the gmag (g_err)
58- 64 F7.4 mag imag [12.84/22] DECam i-band magnitude (i)
66- 71 F6.4 mag e_imag [0.0015/0.5] Uncertainty on the imag (i_err)
73- 77 I5 --- Seq [54/54108] Running sequence number (id)
79 I1 --- Cont [0/1] Control mask (control_mask) (1)
81 I1 --- Cl [0/1] Cluster mask (cluster_mask) (1)
83 I1 --- Mm [0/1] Member mask (member_mask) (2)
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Note (1): The Boolean columns "Cont" and "Cl" are "True" (=1) when a given
source is in the control or cluster CCDs, respectively (see Figure 2).
Note (2): "True" (=1) when the source has a photometric membership probability
>0.5 of belonging to the cluster stellar population (see Section 3.2).
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Byte-by-byte Description of file: gaia.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- Gaia Gaia DR2 identifier (source_id)
21- 35 F15.11 deg RAdeg [173.5/185.5] Barycentric right ascension (ICRS)
at Ep=2015.5 (ra)
37- 51 F15.11 deg DEdeg [-35/-23] Barycentric declination (ICRS)
at Ep=2015.5 (dec)
53- 59 F7.3 mas/yr pmRA [-96.3/22.6] Gaia proper motion in right
ascension (pmRA*cosDE) (pmra)
61- 67 F7.3 mas/yr pmDE [-54.4/29.1] Gaia proper motion in
declination (pmdec)
69- 75 F7.4 mag Gmag [9.3/20.9] Gaia G-band mean magnitude (G0 )
77- 83 F7.4 mag BPmag [9.39/21.27] Gaia BP-band mean magnitude (BP0)
85- 91 F7.4 mag RPmag [9.1/21.3] Gaia RP-band mean magnitude (RP0)
93- 99 F7.4 --- Mm [0/1] Membership probability (member_prob)
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History:
From electronic version of the journal
(End) Emmanuelle Perret [CDS] 06-May-2021