Issue |
A&A
Volume 507, Number 2, November IV 2009
|
|
---|---|---|
Page(s) | L37 - L40 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200913173 | |
Published online | 04 November 2009 |
A&A 507, L37-L40 (2009)
LETTER TO THE EDITOR
SDSS J013655.91+242546.0 - an A-type hyper-velocity star from the outskirts of the
Galaxy
,![[*]](/icons/foot_motif.png)
A. Tillich1 - N. Przybilla1 - R.-D. Scholz2 - U. Heber1
1 - Dr. Remeis-Sternwarte Bamberg, Universität Erlangen-Nürnberg,
Sternwartstr. 7, 96049 Bamberg, Germany
2 - Astrophysikalisches Institut Potsdam, An der Sternwarte 16,
14482 Potsdam, Germany
Received 24 August 2009 / Accepted 24 October 2009
Abstract
Context. Hyper-velocity stars (HVS) are moving so fast that
they are unbound to the Galaxy. Dynamical ejection by a supermassive
black hole is favoured to explain their origin.
Aims. Locating the place of birth of an individual HVS is of utmost importance to understanding the ejection mechanism.
Methods. SDSS J013655.91+242546.0 (J0136+2425 for short)
was found amongst three high-velocity stars (drawn from a sample of
more than 10 000 blue stars), for which proper motions were
measured. A kinematical as well as a quantitative NLTE spectral
analysis was performed. When combined with the radial velocity (RV) and
the spectroscopic distance, the trajectory of the star in the Galactic
potential was reconstructed.
Results. J0136+2425 is found to be an A-type main-sequence star travelling at 590
,
possibly unbound to the Galaxy and originating in the outer Galactic rim nowhere near the Galactic centre.
Conclusions. J0136+2425 is the second HVS candidate with
measured proper motion, besides the massive B star HD 271791,
and also the second for which its proper motion excludes a Galactic
centre origin and, hence, the SMBH slingshot mechanism. Most known HVS
are late B-type stars of about 3 .
With a mass of 2.45
,
J0136+2425 resembles a typical HVS far more than HD 271791 does.
Hence, this is the first time that a typical HVS is found not to
originate in the Galactic centre. Its ejection velocity from the disk
is so high (550
)
that the extreme supernova binary scenario proposed for HD 271791 is very unlikely.
Key words: stars: kinematics - stars: individual: SDSSJ013655.91+242546.0 - stars: atmospheres - line profiles
1 Introduction
Stars travelling so fast that they escape from the Galaxy are an inevitable consequence of the presence of a supermassive black hole (SMBH) in a dense stellar environment (Hills 1988) such as the Galactic centre (GC). Hills (1988) coined the term hyper-velocity star (HVS) for such an object. It took a long time until Brown et al. (2005), Hirsch et al. (2005) and Edelmann et al. (2005) discovered the first three HVSs serendipitously and the interest in these stars grew tremendously. Systematic searches for HVSs took advantage of the huge SDSS database (Adelman-McCarthy et al. 2008) for target selection and revealed a population of HVSs; the latest compilation lists 16 of these stars (Brown et al. 2009a). These surveys are based on RV measurements alone. The known HVSs are non-uniformly distributed on the sky, as are their travel times. Brown et al. (2009b) argue that this anisotropy supports their common origin being the Galactic centre, while Abadi et al. (2009) point out that the overdensity of HVSs in the constellation Leo suggests that the anisotropic distribution and preferred travel time are the result of the tidal disruption of a dwarf galaxy in the Galactic potential.
Another issue is the distance of a HVS star, because blue horizontal branch (BHB)
stars can not be easily distinguished from main-sequence (MS) stars for the HVSs in
question since both types of
stars populate the same region
in the
-
-diagram (Heber et al. 2008b),
but have different distances. It is generally assumed that the stars are main-sequence
and not blue horizontal branch stars. Detailed spectroscopic analyses
have confirmed these assumptions for HVS1, HVS3, HVS7, and HVS8
(López-Morales & Bonanos 2008; Przybilla et al. 2008b; Heber et al. 2008b; Przybilla et al. 2008c).
In the absence of proper motion measurements,
the trajectories have not been derived for any individual HVS so far.
Heber et al. (2008a) succeeded in investigating the trajectory of
the high-velocity B star HD 271791 from accurate proper motions, radial velocity,
and spectroscopic distance. The star was found to be probably unbound to the
Galaxy and to originate in the outer rim of the Galaxy rather than in its centre,
demonstrating that a mechanism other than that of Hills (1988)
must operate. Hence, it is rewarding to measure proper motions of
high-velocity stars. Xue et al. (2008) presented radial velocities
for more than 10 000 blue stars from the SDSS , which are a mix of blue horizontal
branch, blue straggler, and main-sequence stars with effective temperature
between roughly 7000 and 10 000 K according to their colours.
We focused on the 11 fastest stars in terms of large positive Galactic
rest-frame (GRF) velocities to unravel their nature, distance, and
kinematics from detailed quantitative spectral analyses and astrometry.
Three stars exhibited significant proper motions
.
Here we report that J0136+2425 is an A-type
main sequence star travelling at
590
,
possibly unbound to the
Galaxy and originating in the outer Galactic disk.
2 Target selection and proper motion
We selected all stars
with GRF velocities
from the
RV-based sample of Xue et al. (2008) and obtained 11 targets for which we attempted to measure proper motions.
All
available independent position measurements on Schmidt plates
(APM - McMahon et al. 2000;
SSS - Hambly et al. 2001) were collected
and combined with the SDSS and other available positions
(CMC14 Carlsberg-Meridian-Catalog 2006;
2MASS - Cutri et al. 2003;
UKIDSS - Lawrence et al. 2007)
to perform a linear proper motion fit. However, there were even
more Schmidt plate measurements from up to 14 different epochs in the case
of overlapping plates of the Digitised Sky
Surveys
.
FITS images of 15 by 15 arcmin size were extracted from all available plates
and ESO MIDAS tool center/gauss was used to measure positions. For this purpose,
we selected compact background galaxies around each target, identified from
SDSS, to transform the target positions in all the Schmidt plates to the
SDSS system. The small fields allowed us to apply a simple model
(shift+rotation) and to achieve an improved fit
for all our targets (see Fig. 1).
Significant proper motions were found for the three brightest stars only.
For J0136+2425, we obtain
and
.
![]() |
Figure 1: Linear fit of the position measurements for J0136+2425, whereas 1979.74 is the zero epoch. |
Open with DEXTER |
3 Observations and quantitative spectroscopy
In order to exclude RV variability, we reobserved J0136+2425 at ESO
with EFOSC2 mounted on the NTT in October 2008 and with
the TWIN spectrograph at the 3.5 m telescope on Calar Alto in
July 2009; during the latter run, six spectra for J0136+2425
distributed over three days were
obtained. Radial velocities were derived by
-fitting suitable synthetic spectra over the full spectral range.
Since we used many spectral lines, our results differ from that of
Xue et al. (2008) who used the
line only.
The radial velocities from individual spectra agree to within
their respective error limits, indicating that the star is not RV
variable within a few kilometers per second on timescales of days.
A quantitative analysis
was carried
out following the hybrid NLTE approach discussed by Przybilla et al. (2006).
The effective temperature
and the surface gravity
were determined by fits to the Stark-broadened Balmer and Paschen
lines and the ionisation equilibrium of Mg I/II. The stellar
metallicity were derived by model fits to the observed metal-line
spectra. Results are listed in Table 1 and a
comparison of the resulting final synthetic spectrum with
observations in the selected
regions around the Balmer lines, the higher Paschen series,
Mg II
4481 Å, the Mg I b and the near-IR
O I triplets, is shown in Fig. 2.
Overall, excellent agreement is obtained for all strategic spectral lines
throughout the entire wavelength range.
The uncertainties in the stellar parameters were constrained by
the quality of the match of the spectral indicators within the given S/N limitations.
![]() |
Figure 2: Comparison of NLTE spectrum synthesis (thick line) with observation (thin wiggly line) for J0136+2425. |
Open with DEXTER |
Its
and gravity places J0136+2425 on the main sequence (see Fig. 3). In
addition the star is rapidly rotating at 250
and its metallicity is solar,
which strengthens the
conclusion that it is an intermediate-mass A-type main-sequence star of 2.45
as
derived by comparing the position of the star to predictions of the
evolutionary models of Schaller et al. (1992).
In Fig. 3, we compare the position of J0136+2425 in the
(
,
)-diagram to those of five late B-type HVSs for which these
parameters are available (López-Morales & Bonanos 2008; Przybilla et al. 2008b; Heber et al. 2008b; Przybilla et al. 2008c) as well as those of
the massive B star HE 0437-5439 originating in the LMC, and the B giant HD 271791. The
vast majority of HVSs are of late B type because they were discovered by
targeted searches. Hence, we shall term them typical HVSs.
J0136+2425 is slightly cooler and less massive than the other known HVS. However,
its mass and evolutionary lifetime is similar to that of the typical HVSs
(3-4
,
100 Myr), while HE 0437-5439 and HD 271791 are far
more massive (9-11
)
and short-lived (
20 Myr).
Table 1: Results of the spectroscopic and kinematic analysis of J0136+2425.
![]() |
Figure 3:
J0136+2425 in the (
|
Open with DEXTER |
4 Distance, kinematics, and errors
Using the mass, effective temperature, gravity, and extinction-corrected apparent magnitude, we derive the distance following Ramspeck et al. (2001) using the fluxes from the final model spectrum. The distance error is dominated by the gravity error.
Applying the Galactic potential of Allen & Santillan (1991), we calculated orbits and reconstructed the path of the star back to the Galactic plane with the program of Odenkirchen & Brosche (1992). The distance of the GC from the Sun was adopted to be 8.0 kpc and the Sun's motion with respect to the local standard of rest was taken from Dehnen & Binney (1998). Since the RV is well known, the error in the space motion is made up of that of the distance, which is controlled mainly by the gravity error, and those of the proper motion components. Varying these three quantities within their respective errors by applying a Monte Carlo procedure with a depth of 1000, we determined the intersection area of the trajectories with the Galactic plane and the time-of-flight. From these Monte Carlo simulations, we derived the median GRF velocities at the present location and their distibution (see Fig. 4) and compared these with the local escape velocity calculated from the Galactic potential of Allen & Santillan (1991).
![]() |
Figure 4: Galactic rest-frame velocity distribution for J0136+2425. |
Open with DEXTER |
For J0136+2425, the GRF velocity of 587
+144-89
slightly exceeds the local
escape velocity. Whether the star is bound to the Galaxy depends on the Galactic
potential adopted, in particular for the dark matter halo, as pointed out by
Abadi et al. (2009). Allen & Santillan (1991) adopted a halo mass out
to 100 kpc of
.
Xue et al. (2008)
derived a
somewhat lower mass
whereas
Abadi et al. (2009) favoured a higher one of
.
If the former were correct,
J0136+2425 would be unbound, while it would be bound in the latter case.
As can be seen in Fig. 5,
its place of origin
is found to be in the outer part of the Galactic plane
at Galactic radii between 12.5 kpc and 18 kpc, nowhere near the GC.
The time of flight (
Myr) is much shorter than the star's lifetime (450 Myr).
![]() |
Figure 5:
Upper panel: trajectory for J0136+2425 with place of birth marked in green (3 |
Open with DEXTER |
5 Conclusion
We have reported the quantitative spectral analysis of a high-velocity star from the sample of faint blue stars in the halo of Xue et al. (2008). The radial velocity, proper motion, and spectroscopic distance were derived and a detailed kinematical analysis was performed using the Galactic potential of Allen & Santillan (1991).
J0136+2425 was found to be a rapidly rotating A star of solar composition and
therefore classified as a main-sequence star of 2.45 .
The kinematic
analysis
excludes an origin at the GC and hence the Hills mechanism for
ejection of the star.
Its place of origin
was found to be in the outer part of the Galactic plane
at Galactic radii between 12.5 kpc and 18 kpc. This is very similar to the case
of the massive B star HD 271791 (Heber et al. 2008a), which was the first
ultra-high-velocity star whose proper motion excludes a GC origin.
The ejection mechanism for the star was proposed to be an extreme binary supernova,
which also explained its enrichment in
elements.
However, we find no evidence of
-enhancement in J0136+2425 from the
existing spectra. The star is more metal-rich than expected on average for an
object born at such a large Galactocentric distance. Due to the presence of
Galactic abundance gradients (Rudolph et al. 2006), we expect the outer parts of
the Galaxy to be less metal-rich than the Sun.
The metallicity of J0136+2425 is compatible with the high end of the
abundance distribution in the outer disk.
The required ejection velocity from the disk is so high
(550
)
that an extreme supernova binary scenario as proposed for HD 271791
(Przybilla et al. 2008a) is very unlikely.
Most known HVS are late B-type
stars of about 3 .
With a mass of 2.45
,
J0136+2425
resembles such a typical HVS much more
than HD 271791 (M=11
)
does.
Hence, this isthe first time that a typical HVS is found
not to originate in the GC and excludes the SMBH slingshot
mechanism. Hence, typical HVS may have been ejected by different
mechanisms other than that proposed by Hills.
Once more, this calls for an alternative ejection scenario to the Hills mechanism
such as dynamical ejection from
clusters or binary supernovae (see Gvaramadze et al. 2009).
Hence, we are left with the dynamical ejection scenario or tidal disruption
of a satellite galaxy, as proposed by Abadi et al. (2009).
They noticed a clustering
of more than half of the known HVS in a region of 26-diameter in the
constellation of Leo
(
,
).
They suggested that if these star
stems from a disruption event of a dwarf galaxy, more high velocity stars
should be found in that area. However, J0136+2425 (
,
)
is
located far from Leo and is therefore probably unrelated.
A.T. acknowledges funding by the Deutsche Forschungsgemeinschaft through grant HE1356/45-1. We are very grateful to Stephan Geier for stimulating discussions and advice. Our thanks go to S. Müller and T. Kupfer for observing and reducing the data from DSAZ.
References
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Footnotes
- ...
Galaxy
- Based on data collected at the European Southern Observatory, Chile. Program ID: 082.D-0649.
- ...
- Based on observations at the 3.5 m telescope at DSAZ observatory (Calar Alto) in Spain. Program ID: H09-3.5-028.
- ... motions
- Two of the stars are found to be a metal-poor straggler of population II and a spectroscopic binary, as described in a forthcoming paper.
- ...
Surveys
- http://archive.stsci.edu/cgi-bin/dss_plate_finder
All Tables
Table 1: Results of the spectroscopic and kinematic analysis of J0136+2425.
All Figures
![]() |
Figure 1: Linear fit of the position measurements for J0136+2425, whereas 1979.74 is the zero epoch. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Comparison of NLTE spectrum synthesis (thick line) with observation (thin wiggly line) for J0136+2425. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
J0136+2425 in the (
|
Open with DEXTER | |
In the text |
![]() |
Figure 4: Galactic rest-frame velocity distribution for J0136+2425. |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Upper panel: trajectory for J0136+2425 with place of birth marked in green (3 |
Open with DEXTER | |
In the text |
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