Chemical Composition of Young Stars in the Leading
Arm of the Magellanic System
Lan Zhang1,2,3, Christian Moni Bidin4, Dana I. Casetti-Dinescu5,6, Réne A. Méndez3, Terrence M. Girard, Vladimir I. Korchagin7, Katherine
Vieira8, William F. van Altena6 & Gang Zhao1
Key Lab of Optical Astronomy, National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, 100012 Beijing, China
2
CAS South America Center for Astronomy, Camino El observatorio #1515, Las Condes, Santiago, Chile
3
Departamento de Astronomia Universidad de Chile, Camino El observatorio #1515, Las Condes, Santiago, Chile
4
Astronomy Department, Yale University, 260 Whitney Ave. , New Haven, CT 06511, USA
5
Department of Physics, Southern Connecticut State University, 501 Crescent St., New Haven, CT 06515, USA
6
Instituto de Astronomía, Universidad Católica del Norte, Av. Angomos 0610, Antofagasta, Chile
7
Institute of Physics, Southern Federal University, Stachki st. 124, 344090, Rostov-on-Don, Russia
8
Centro de Investigaciones de Astronomá, Apartado Postal 264, Mérida 5101-A, Venezuela
CD14-A5
8
CD14-A8
5
CD14-A15
4
The fitting process for the observed spectra is
shown in Fig. 2.
RESULTS
The abundance patterns are shown in Fig. 2.
Mg abundance of all B-type stars from our
sample are subsolar or near-solar within uncertainties. CD14-A08, the only O-type star,
has super-solar Mg abundance. He abundance
appears super-solar in all B-type stars, except CD14-B03 which is slightly sub-solar, but
within uncertainties may be solar. C abundances of the B-type stars show enrichments
in different degrees above solar, except CD14A11 which is near solar.
[Mg/H] as a functions of RV and distance mod-
2.0
1.5
CD14-B14
4080
Figure 1:
Hγ
4320
4350
HeI
4380
λ [Å]
[X/H]
NLTE, Z/Z ¯ =0 model atmosphere
NGC 4775 (in the MW)
NGC 330 (in the SMC)
[He/H], LTE, Z/Z ¯ =0 model atmosphere
NLTE, Z/Z ¯ =0.5 model atmosphere
NGC 2004 (in the LMC)
solar abundance
CD14-A05
CD14-A08
CD14-A11
CD14-A12
CD14-A15
CD14-B02
CD14-B03
CNO
4440
Ne Mg
Atomic Number
CD14-B14
Si
S
He
CNO
Ne Mg
Atomic Number
Si
S
1.5
1.0
0.5
0.0
0.5
1.0
1.5
1.0
0.5
0.0
0.5
1.0
1.5
1.0
0.5
0.0
0.5
1.0
1.5
1.0
0.5
0.0
0.5
1.0
disk/MW; and CD14-A05, CD14-A15, CD14B02, CD14-B03, and CD14-B14 as members of
the LA.
REFERENCES
Figure 3:
[Mg/H] as a function of RV and distance modulus.
We also calculate the average [Mg/H] for the
two groups of stars, we obtain a value of
−0.42±0.16 dex for the LA group, and −0.07±
0.06 for the MW group. The average [Mg/H]
of the five stars in the LA group is significantly
lower than that of the MW group and in good
agreement with that of cluster NGC 2004 in
the LMC (−0.45 ± 0.10). Also, the large abundance scatter of the LA group indicates that
LA III
5
125
LA II
75
LA I
25
75
150
RVs [km s−1 ]
25
20
15
LA II
15
10
LA I
25
5
225
225
25
5
175
15
LA III
5
225
5
0
Figure 2:
HeI
HeI
0.5
4910
4940 5871 5875 5879
λ [Å]
Hβ
4850
4880
Examples of fits of the most prominent spectral features with the best fitted stellar atmospheric parameters.
km s−1 appear to show lower Mg abundance.
Five B-type stars that have radial velocities
compatible with membership to the LA have an
average [Mg/H] of −0.42 ± 0.16, significantly
lower than the average of the remaining two
[Mg/H] = −0.07 ± 0.06 that are kinematical
members of the Galactic disk.
He
4410
HeI
4820
Distance [kpc]
4050
Hδ HeI HeI
4110
4140
10
15
Distance [kpc]
5
20
20
Abundance results derived using the stellar parame-
Distance [kpc]
HeI HeI
0
4020
150
75
ters.
0
60
DISCUSSIONS
We then explore the RV and distance properties of our data on the MS coordinate system (ΛMS, BMS). Fig. 4 shows RV and distance
as functions of (ΛMS, BMS), respectively. The
two stars with RV < 100 km s−1 have small
distances, and are at a similar location in the
(ΛMS, BMS) plane. While for the other five stars
whose RV > 100 km s−1, the RV and distance
decrease with decreasing Magellanic longitude.
This RV profile and the shape of the distance
profile are quite similar to the ones from previous H I gas studies and N -body simulation for
the leading material. Therefore, we will consider stars with RVs in excess of 100 km s−1
[Mg/H] is significantly lower than
that of the stars which are
representative of the Galactic disk.
2 Both RV and distance decrease with
decreasing Magellanic longitude.
3 The average age and its small scatter
suggest a single star-forming episode
∼ 70 Myr ago in the LA.
1
1.0
[X/H]
• Model atmospheres: interpolated from
comprehensive grids of metal line-blanketed,
NLTE, plane-parallel, hydrostatic model
atmospheres generated by TLUSTY (Lanz
& Hubeny, 2003, 2007)
• Line synthesis: a general spectrum synthesis
program, SYNSPEC (developed by Ivan
Hubeny & Thierry Lanz)
Among the seven B-type stars, five
stars have kinematics compatible with
LA membership. For the five possible
LA member stars,
2.5
CD14-B3
1
METHODS
The stellar atmospheric parameters (Teff , log g,
[He/H], & v sin i) and abundances of other six
elements (C, N, O, Mg, Si, S) of the target stars
were measured by fitting the observed hydrogen and helium lines with synthetic spectra.
3.0
CD14-B2
3
1.5
1.0
0.5
0.0
0.5
1.0
1.5
1.0
0.5
0.0
0.5
1.0
1.5
1.0
0.5
0.0
0.5
1.0
1.5
1.0
0.5
0.0
0.5
1.0
CONCLUSIONS
3.5
BMS
• blue side (3200 − 5000 Å): R ∼ 33000 and
S/N ∼ 26.
• red side (4900 − 10000 Å): R ∼ 29000 and
S/N ∼ 28.
uncertainty of the [Mg/H] abundance determination of CD14-A05 for instance, we find
it is statistically consistent at 90% confidence
with that of cluster NGC 330 in the SMC
(−0.86 ± 0.12). Thus, we cannot exclude
the possibility that more metal-poor, SMC-like
material could have participated in the formation of CD14-A05 (LA I) and perhaps CD14B14 (LA II).
4.0
RVs [km s−1 ]
High-resolution spectra were obtained with the
MIKE instrument on the 6.5m Clay telescope
in March of 2014 for these eight stars.
4.5
CD14-A12
2
• five stars are below the Galactic plane
(“CD14-A**”), at Magellanic coordinates
(ΛM , BM ) ∼ (15◦, −22◦)
• three stars are above the Galactic plane
(“CD14-B**”), at (ΛM , BM ) ∼ (42◦, −8◦).
5.0
CD14-A11
6
We select eight stars from the pilot,
intermediated-resolution spectral study of
Casetti-Dinescu et al. (2014, CD14)
Syn. Spec.
5.5
7
OBSERVATIONS
Obs. Spec.
RVs [km s−1 ]
• Determine chemical abundances, radial
velocities, distances and ages for eight Oand B- type stars from high-resolution
spectra obtained with the MIKE
instrument on the 6.5m Clay telescope;
• Identify LA members with these
kinematical and abundance information;
• Understand the origin of these stars and
thus further constrain the formation and
evolution history of the LA;
• Explore whether there is a chemical
difference between young stars that are
kinematical members of the LA and
those that are not.
to be members of the LA, while the remaining stars are assumed members of the disk of
the MW. This leaves two stars, CD14-A11 and
CD14-A12 as representative of the Galactic
BMS
OBJECTIVES
ulus for our target stars are shown in Fig. 3. No
obvious trend of [Mg/H] against the RV or the
distance modulus is seen in our data. However,
there is a suggestion that stars with RV > 100
Normalized Flux
1
Figure 4:
50
40
30
ΛMS
20
10
0
15
10
5
60
50
40
30
ΛMS
20
10
0
The RV and distance distribution on the MS coordi-
nate system.
the source of LA material is chemically complex. In Fig. 3, it can be seen that individual
[Mg/H] abundances of four LA members are
close to that of the LMC cluster. This suggests that the young LA members are formed
from LMC-like material. One star, CD14-B02,
has a [Mg/H] abundance close to that of cluster
NGC 4755 in the MW (−0.35±0.12), although
kinematically it seems to belong to the LA. Its
location in LA III would suggest a higher RV,
more like that of CD14-B03 and CD14-B14 together with a rather small distance among the
five kinematical LA members, CD14-B02 may
be atypical of LA members. Considering the
Casetti-Dinescu, D. I., Majewski, S. R., Girard, T. M., et al. 2006,
AJ, 132, 2082
Casetti-Dinescu, D. I., Moni Bidin, C., Girard, T. M., et al. 2014,
ApJL, 784, L37
Lanz, T., & Hubeny, I. 2003, ApJS, 146, 417
—. 2007, ApJS, 169, 83
McClure-Griffiths, N. M., Pisano, D. J., Calabretta, M. R., et al.
2009, ApJS, 181, 398
Przybilla, N., Nieva, M. F., Heber, U., et al. 2008, A&A, 480, L37
Trundle, C., Dufton, P. L., Hunter, I., et al. 2007, A&A, 471, 625
Kapteyn’s Selected Areas
We used MMT Hectospec in February of
2016 to perform low resolution (R = 2500)
spectra for ∼ 500 stars in one Kapteyn’s
Selected Areas (SA) 100 and to measure
their radial velocities and astrophysical parameters. The data reduction has already been finished. Combing with precise
(1 − 3 mas yr−1 at V∼19-21) proper motions from the SA survey of Casetti-Dinescu
et al. (2006), we will select Monoceros Ring
candidates, and derive full six-dimensional
phase-space coordinates for them. These
data will help 1) constrain dynamical models and pinpoint the characteristics of the
event that caused the perturbation of the
Galactic disk; 2) better understand whether
the excess populations we see are distinct from the canonical populations of the
Galaxy, and thus distinguishing whether
the stellar excess is predominantly an accreted population, or scattered Galactic
disk stars.