A&A 423, 155-167 (2004)
DOI: 10.1051/0004-6361:20034147
J. P. Maillard1 - T. Paumard1 - S. R. Stolovy2 - F. Rigaut3
1 - Institut d'Astrophysique de Paris (CNRS), 98bis Bd Arago,
75014 Paris, France
2 - Spitzer Science Center, CalTech, MS 220-6, Pasadena, CA 91125, USA
3 - Gemini North Headquarter, Hilo, HI 96720, USA
Received 1 August 2003 / Accepted 8 April 2004
Abstract
High spatial resolution observations in the 1 to 3.5 m
region of the Galactic Center source known historically as IRS 13
are presented. They include ground-based adaptive optics images in the H,
Kp (2.12/0.4
m) and L bands, HST-NICMOS data in filters between 1.1
and 2.2
m, and integral field spectroscopic data from BEAR, an Imaging FTS, in the He I 2.06
m and the Br
line
regions. Analysis of all these data provides a completely new picture of
the main component, IRS 13E, which appears as a cluster of seven
individual stars within a projected diameter of
0.5
(0.02 pc). The brightest sources, 13E1, 13E2, 13E3 which is detected as a binary,
and 13E4, are all massive stars of different type. The star 13E1 is a
luminous, blue object, with no detected emission line. 13E2 and 13E4 are
two hot, high-mass emission line stars, 13E2 being at the WR stage and 13E4
a massive O-type star. In contrast, 13E3A and B are extremely red objects,
proposed as other examples of dusty WR stars, like IRS 21 (Tanner
et al. 2002). All these sources have a common westward proper
motion (Ott et al. 2003) indicating they are bounded. Two other
sources, detected after deconvolution of the AO images in the H and Kp bands, are also identified. One, that we call 13E5, is a red source similar
to 13E3A and B, while the other one, 13E6, is probably a main sequence
O star in front of the cluster. Considering this exceptional concentration
of comoving massive hot stars, IRS 13E is proposed as the remaining core of
a massive star cluster, which could harbor an intermediate-mass black hole (IMBH) (Portegies Zwart & McMillan 2002) of
1300
.
This detection plays in favor of a scenario,
first suggested by Gerhard (2001), in which the helium stars and
the other hot stars in the central parsec originate from the stripping of a
massive cluster formed several tens of pc from the center. This cluster
would have spiraled towards SgrA
,
and IRS 13E would be its
remnant. Furthermore, IRS 13E might be the second black hole needed
according to a model by Hansen & Milosavljevic (2003) to drag
massive main-sequence stars, in the required timescale, very close to the
massive black hole. The detection of a discrete X-ray emission (Baganoff
et al. 2003) at the IRS 13 position (within the positional
accuracy) is examined in this context.
Key words: instrumentation: adaptive optics - infrared: stars - Galaxy: center - stars: Wolf-Rayet
In the early images of the central region of the GC recorded in the near
infrared, at the best seeing-limited resolution, several bright point
sources dominate the
field centered on
SgrA
.
With the radical improvement of angular resolution through multiple
short exposures with shift-and-add (SHARP camera, Eckart et al. 1995) or speckle techniques (Ghez et al. 1999), with the
images from the NICMOS cameras on board HST (Stolovy et al. 1999),
to the advent of adaptive optics correctors behind large telescopes as NAOS
(Ott et al. 2003), a more complex vision of this crowded field has
emerged. At a spatial resolution in the best case of 0.05
(Ghez
et al. 1999) new, faint stellar sources appear and the
early-identified sources are often resolved into several
components. Therefore, the observed spectra, generally made at a resolution
limited at best by a slit not less than 1
wide, are actually
composites of emission from stellar objects of different spectral type as
well as from emission lines from the surrounding interstellar medium (ISM).
The stellar type identified from these spectra can be wrong if attributed
to a single source, which can erroneously look unusual. This care led to
the paper on the revised identifications of the central cluster of
He I stars by Paumard et al. (2001) made from slitless
integral field spectroscopy, with BEAR an imaging Fourier Transform
Spectrometer (Maillard 2000). This instrument makes it possible
to obtain near-infrared spectroscopy at the seeing-limited resolution of
Mauna Kea (i.e.
0.5
), which represents a significant
improvement. However, this resolution is not sufficient for such a crowded
field. Therefore, the data were compared to adaptive optics (AO) images in
the K band of the same field, with a spatial resolution of
0.15
,
in order to check whether the emission sources at the
angular resolution of seeing were single stars or not.
Among the sources studied in this paper the object historically named IRS 13, is a typical example. Located approximately 3.6
south-west
of SgrA
,
at the edge of the circular structure of the SgrA West
HII region called the Minicavity, it appears in early works at
near-infrared wavelengths, in J, H, K (Rieke et al. 1989) and L(Allen & Sanders 1986), as a bright spot. However, from a
discussion on the sources of energy at the Galactic Center, Rieke et al. (1989) note there are separate luminosity sources in the
core of the 10
m sources 1, 9 and 13, because, particularly
for IRS 13, the energy distribution from 1 to 5
m presents a sudden,
steep increase beyond 3
m. A first work at subarcsecond resolution, by
lunar occultation in the K band (Simon et al. 1990), indicates that IRS 13 resolves into a pair of sources, separated by
1.2
,
which were henceforth designated as IRS 13E and 13W. In the photometric
survey of Ott et al. (1999) from SHARP imaging data (Eckart et al. 1995), at a limit of resolution of
0.15
obtained
by deconvolution, two equally bright components (
)
are
reported, IRS 13E1 and 13E2, with a separation of 0.2
.
Paumard
et al. (2001) published the first AO map of IRS 13E in the K band,
extracted from a larger image of the central region obtained with the
CFHT-AO system (Lai et al. 1997). We noted that a fainter third
source, we called IRS 13E3, was also present, forming a kind of
equilateral triangle with 13E1 and 13E2. In parallel, Clénet et al. (2001) published a photometric analysis of the same data. For IRS 13E they reported three components in the K band, noted 13E1, 13E2 and 13N. From the given offset coordinates the source called 13N in this paper
is not exactly coincident with the source we had previously called 13E3
(Paumard et al. 2001).
In the meantime, spectroscopic works on the stellar population of the
central parsec were conducted. Several spectra of IRS 13 (Blum et al. 1995; Libonate et al. 1995; Tamblyn et al. 1996),
and specifically of IRS 13E (Krabbe et al. 1995; Genzel et al. 1996; Najarro et al. 1997), and of IRS 13W
(Krabbe et al. 1995) have been published. They cover mostly the K band, i.e. all or part of the 1.95-2.45 m range, and one the 1.57-1.75
m region of the H band (Libonate et al. 1995). From
these spectra IRS 13W is unambiguously a cool star with the strong vib-rot
CO signatures at 2.3
m. Emission lines were detected on IRS 13E,
typical of a luminous, hot star: strong He I 2.058, 2.112
m,
Br
line and other Brackett lines up to Br12, plus weaker lines of [Fe II], [Fe III], and a weak emission at 2.189
m
attributed to He II. The 7-10 line is the most intense of this ion
in the K band, and is supposed unblended, while the 8-14 line cannot be
distinguished from Br
of which it is separated by less than 2 cm-1. The He II line if detected is a precious type
indicator in the classification of WR stars (Tamblyn et al. 1996;
Figer et al. 1997). From these spectral characteristics Libonate
et al. (1995) concluded the IRS 13 spectrum bears a strong
resemblance to the low-resolution K-band spectra of P Cygni (a LBV) and the AF source. Krabbe et al. (1995) proposed IRS 13E as a WN9 star. From a K-band spectrum of the Minicavity Lutz et al. (1993)
attributed to [Fe III] several emission lines (2.145, 2.218, 2.242, 2.348
m). Images at 2.218
m show that this line is present all
over the Minicavity, with a particular enhancement at the position of IRS 13. From NICMOS data in the F164N filter (Stolovy et al. 1999) the lower ionized Fe species, [Fe II] is detected in
emission in the central ISM, particularly strong at the edge of the
Minicavity, but remarkably absent at the position of IRS 13E. As mentioned
in this paper the ionization condition for [Fe III] requires 16.2 eV while only 7.2 eV are required for [Fe II]. Hard
ionization radiation is likely originating from IRS 13E. Hence, it can
concluded that these iron lines are not of stellar origin but are present
in the surrounding ionized gas.
From the extraction of the He I 2.058 m line profile at high
spectral resolution from the BEAR data, Paumard et al. (2001)
concluded that one of the three sources identified as forming IRS 13E,
instead of a LBV-type star should be already at the WR stage. The main
argument was the width of the observed emission line profile (
km s-1), typical of WR stars, making the source belonging to a
class of broad-line stars including 8 other stars in the central cluster of
helium stars. It was also measured that the broad-line stars were in
average weaker by
2.4 mag in the K band than the rest of helium
sources which were characterized by a narrow He I emission
line. With a
for IRS 13E3 consistent with the magnitude measured
for the other broad-line stars, and 13E1 and 13E2 much brighter, it was
proposed that IRS 13E3 should be the WR-type helium emitter.
It was not possible to precise the 13E1 and 13E2 stellar type.
In the centimetric domain, Zhao & Goss (1998) presented the
detection of IRS 13 at 7 and 13 mm with the VLA, at a resolution
of 0.06
,
which they reported to be the brightest radio continuum
source after SgrA
at the Galactic Center. They resolved the source
into two components, one with no significant proper motion while the other
one is moving south at a rate 6.2
1.1 mas yr-1. They called
the two compact H II regions IRS 13E and IRS 13W, which
was improper, since these denominations had already been given to infrared
sources at IRS 13 as reminded above, with which they are not
coincident. However, this detection is another element which makes this
source special.
The detection of a bright, discrete X-ray emission source (Baganoff et al. 2003; Muno et al. 2003) at the IRS 13 position
within the positional accuracy - source CXOGC J174539.7-290029 - among the
brightest sources within the central parsec besides SgrA,
is a last
element contributing to make this source an object of interest. This coincidence
already triggered the interest of Coker et al. (2002) who presented
the first Chandra X-ray spectrum of IRS 13 and deduced that it was
consistent with a highly absorbed X-ray binary system. They concluded that
IRS 13E2 should be a compact post-LBV binary whose colliding winds were
the source of the X-ray emission.
As a conclusion, the origin of the brightness of IRS 13, one of the
brightest objects at all wavelengths in the vicinity of SgrA,
has been a
matter of debate. We examined all the high-angular resolution images in
the near infrared currently available. This analysis made it possible to
build a completely new picture of IRS 13 which is presented in this
paper. We will show that the peculiar spectral energy distribution (SED)
previously reported is well explained by the nature of the individual
sources which compose IRS 13. Unexpectedly, this analysis provides new
insight to the unsolved question of the formation of the central massive
cluster. It has also given the opportunity through the study of the stars
in the IRS 13 field to characterize a sample of the star population in the
central parsec. A preliminary version of the paper was presented at the
Galactic Center Workshop 2002 (Maillard et al. 2003).
Ground-based AO images from several telescopes and space-based NICMOS data containing IRS 13 in their field have been gathered. Their characteristics are given in Table 1.
We have analyzed data from three different AO systems: PUEO/CFHT (Lai et al. 1997), Hokupa'a/Gemini North (Graves et al. 1998), and
Adonis/ESO 3.6-m telescope (Beuzit et al. 1997). The CFHT data, in
the K band, were obtained on 26 June, 1998. They were already presented and
analyzed in detail in Paumard et al. (2001). The total field
covers
centered on SgrA
.
The FWHM of the
Point Spread Function (PSF) varies from 0.13
to 0.20
in
the field since PUEO has a visible wavefront sensor, requiring to use a
star 20
to the north-east of the field center. The
L-band data were obtained with the ADONIS visible wavefront sensor in 2000,
from 20 to 22 May, on the ESO 3.6-m telescope. These data are described in
Clénet et al. (2001). The final L-band image we used has a PSF
with a FWHM of 0.291
and covers a field of
centered on SgrA
.
The Gemini North
data were part of the AO demonstration run conducted by F. Rigaut on the
Galactic Center in July 2000. The data were obtained with the Hokupa'a AO system and the QUIRC camera (Graves et al. 1998) in the Kp and the
H band (Table 1) respectively 3 and 6 July in field 1, centered
on SgrA
.
The field coverage of each image is
.
For the H image the FWHM of the PSF varies from 0.115 to 0.19
and
for the Kp image from 0.12 to 0.18
.
In the vicinity of IRS 13 the
measured FWHMs are respectively 0.180
and 0.172
.
These AO data are characterized by a low Strehl ratio of 2.5
in H and 7
in Kp. The small portion of the AO image in the Kp filter analyzed in the
paper is shown in Fig. 1.
Table 1: Instruments, filter properties and year of data acquisition of the high-resolution images of the IRS 13 field.
Six filters, coded F110M, F145M, F160W, F187N, F190N, and F222M were used
in observing the stars in the inner parsec of the Galactic Center with the
NICMOS cameras on board HST, as part of three independent Galactic Center
programs (7214, 7225, and 7842). These data were taken between Aug. 1997
and Aug. 1998. The filter properties (central wavelength, bandwidth,
zero-magnitude flux, pixel size) and the data processing of the raw data
are described in Rieke (1999) and Stolovy et al. (1999).
Although each program covered a larger region, we extracted a small portion
of
centered on IRS 13E of the image
from each filter. All these diffraction-limited images were particularly
useful to derive the SED of the stellar components of IRS 13 and its
environment.
Apart from wide (W) or medium (M) bandpass filters, F187N is a 1 narrow-band filter centered on the Pa
line. By subtraction of F190N, a
comparable narrow-band filter in the nearby continuum, the distribution of
the ionized gas in the inner parsecs was obtained (Stolovy et al. 1999). In this map are also discrete emission sources which must
come from Pa
emission in the atmosphere of hot stars. We used a
best-fit scale factor between F187N and F190N in order to minimize both
positive and negative stellar residuals of the stars in the central parsec.
Negative residuals can arise in stars with Pa
absorption and/or in
stars with local excess extinction along the line of sight. For this paper,
we identified emission line stars as those with very significant
F187N/F190N ratios (exceeding 1.2). The portion of the resulting Pa
image with the IRS 13 complex was essential in determining which of the
individual stars was an emission line star (Fig. 3).
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Figure 1:
IRS 13 field from the Gemini AO image in the Kp band. IRS 13E is the central,
compact group of stars and IRS 13W the brightest source ![]() ![]() ![]() |
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Figure 2:
The
Fig. 1 field with the star detected after deconvolution by
the MCS code (Sect. 2.3). The vector associated with most of the
stars represents in amplitude the velocity and the direction of proper
motions measured from SHARP data by Ott et al. (2003). For E3A and E3B, only the proper motion of the center of light is determined. The
amplitudes reported in Table 6 for the four brightest sources
of IRS 13E give the scale of the proper motion vectors. The cross marked
X represents the nominal position of the X-ray source at the center of an
error box of ![]() ![]() |
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Figure 3:
At left,
NICMOS images of IRS 13E in the Pa![]() ![]() ![]() |
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The initial processing of each dataset is not described here. It can be found in the papers cited with the presentation of each of them. Below are listed the main operations which were performed subsequently to the initial data reduction.
To perform the extraction of the individual stars from the AO and NICMOS data, StarFinder, an IDL procedure (Diolaiti et al. 2000) specially written for AO data, was first used. The preliminary operation consists of building a good PSF by averaging several isolated and bright stars in each image. By adjusting the PSF to the local peaks in the field the exact position and the flux of the corresponding stars are retrieved by the procedure.
The image residuals left by applying this procedure led us to
suspect that more sources might be present in the IRS 13 complex, which
could not be detected because the spatial resolution was not high
enough to separate them. To improve the star detection we applied a new
deconvolution code called MCS (Magain et al. 1998). Contrary to the
StarFinder procedure, the MCS program uses an analytical PSF (a
Moffat-type function). The final PSF is chosen prior to deconvolution, with
a width compatible with the original image sampling. Thus, the final PSF can be narrower than the observed one without violating the Shannon
sampling theorem. The contribution of a continuous background is also
matched to the image with an adjustable smoothing. The sampling was high
enough for the Gemini AO images, in the H and the Kp bands, to provide a
substantial gain in resolution. Because of the sampling of the L-band image
a more limited gain was obtained, which was useful
anyway. For the H and the Kp images the width of the synthetic PSF is equal
to 0.040
and to 0.192
for the L-band image, i.e. a gain
in resolution respectively of a factor 4.5 in H, 4.3 in Kp and 1.5 in L. However, this method applied to the IRS 13 images demanded many trials
to reach a stable and plausible solution because of the presence of a non
uniform background in which the sources are embedded. The convergence was
helped by the comparison between the solutions for the H and Kp filters. We
imposed the sources to be detected in both bands, to avoid the risk of
taking deconvolution artifacts for sources. The two filter bandpasses are
almost adjacent which makes pertinent the detection in both filters, even
for very red or very blue objects. This code was not applied to the CFHT
K-band nor to the NICMOS images, which both PSFs show significant secondary
rings, because they are not correctly handled by this code, written for
seeing-limited images. Anyway, it was applied on the Gemini/Hokupa'a data
because of their low Strehl ratio.
All the NICMOS data were calibrated as described by Rieke
(1999). The calibration of the sources in the L band was based
on the photometry of IRS 13W reported by Clénet et al. (2001), a
prominent, isolated source in this band. The Gemini data in the H and the
Kp filters were not calibrated. In order to minimize photometric
uncertainties between data from various origins we calibrated the two filters by interpolation from the NICMOS data which offer a set of filters
close to H and Kp. Based on IRS 13E1, a bright and well-measured star by
MCS deconvolution in the stellar complex, and taking into account the
exact filter bandpasses, we applied a strict linear interpolation between
the F160W and F190N flux of this star for the H band (central wavelength 1.65 m), between F190N and F222M for the Kp band (central wavelength 2.12
m, see Table 1). With this source calibrated that
way in the H and the Kp filters the flux measurements of all the other
sources in the two filters were calibrated. The intensities of the
detected sources in the 8 filters were expressed in
Jy as were
originally the NICMOS data.
We obtained from Ott et al. (2003), who have conducted an analysis of ten years of SHARP data, providing more than 1000 proper motions in the central parsec, the proper motions of the main components of IRS 13 and of its nearby field.
Even though the BEAR data are not at the spatial resolution of the NICMOS
and AO data, the Br
and 2.06
m He I line profile at IRS 13E were used as complementary information to help determine the
spectral type of the underlying stars. The IRS 13 complex is located in a
region of interstellar emission, intense in Br
(Morris & Maillard
2000; Paumard et al. 2003a) as well as in the 2.06
m helium line (Paumard et al. 2001). Hence, the line
profiles had to be corrected to remove the ISM contribution, made here of
two main velocity components (Fig. 4 - central panel),
superimposed to the stellar profile. This operation required special
attention for the Br
profile since the ISM emission is locally very
intense. Only the high resolution (21.3 km s-1) made it possible to separate,
with some approximation anyway, the stellar profile which is naturally
broader, from the interstellar components.
Twenty sources in total (Table 2) are detected both in H and Kp
after MCS deconvolution of the Gemini data in the small IRS 13 field, which
are identified in Fig. 2. All these sources were searched
for with StarFinder for the filters where the deconvolution operation
could not be applied, and again with the MCS code in the L band. Empty
positions in Table 2 indicate that the source is not detectable
in a specific filter. An upper limit is given at some positions for the
extreme filters (1.1 m and L) where the detection is the most
difficult. This limit is not uniform in the field depending on the
proximity of another bright source, particularly in L.
Table 2:
Photometric measurements in all the filters, in log (Jy), of
all the stars detected by MCS deconvolution (Sect. 2.3) of the
IRS 13 field from the H and Kp Gemini AO images. Measurements in the NICMOS filters (F) were obtained by StarFinder, and in the L band by MCS analysis. The sources are listed by
order of decreasing brightness in the Kp band, except IRS 13W, which is
listed first. Values preceded by < in the 1.1
m and the L bands are
minima of detection, i.e. flux upper limits.
In conclusion, IRS 13E is resolved into a compact cluster of at least 7 objects encircled within 0.5
(Fig. 2). The two brightest sources, 13E1 and 13E2, had already been identified in Ott et al. (1999) and in Paumard et al. (2001). The one we had noted 13E3 appears double after deconvolution. Thus, we call the two components 13E3A and 13E3B. By continuity, the closest bright source, north of 13E3, is
called 13E4. Two other sources revealed by deconvolution are called 13E5
and 13E6.
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Figure 4:
He I 2.06 ![]() ![]() ![]() ![]() ![]() ![]() |
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The positions of the sources in the
IRS 13
field in offsets from SgrA
are given in Table 3. The
astrometry was retrieved from the results of the deconvolution of the
portion of the Gemini Kp-band image. The precision of the positions
depends on the brightness of the sources. For the brightest sources the
relative 1-
position error is equal to 1.0 mas. From these
positions the projected angular separations between the main IRS 13E
components are given in Table 4, translated to AU, by taking a
distance to the GC of 8 kpc (Reid 1993). All the sources of the small
field are detected by the reprocessing of the SHARP data we obtained prior
to publication from Ott et al. (2003), except that a single source
is given for E3A and E3B (
118) and E6 is not detected.
Table 3 offsets are estimated from the positions of isolated
stars around IRS 13E published by Ott et al. (1999) which, in this
paper are given relative to IRS 7. The offset of IRS 7 from
SgrA
estimated from VLA measurements, taken from Menten et al. (1997) is added. The identification of the sources is presented
in Fig. 2.
Table 3:
Offsets from SgrAa of the sources shown in
Fig. 2.
Table 4:
Projected separation of the main IRS 13E sources. From the
astrometry the precision on the distance betwen two sources is 15 AU.
We used this astrometry to examine the position of the X-ray source CXOGC J174539.7-290029 whose IRS 13 is proposed as the optical counterpart by
Baganoff et al. (2003). The X-ray source was placed in
Fig. 2 by estimating the offset of the source with respect
to SgrA
from the coordinates of both sources reported in Baganoff
et al. The 1-
error reported on the coordinates of each X-ray
source is of
0.2
in right ascension and
0.1
in declination. By combining the astrometric uncertainties on the two source positions we drew a minimum error box of
0.3
around
the nominal position. The resulting position falls outside the center of IRS 13E, which will be discussed in Sect. 6.
Table 2 has been used to derive the standard photometry in H, K, and L bands and the color indices of all the sources identified in Fig. 2. The Kp filter (Table 1) which is close and has a width comparable to K was used. It is referred to as K in the results presented in Table 5.
Table 5: H, K and L photometry of the IRS 13E cluster and of the nearby field stars from Table 2.
From the results communicated by Ott et al. (2003), the direction
and amplitude of the proper motions for the main IRS 13E sources and most
of the stars in the 2.5
field are represented in
Fig. 2. Note that the five sources, 13E1, 13E2, 13E3A, 13E3B
(proper motion is only given for the center of light of 13E3A and B) and 13E4 are all moving West, with a similar velocity, while all the nearby
stars have very different directions and amplitudes. The amplitude of the
proper motions of the four main sources, in angular motion per year (for a
GC distance of 8 kpc) and in velocity, are presented in
Table 6. The reported uncertainties come from the mean error on
the proper motion vector coordinates adjusted on the set of positions. The
rms uncertainty on 13E3 proper motion is the largest one, because of the
difficulty of measuring accurate positions from several epochs of SHARP data for a weak source so close to much brighter sources, 13E2 and 13E4. The error becomes large for data recorded with poor seeing
conditions.
Table 6: Amplitude of proper motions of the main IRS 13E sources (from Ott et al. 2003).
The Pa
emission line is tracing also in the two central parsecs the HII region called SgrA West or commonly the Minispiral. IRS 13E is
located just at the northern end of a bright emission arc, part of the
circular structure open toward SgrA
,
called the Minicavity. In Stolovy
et al. (1999) the bubble-like feature is proposed as created by
the wind from a star identified by a weak Pa
point source, since
located very near the geometric center of the Minicavity. A proper motion
of the edge of the Minicavity of
200 km s-1 is measured by Zhao &
Goss (1998). In Paumard et al. (2003a), it is shown that
the Minicavity is embedded inside a non-planar gas flow of the Minispiral,
called the Northern Arm. In the field of the Minicavity another
velocity structure is identified, stretched toward northwest, called the
Bar, roughly perpendicular to the bright edge of the Northern Arm.
On the line of sight of IRS 13E these two velocity structures produce the
two main components in the line profile of the ionized gas shown in
Fig. 4 central panel. The fastest component at -250 km s-1 is due to the motion of the gas disturbed by the Minicavity, and the slowest
component, at -39 km s-1, to the Bar. On a morphological basis IRS 13E would
seem to belong to the bright arc of the gas shocked at the edge of the
Minicavity. On the intensity map of the Bar, isolated by this
multi-component analysis, a small region just centered on the IRS 13E position appears locally enhanced. Therefore, the observed brightness of
the ionized gas around the position of IRS 13E is due to the addition of
two contributions on the line of sight: the edge of the Minicavity and the
locally excited gas of the Bar. Only this latter component is due to the
presence of the IRS 13E sources, by their strong ionizing flux.
Hence, IRS13E should be located close to or inside the Bar, which lays
behind the Minicavity (Paumard et al. 2003a).
As shown in Fig. 3, only two of the IRS 13E components,
IRS 13E2 and 13E4, remain after subtraction of the F190N continuum
from the F187N filter which contains Pa.
These two stars are
unambiguously emission line objects, with the integrated line intensity
at IRS 13E2 brighter by a factor
than at IRS 13E4.
The profiles of these two emission lines were obtained from spectra
extracted on the same aperture size ( pixels i.e.
)
from each BEAR data cube, at the IRS 13E position. With the spatial resolution of the BEAR data the contribution of
the two emission line stars 13E2 and 13E4 is mixed, to which is added the
ISM emission. After the most plausible subtraction to each line profile of
this latter contribution, shown on the central panel of Fig. 4,
possible thanks to the spectral resolution of the data, Br
appears
much narrower than the He I 2.06
m line. With a FWHM of 215 km s-1, compared to
900 km s-1 for the helium line, as already
measured in Paumard et al. (2001), the two lines should belong to
two different sources, which is discussed in Sect. 4.2.
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Figure 5:
Dereddened SED of the IRS 13 sources in W cm-2 ![]() ![]() ![]() ![]() |
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The photometric measurements presented in Table 2 were used to
build the SED between 1 and 4 m of the IRS 13 components
(Fig. 5), and also of all the stars detected in the
surrounding field. To achieve the final goal of determining the spectral
type of these stars, a dereddening has to be applied over this range. The
extinction over the central parsecs is highly variable (Blum et al. 1996; Rieke 1999; Scoville et al. 2003). We
took the most recently published law, which is a merging of previous works
(Moneti et al. 2001), and adjusted
,
making first the
simplifying assumption that the reddening toward IRS 13E and the few
arcsecs around would be identical for all the sources. For this adjustment
we were helped by two constraints: IRS 13E2 and 13E4 are emission line
sources (Fig. 3), therefore hot sources with a blue SED, while
IRS 13W is a cool star (Krabbe et al. 1995). Black-body curves
were fitted to the data.
was adjusted in order to fulfill the two
constraints. Higher values of
make the sources bluer, while lower
values make the sources redder. Temperatures higher than 25 000 K had to
be introduced for IRS 13E1, 13E2 and 13E4, which correspond to the
Rayleigh-Jeans regime in this spectral range. In that case, only a lower
limit of
can be derived. The slope of the SED becomes constant in
a log [F(
)] diagram. For these stars the adjustment of
makes
it possible to bring the SED parallel to the data points, providing the
strongest constraint on
.
Finally, a value of
was
adopted from the three hot stars, 13E1, 13E2 and 13E4. A mean value of
had been determined by Rieke (1999) from a survey of the
stars in the central parsec, which excluded objects like IRS 13 from the
color-magnitude diagram because of the surrounding dust. We confirm a much
higher
value for IRS 13E. On the other hand, for IRS 13E6
was too high for a good fit to the data. A value of 29 was more
appropriate. On the contrary, we will show in Sect. 4.7 that a
value of
had to be adopted for IRS 13W.
However, it appeared that the fit of the dereddened data from
Table 2 was not possible with a single temperature for the IRS 13 sources. Except 13E6, they show an infrared excess. Then, the fit was made
as a sum of two black-body curves,
.
The four parameters of adjustment obtained for each star are
reported in Table 7. The final SEDs are shown in
Fig. 5. From all the results presented above a spectral
classification of the stars detected in the IRS 13E cluster is proposed.
Table 7: Fitting parameters of the SED of the IRS 13 sources. The spectral type of each source, as discussed in Sect. 4, is summarized in the last column.
This source is characterized by:
A decisive element is reported by Clénet et al. (2003). From
spectro-imaging with a FP in the 2.06 m helium line behind the
CFHT-AO system they indicate that IRS 13E2 is the only helium emitter. The
radical difference of linewidth between the He I 2.06
m and
Br
stellar line profiles at IRS 13E (Fig. 4) brings
another piece of information. A similar difference of linewidth of the
He I 2.06
m lines is observed among the hot stars of the
central cluster (Paumard et al. 2001, 2003b), leading to the
identification of two classes of massive stars. Applying this criterion to 13E2 and 13E4, and taking into account the detection of He I 2.06
m only at IRS 13E2, it can be concluded that IRS 13E2 as a
strong helium emitter with broad line is a late-type WR star, probably of
WN type (Figer et al. 1999) since no detection of C III or
C IV typical of WC type is reported from earlier K-band spectra. A
K estimated for the source (Table 7) is
consistent with this identification. IRS 13E4 with also a
K (Table 7), but no helium emission, source of the
narrower Br
profile, is more likely a less evolved star than
IRS 13E2. From its SED and its absolute brightness in K, fainter than IRS 13E1, IRS 13E4 can be reasonably proposed as a O5IIIe star or just
reaching the LBV stage but in a high extinction phase to explain its weakness.
However, there is an apparent contradiction between the brightness of IRS 13E2 in Pa
in Fig. 3 and the Br
profile at IRS 13E
from the BEAR data after correction for the ISM emission
(Fig. 4). As Pa
appears strong at IRS 13E2 in
Fig. 3, the Br
profile at IRS 13E should be dominated by
the emission from this source and be as wide as the He I 2.06
m profile. That is not what is observed as shown in
Fig. 4. To reconcile these two facts, it must be noticed that
Pa
is far from a perfect indicator with narrow-band imaging technique,
to distinguish between hydrogen-rich and helium-rich emitters since the
Pa
line (1.8751
m) is blended with a strong helium line,
He I (4-3) at 1.8697
m. There is another helium line within the
bandpass of the continuum filter, at 1.9089
m, but which is weak and
will contribute to subtract only a little of the helium emission. All these
features are well seen in the CGS4 spectrum of the AF star around Pa
,
presented by Najarro et al. (1994), which is another helium star
belonging to the class of the broad-line stars (Paumard et al. 2001, 2003b). Hence, the intensity in the F187N-F190N image at the star position cannot be considered as a fully reliable
measurement of the true Pa
emission in the stellar atmosphere. The
bright spot at IRS 13E2 in Fig. 3 is likely due to the He I 1.8697
m line and with some contribution of the Pa
emission. That is consistent with the Br
profile shown in
Fig. 4 which can be decomposed into a narrow line, whose origin
must be IRS 13E4, and a fainter, broad component of same width as the
He I 2.06
m line, which should be the contribution of IRS 13E2 to the observed profile. The residual Br
profile will
show a P Cyg profile, typical of hot stars with an atmosphere in expansion,
consistent with the spectral type attributed to IRS E4.
IRS 13E3 resolves into a double source in the deconvolved AO images in Hand Kp (Fig. 2). Their projected separation is equal to 600 AU (Table 4). The two sources are photometrically
quite identical. They are extremely red objects as indicated by the
measurements in Table 5, and from their SED (Fig. 5). They are faint in the H band and prominent in the
L band. We measure a K-L color index of
5 mag. for the two components (Table 5). Several sources in the inner parsec,
mainly located along the Northern Arm (IRS 1W, 2, 3, 5, 10W, 21),
are also very red objects, with K-L > 3, reported in Clénet et al. (2001). IRS 1W and IRS 21 have featureless spectra in
the K band (Blum et al. 1996). IRS 21 has been studied in detail,
from 2 to 25
m by Tanner et al. (2002). They have fitted
its SED by a two-component model, the near-infrared scattered light from
the central source peaking at
3.8
m (760 K), and the
mid-infrared re-emitted light from the dust shell at
250 K. They
conclude that IRS 21 is a dusty WR star, experiencing rapid mass loss as
well as the other luminous Northern Arm sources (Tanner et al. 2003). The IRS 13E3 SEDs are also fitted by two components
(Table 7), but with respectively 3800 K and 600 K. IRS 13E3A and E3B are likely sources of the same type as IRS 21 and the other Northern Arm sources. The higher temperature of the
infrared component can come from the additional heating of the dust shell
by the very close, massive blue stars, IRS 13E1, 13E2 and 13E4. That is
also consistent with IRS 13 being not a prominent source at 12.5
m
on the images in this band (Tanner et al. 2002) compared to the
other Northern Arm sources.
One of the sources revealed by deconvolution of the AO images (Fig. 2) we propose to name IRS 13E5, is also present in the SHARP data (Ott et al. 2003). From the dereddened photometry (Fig. 5) this source has a SED similar to IRS 13E3A and E3B, being roughly a factor 2 fainter than each of the IRS 13E3 components. Its SED is also fitted by two thermal components, with temperatures of the same order as for 13E3A and E3B (Table 7). From this similarity we propose that IRS 13E5 is another example of dusty WR star, possibly more embedded, behind IRS 13E3A and E3B.
IRS 13E6 is detected in H, in the F160W filter, a broader H filter, and
near the detection limit of the Kp band (Table 2) and not in L.
It is not detected by Ott et al. (2003) whose data come from K-band
imaging on the NTT. With
K and
(Table 5) 13E6 is a weak, hot star, 3.5 mag fainter than IRS 13E1. It can be considered close to a main sequence O5V star. In
Fig. 5 the best fit to the data was obtained with a value
of
lower than for the other IRS 13E sources. It should mean that
IRS 13E6 is located on the line of sight, but in front of the IRS 13E complex.
![]() |
Figure 6:
Map of the residual fitted continuum in the deconvolution
operation of the L-band image with the MCS code. The identified stars
in this band (Table 2) have been subtracted explaining the
dark holes in the map (e.g. at the W position). Source positions are
marked by crosses with their names. To limit confusion, for the E3 binary only E3B position is indicated. The apparent halo might be not
purely thermal emission of dust and contain more fainter embedded
sources (Eckart et al. 2003). An estimation of the halo
contrast is given for the brightest pixel (position -3.1
![]() ![]() |
Open with DEXTER |
This star is associated with IRS 13 only for historical reasons. We kept
it in all the study since its cool stellar type, confirmed by the presence
of CO in its K-band spectrum (Krabbe et al. 1995), was a constraint
on the determination of the local value of .
In the deconvolved
Kp-band image 13W shows a larger size than the PSF which might be
indicative of a dusty envelope, attested also by a significant infrared
excess (Table 7). However, in the first analysis with the
assumption of the same
value for all the IRS 13 sources a
of
2600 K was obtained for 13W. With this temperature the cool star
should be of Mira-type. A H+K spectrum would show the deep absorption of
water vapor on each side of the K band. With the NICMOS data we have a
photometric measurement at 1.90
m, just in the water vapor
band. Figure 5 shows a smooth distribution of data
points. Thus, we must conclude that this temperature is too low. To
increase this temperature in the fit a higher
value must be assumed
locally. With the four parameters of the fit (Table 7) plus
the solution is not unique. A plausible solution is obtained with
and
K. A higher temperature would require a
higher
value which would become inconsistent with the non detection
of IRS 13W at 1.1
m (Table 2). With this
temperature and its luminosity IRS 13W can be assumed to be a M 3 giant
star.
IRS 13E appears as only composed of hot, massive stars. This
concentration within 0.5
cannot be fortuitous. Further deep
high-spatial resolution imaging could eventually reveal more components
(Eckart et al. 2003). The common westward direction and similar
amplitude of the proper motions with a mean value of
280 km s-1 for
the main components (Sect. 3.4 and Table 6) is a
decisive argument to indicate that 13E1, 13E2, 13E3A/B and 13E4 are
physically bound. Regarding 13E5 for which a proper motion cannot be
currently measured to associate it unambiguously to the cluster, its
spectral type, identical to 13E3A/B, leads us to conclude that this source
belongs to IRS 13E too. For IRS 13E6 the proper motion is not available
either, but the value of
6 mag lower than for IRS 13E likely
indicates that the source does not belong to the cluster. The source is
another O-type star, just located on the line of sight. However, in
conclusion, the source historically called IRS 13E is a compact, massive
star cluster with at least six members. That also means a young star
cluster of a few 106 yr old, since several members are identified as
having already reached the WR stage.
The compactness of the cluster and the common proper motion of the
components raise the question of the force which keeps the massive stars
bound. The hypothesis of the presence of a dark, massive source, a stellar
black hole at the center of IRS 13E is natural. An intermediate-mass black
hole (IMBH 103 to 10
)
is supposed to form by runaway
growth in massive, young stellar clusters as a result of stellar
collisions in the cluster center, as was modeled by Portegies Zwart &
McMillan (2002). Constraints on the possible central mass could
be obtained from the radial velocities of the sources. It is estimated
only for the two emission line stars, with a positive velocity of
30 km s-1 for IRS 13E2 and a negative velocity of
30 km s-1 for IRS 13E4 (Fig. 4). Associated with
parallel and equal proper motion vectors (Fig. 2), this
suggests that both stars orbit around the center of mass in a plane
orthogonal to the plane of the sky. Assuming that both stars orbit around
the black hole in a symmetrical fashion on circular orbits, half the
projected separation between IRS 13E2 and IRS 13E4 (Table 4)
gives an orbit radius of 1300 AU
where i is the angle that the
line containing both stars makes with the plane of the sky. The radial
velocity of each star (
30 km s-1) can be used as an estimate
for their orbital velocity. A period of rotation of
1295 yr is
derived. Then, the 3rd Kepler's law gives directly a total mass of
kpc. We can also release the
circular orbits hypothesis, and only assume that the system is bound,
which means that the potential energy of each stars is greater than its
kinetic energy. This gives a lower limit to the black hole mass, half the
previous estimate:
kpc. The
dependence of these two values is quadratic on the orbital velocity of the
stars and linear on their distance to the black hole, so that the
constraint
can be considered rather robust.
It falls within the range derived by Portegies Zwart & McMillan
(2002) for a black hole formed in the core of a dense star
cluster with massive stars of initial mass
50
.
The large number of massive stars in the central parsec, which are very
rare elsewhere in the Galaxy, remains one of the major mysteries of this
region. Since star formation would be difficult due to the strong tidal
forces from the SgrA
black hole, Gerhard (2001) made the
interesting hypothesis that the central parsec He I stars, the most
prominent of the massive young stars, might be the remains of a dissolved,
young cluster, which originally formed further away from SgrA
.
He argued
that the Arches and the Quintuplet clusters, located within a projected
distance of
30 pc from SgrA
,
testify that star formation by
cluster of massive stars has been occurring in the nuclear disk of the
Galaxy. If one examines Table 7, IRS 13E appears as a kind of
summary of all the spectral types of young stars observed in the central
parsec from O to WR. Among the helium stars only IRS 13E appeared multiple,
which motivated the current study. With the hypothesis that IRS 13E might
be the remaining core of a massive star cluster, was this cluster the source
of the population of massive stars observed in the central parsec?
Morris (1993) has argued that it would take longer than the
lifetime of massive young stars to transport them inward within the central
parsec if they formed at too large distance. The same argument is applied
again by Figer et al. (2000), who claim that the SgrA
cluster
(Genzel et al. 1996; Ghez et al. 1999; Gezari et al. 2002) could not have formed more than 0.1 pc from the center, and
then, the initial clump should have an exceptional density
cm3. Gerhard (2001) discarded this
argument by a revision of the conditions for a cluster formed at 30 pc to
spiral into the center within the lifetime of its most massive stars. The
main condition is that the initial mass of the parent cloud in which the
cluster forms must be massive enough (
)
to survive the evaporation in the strong tidal field of the nuclear
bulge. He concludes that clusters significantly more massive than the
Arches cluster and formed a little closer than 30 pc can reach the central
parsec in due time. In order to test this statement Kim & Morris
(2003) have made several simulations, for different masses
(10
and 10
)
and different initial orbit radii (2.5 to 30 pc), of the dynamical friction on a star cluster near SgrA
.
They came to the conclusion that some simulations can be regarded as
candidates for the origin of the central parsec cluster, but that the required conditions are extreme, with an initial mass of the
cluster of 10
or a very dense core
pc-3 (Kim et al. 2003). A mass of the
cluster of 10
supposes a very large number of particles,
with some of initial mass
10
to reach rapidly the WR stage. Compared to the relatively small number of detected helium stars,
concentrated in the central parsec,
19 from the revision by Paumard
et al. (2003b), plus few more dusty WRs, a very large quantity of
O-type stars (>105) should be detected, which is not the case from
the proportion of such stars we count in the IRS 13 test field (Sect. 4.8). This might seem an argument
against this scenario.
However, another analytic work by McMillan & Portegies Zwart
(2003) has reconsidered the fate of a star cluster near the
central dark mass. They tried to address the problem more completely by
taking into account, in addition to the initial mass and the distance to
the center, the original mass function of the cluster, the initial cluster
radius and the stellar evolution through mass loss during the inspiral
time of the cluster. They conclude that star clusters born with masses 10
within 20 pc from the center, with a half-mass
radii of
0.2 pc can reach a final distance of 1 pc within 10 Myr.
As a secondary conclusion, they assess that from their mass and their
distances, the Arches and the Quintuplet clusters, will never reach the
vicinity of SgrA
.
This latter work makes the origin of the central,
massive star cluster by the dissolution of a compact cluster in the
galactic tidal field more plausible, not requiring extreme mass conditions
as in the simulation of Kim et al. (2003). In conclusion, we
propose that the IRS 13E cluster, by its unique location and composition,
is the possible core of an earlier massive star cluster, formed about 10 Myr ago, within 20 pc of SgrA
,
with a mass of
10
,
which was the progenitor of the entire hot
star population, from WR to O-type stars, observed today in the central
parsecs of the Galaxy. In addition, the hypothesis of IRS 13E harboring a
IMBH as consequences on this scenario.
A recent paper by Hansen & Milosavljevic (2003) came to
our attention when the current work on IRS 13 was completed. The authors
try to solve the difficulty of bringing disparate groups of hot stars, the
WR-type stars, the LBV-type stars and the S-cluster, at their observed
location within the timescale required within a single evolutionary
scenario. They argue that massive star clusters can sink within the
required star lifetime but are tidally disrupted at a distance greater than 1 parsec from SgrA,
from which it would result a population of sources
with low binding energy orbits unlike those of the helium stars and
particularly of the S-cluster stars orbiting within 0.1 pc of SgrA
*. For
this purpose they propose a model with an infalling IMBH. The stellar
orbits continue to evolve by undergoing close encounters with the IMBH, bringing
some stars near enough to be trapped by the massive BH. This model appears as a
refinement of the same idea of an origin of the young star population in
the central parsec as formed at a distance of the SgrA
> 10 pc where
star formation can occur, in a dense star cluster sinking towards SgrA
.
We
propose IRS 13E with a IMBH as the possible remnant of this initial cluster.
The presence of a compact star cluster of six hot, massive stars at the
position of IRS 13E from high-resolution near-infrared observations is
demonstrated. The spectral types of the various members range from O to WR,
including dusty WRs. Proper motion measurements indicate that the brightest
stars are co-moving suggesting that the members of the cluster are bound by
a central IMBH with a mass 1300
.
Such a secondary
black-hole in the vicinity of SgrA
could be the element needed to
explain the population of massive young stars observed today (Hansen &
Milosavljevic 2003).
To precise the spectral type of the components, better constrain the mass
of the IMBH, spectroscopy in the 1-5 m range of all the individual
sources within IRS 13E, at angular resolution as good as 0.1
,
is
required. In the L and M bands it should confirm the expected featureless
spectrum, except dust signatures, of the IRS 13E3 and 13E5 objects. These
studies will need near-infrared 3D spectrometers behind an AO system on a
8-m telescope, like SINFONI (Mengel et al. 2000 and AMBER behind
VLTI (Petrov et al. 2000). Deeper AO imaging, as already obtained
with NAOS/CONICA north of the IRS 13E center (Eckart et al. 2003), could make it possible to detect more members of the
cluster. Proper motions of the fainter members would help to confirm
which of the individual sources are kinematically bound together.
Theoretical work is needed to confirm whether IRS 13E can be the remnant
of a massive cluster. More generally, to address the problem of recent
star formation in the vicinity of SgrA
a full census of the hot star
population, illustrated in this paper on a small field, remains to be
completed. It can be done by deep AO imaging to the condition to acquire
data down to 1
m. Up to now, such data, which appeared essential as
shown in the current analysis, have only been possible with NICMOS on HST. Search for optical counterparts of the star-like X-ray sources
detected by Chandra in the central parsec is another objective.
Acknowledgements
We gratefully acknowledge helpful discussions with R. Coker which stimulated the close examination of the high-resolution data available on IRS 13. We want to warmly thank Y. Clénet (Meudon Observatory) who made available to us his L band AO data. Thanks to the organizing committee led by Tom Geballe, the GC02 Workshop in Kona (Nov. 2002) was an ideal place to improve various issues raised in this paper. Special thanks are also due to T. Ott who sent us the proper motions of the sources of the IRS 13 field, prior to publication.