J. Moultaka1 - A. Eckart1 - T. Viehmann1 - N. Mouawad1 - C. Straubmeier1 - T. Ott2 - R. Schödel1
1 - I Physikalishes Institut,
Zülpicher Str. 77,
50937 Köln, Germany
2 -
Max Planck Institut für extraterrestrische Physik,
Giessenbachstrasse, 85748 Garching, Germany
Received 4 December 2003 / Accepted 3 June 2004
Abstract
We present the first L-band spectroscopic observations of a dozen stellar sources in the central 0.5 pc of the GC stellar cluster that are
bright in the 2-4 m wavelength domain. The L-band data were
taken with ISAAC at the VLT UT1 (Antu). With the aid of additional
K-band spectroscopic data we derive the optical depth spectra of the
sources after fitting their continuum emission with a single reddened
blackbody continuum. We also derive intrinsic source spectra by
correcting the line of sight extinction via the optical depth spectrum of a late type star that is most likely not affected by local dust
emission or extinction at the Galactic Center. The good agreement
between the two approaches shows that the overall variation of the
line-of-sight extinction across the central 0.5 pc is
mag. The extinction-corrected spectra of the
hot He-stars resemble pure Rayleigh-Jeans continuum
spectra. The intrinsic spectra of all other sources are in agreement
with being the result of the continuum emission and absorption features due to the dust
in which they are embedded. We interprete both facts as evidence that
a significant amount of the absorption takes place within the central
parsec of the Galactic Center and is most likely associated with the
individual sources there. We find absorption features at
m,
m, and
m wavelength. Correlations
between all
three features show that they are very likely to arise in
the ISM of the central 0.5 pc.
Spatially highly
variable hydrogen emission lines seen towards the individual sources
give evidence of the complex density and temperature structure of the
mini-spiral. The featureless K-band
spectra of sources like IRS 21 and IRS 1W are consistent with these sources being massive hot stars embedded in the bow shock created by their motion
through the dust and gas of the mini-spiral. The
bow shock scenario may be applicable to most of the dust-embedded
sources in the central stellar cluster. Spectroscopy of high
MIR-excess sources 0.5
north of the IRS 13 complex is largely
consistent with them being YSOs. However, a bow-shock nature of these
sources cannot be excluded. The L-band spectrum at the location of SgrA* closely resembles that of a hot O-type star, such as S2, which
was very close to Sgr A* at the time of our observations.
Key words: Galaxy: center - galaxies: nuclei - infrared: stars - infrared: ISM
Near-infrared diffraction limited imaging over the past 10 years
(Eckart & Genzel 1996; Genzel et al. 1997; Ghez et al. 1998, 2000;
Eckart et al. 2002; Schödel et al. 2002, 2003; Ghez et al. 2003)
has yielded convincing evidence for a
black
hole at the center of the Milky Way. This finding is supported by the
discovery of a variable X-ray and NIR source at the position of SgrA*
(Baganoff et al. 2001; Genzel et al. 2003a). Most intriguingly,
near-infrared imaging and spectroscopic observations have provided
evidence for recent star formation in the central parsec of the Milky
Way, an environment previously thought hostile to star formation
because of the tidal field of the black hole, intense stellar winds,
and strong magnetic fields.
At a distance of 8 kpc (Eisenhauer et al. 2003), the Galactic Center is
surrounded by a circumnuclear ring of dense gas and dust showing
clumpy extinction (Güsten et al. 1987). Inside this ring, there is a
central cavity of about 3 pc diameter that contains mainly ionized or
atomic gas. The visual extinction estimates towards prominent sources
within the central stellar cluster range between 20 and 50 mag
with a median around 30 mag (see Rieke et al. 1989;
Chan et al. 1997; Scoville et al. 2003). In addition Scoville et al. (2003) showed that the extinction is smoothly distributed across
the central 10 to 20 arcsec with no indication of concentrations
of extinction on scales of about 1
to 2
.
The visual extinction by 30 mag along the line of sight
toward the Galactic Center (GC) is mostly due to the diffuse
interstellar medium (ISM) (Lebofsky 1979) and in part to
dense molecular gas (Gerakines et al. 1999; de Graauw et al. 1996;
Lutz et al. 1996). The absorbing gas is cold (10 K) and the
abundances of important molecular species are similar to those in the solar
neighborhood (Moneti et al. 2001a; Chiar et al. 2000).
In addition Blum et al. (1996) and Clénet et al. (2001) concluded
that the colours of individual dusty sources within the central stellar
cluster contain a substantial contribution from intrinsic reddening.
The entire central parsec of our Galaxy is powered by
a cluster of young and massive stars (Blum et al. 1988; Krabbe et al. 1995;
Genzel et al. 1996; Eckart et al. 1999; Clénet et al. 2001).
Within that cluster the 7 most luminous (
L> 105.75 ),
moderately hot (
T< 104.5 K) blue supergiants
contribute half of the ionizing luminosity of that region
(Najarro et al. 1997; Krabbe et al. 1995; Blum et al. 1995).
Such massive and hot stars were also found in dense clusters
within the Galactic bulge, i.e. the Arches cluster (Cotera et al. 1992,
see also Figer et al. 2002 and references therein) and the
Quintuplet cluster (e.g. Figer et al. 1997).
In addition to the massive blue supergiants, a population of dusty sources associated with bright dust emission can be found in the Galactic Center stellar cluster. After initial work by Becklin & Neugebauer (1968, 1969) the first individual mid-infrared sources in the central stellar cluster (among them IRS 1, 3 and others) were reported by Rieke & Low (1973) and Becklin & Neugebauer (1975). Later, IRS 1 was resolved into multiple components by Storey & Allen (1983), Rieke et al. (1989), Simon et al. (1990), and Herbst et al. (1993). Further high resolution imaging by Tollestrup et al. (1989) resolved IRS 6 and IRS 12 into multiple components.
In this paper, we discuss MIR sources that are located well within the
central stellar cluster at projected distances from Sgr A* of less
than 0.5 pc (Fig. 1). Several sources like IRS 1, 3, and 21 are dominated by dust emission and are strong at a
wavelength of 10 m, whereas the supergiant IRS 7 is brightest at 2.2
m. The nature of the dust-enshrouded sources is still
unknown. Among the best studied cases is IRS 21, which is
strongly polarized (17% at 2
m; Eckart et al. 1995; Ott et al. 1999; Krabbe et al. 1995). Initially, Gezari et al. (1985)
suggested that IRS 21 is an externally heated, high-density dust
clump. Given the MIR excess and the featureless NIR spectra several
other classifications have been proposed, including an embedded
early-type star and a protostar (Blum et al. 1988; Krabbe et al. 1995;
Genzel et al. 1996; Clénet et al. 2001). Tanner et al. (2002)
suggest that IRS 21 is an optically thick dust shell surrounding a
mass-losing source, such as a dusty recently formed WC9 Wolf-Rayet
star. Tanner et al. (2002, 2003) indicate that the extended dust
emission of most of the central sources is consistent with bow shocks
created by the motion of massive hot stars through the dust and gas of
the mini-spiral.
One way of investigating the nature of these bright NIR/MIR sources is
by imaging and spectroscopy in the 2 to 4 m wavelength range.
In addition to hydrogen and helium recombination lines,
this wavelength domain is dominated by strong absorption features
due to abundant molecules (NH3, CH3OH, H2O, CO, CO2 etc.), functional groups (like NH2, CH2), and ices.
Here H2O ice enriched with molecular material is of special importance.
Liquid, crystalline, amorphous water ice as well as
trapped water ice in SiO condensate (Wada et al. 1991)
give rise to a rich variety in shapes of a prominent feature
with its deepest absorption at 2.94-3.00
m
(e.g. Wada et al. 1991). The variety in shapes of the water ice feature is dependent not only on temperature, but also on annealing history and on the ice composition etc. (Hagen et al. 1983; Tielens & Hagen 1982; Tielens et al. 1983; Kitta & Kratschmer 1983; Hudgins et al. 1993; Maldoni et al. 1998).
The emission of dust and the absorption features of ices are important diagnostic tools for the investigation of the interstellar medium and circumstellar environments of individual sources.
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Figure 1: L-band image as obtained with ISAAC on the VLT UT1. The four slit positions chosen for the spectroscopic observations are also shown. They all include the position of SgrA*. The position of the CO-star used for the calibration along the line of sight is shown as well. |
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Infrared sources towards the Galactic Center show a wealth of ice
absorption features (Butchart et al. 1986; Sandford et al. 1991)
indicative of a broad range of organic material mostly in the diffuse
interstellar medium. Aliphatic hydrocarbons are characterized by
their CH2 (methylene) and CH3 (methyl) stretching modes around 3.4 m (Sandford et al. 1991; Pendleton et al. 1994). Aromatic
hydrocarbons are detected via their CH and CC stretching modes at 3.28 and 6.2
m. (Chiar et al. 2000; Pendleton et al. 1994). An
absorption feature at 3.25
m has been found towards dense
molecular clouds. It is attributed to aromatic hydrocarbon molecules
at low temperatures (Sellgren et al. 1995; Brooke et al. 1999).
Differences in the exact central wavelength and profile width of the
absorption near 3.3
m are mostly attributed to differences in
temperature and/or carrier of the absorbing molecules in these
regions.
In this paper we present 3 to 4 m imaging and spectroscopy
data combined with near-infrared 2.2
m spectroscopy of the
strongest mid-infrared sources in the central stellar cluster. In addition to the previously published L-band observations of IRS 1W, IRS 3 and IRS 7 we provide the first L-band spectra of 9 other MIR sources: IRS 9, IRS 13, IRS 13N, IRS 21, IRS 29 and IRS 16 C, CC, NE and SW. These
data on sources located in the central 0.5 pc of the GC enable us to study
the properties of the local interstellar medium and of circumstellar
matter in this region.
To investigate the nature of the dust-embedded sources within the central 0.5 pc of the nuclear stellar cluster we used near- and mid-infrared imaging and spectroscopy. In the following we describe the instrumentation and the data reduction.
Mid-infrared imaging and spectroscopy was obtained using the ISAAC instrument on the ESO Very Large Telescope (VLT) unit telescope UT1 (Antu), at the Paranal observatory in Chile. We have performed spectroscopic and imaging observations of the Galactic Center during May 23-30, 2002, as part of a monitoring program of SgrA* (Eckart et al. 2003; Baganoff et al. 2003). All the data reduction has been performed using routines from the IRAF and MIDAS software packages.
For the imaging in the L-band, paired flat fielded images at different
chopper throws (18
)
and chopping position angles (0
to 180
)
were subtracted from each other, resulting in frames
containing a positive and a (shifted) negative image. The frames were
then shifted to a common reference point that coincides with a
positive image of a source. Subsequently, frames belonging to the
same batch, i.e. taken sequentially with identical or different
chopper throws and/or chopping angles were combined by calculating the
median. Since the images were moved to a common reference point this
procedure eliminates the negative "shadows'' generated by the
subtractions. This procedure also effectively removes cosmic rays and
bad pixel structures. Such a batch typically consists of up to 40 images, and each resulting combined image covers an integration time
of approximately 35 min. For the present investigation we used
the images with the best seeing and image quality with an angular
resolution of about 0.4
.
The absolute L-band flux calibration (see
magnitudes listed in Table 1) was performed using the
fluxes of several bright well isolated objects also measured by
Clénet et al. (2001).
The spectroscopic observations were performed with the long-wavelength
(LWS3) and low resolution (LW) mode using the SL filter covering the wavelength range of 2.7
m and
4.4
m, respectively. The use of a 0.6
slit width
implied a spectral resolution (
)
of R=600 in
that wavelength domain. The seeing at this time was in the range between 0.4 and 0.9 arcsec. To compensate for the thermal
background, separate chopped observations were carried out using
chopper throws of
18 arcsec and random nodding within 2 arcsec along the slit. We adopted 4 different slit positions that all included the SgrA* location
(see Fig. 1). The resulting images were divided by
flat-fields, corrected for cosmic rays, for sky lines and for dispersion-related distortion. The wavelength calibration was performed
using a xenon-argon lamp.
Two chopped frames (with shifted image positions) were then subtracted from each other to provide a single frame containing two negative trace images and a positive one with twice the intensity of the negative images. After extraction of the individual source spectra they were corrected for wavelength dependent sensitivity, atmospheric transmission, and telluric lines using two standard stars HD 194636 (B4V) and HD 148703 (B2III-IV).
The spectra were normalized to the given K- and L-band magnitudes listed in Table 1.
Table 1:
Results from fitting a single reddened blackbody to the K- and L-band spectra.
In addition to the blackbody temperature and the K-band extinction obtained from the fitting procedure, we list the L-band
magnitudes (0.2 mag uncertainty) we derived from our images.
The near-infrared data in the K-band (2.2 m) were obtained via
integral field spectroscopy using the imaging spectrograph 3D (Weitzel et al. 1996) combined with the tip/tilt corrector ROGUE
(Thatte et al. 1995) at the ESO/MPG 2.2 m located at La Silla.
This instrument allows observations of a continuous 2-dimensional
field (
pixels) while providing spectral information
for each spatial image element.
These seeing-limited observations were done
with a pixel scale of
resulting in a field
of view of
for individual pointings.
Using a spectral resolution of R=2000
each half of the K-band had to be covered separately.
The observations were centered on the IRS 16 cluster.
In March 1996, the central parsec was observed with a spectral coverage
from 1.97 to 2.21 m (lower half of the K-band).
In total 17 different but overlapping regions were observed.
The total sky area covered amounts to
in the East-West
and
in the North-South direction.
The upper half of the K-band from 2.18 to 2.45
m was
observed during a second observing run in April 1996.
Here a total of 52 overlapping regions was observed.
This resulted in an area of
in the North-South direction
and
in East-West.
For further details of the observations see
Genzel et al. (1997), Ott et al. (2003).
To calibrate the wavelength scale, spectral lamp data (argon lamps in this case) were taken at the beginning or the end of each observing night. As a further step, calibration sources with a known spectrum were observed at a similar airmass as the Galactic Center. These standard stars were divided by a spectrum of the same stellar type (Kleinmann & Hall 1986) to remove stellar features, resulting in an atmospheric transmission spectrum. The source data were then divided by this spectrum.
The patchy extinction towards the GC stellar cluster (see e.g. Scoville et al. 2003) demands a careful calibration of spectral data. Therefore, the correction for the extinction along the line of sight has been carried out using the method described in Sect. 3.1 and tested using another, independent method described in Sect. 3.2. A cross-check shows that both our two approaches agree very well (see the results in Sect. 3.2.2 and the conclusion (Sect. 3.3) at the end of this section) and that the extraction of the optical depth spectra described in Sect. 4 can be done safely.
To estimate the extinction towards the individual sources and
to determine the approximate continuum shape of the observed spectra,
we have performed simultaneous fits of our K- and L-band data with
single, reddened blackbody spectra. In this process we have
considered the continuum emission, fitted locally around m,
m, and
m wavelength, as representing the
intrinsic, reddened continuum of the sources. To find the
best fit, the temperature of the blackbody was allowed to vary within
given limits with a step size of 100 K. For the helium stars these limits
were 20 000 and 26 000 K (Najarro et al. 1997). For the remaining
objects the limits were 200 K and 4000 K. We allowed the K-band
absorption AK to vary over a range between 2.7 and 4.5 mag with
a step sizes of 0.05 mag assuming the extinction law
stated by Martin & Whittet (1990).
We extended the fitting range towards higher AK values to
allow a search for higher extinction towards the dusty sources.
In general the fitting is not very sensitive with respect to the
determination of AK due to the strong H2O absorption
feature that dominates almost the entire L-band.
There are several potential problems that one has to consider when using this procedure (see also discussion in Chiar et al. 2002):
We have to take into account that our K- and L-band data have not been taken at the same epochs and in some cases their relative calibration may be affected by variability. Some sources in the central parsec of the Milky Way are variable on time scales of months to years (Ott et al. 1999; Blum et al. 1996; Tamura et al. 1994, 1996). To achieve a successful combined fit over both bands we therefore used different overall flux calibrations of the spectra. For the K-band fluxes, we considered data from 5 different references: Becklin et al. (1978), Tollestrup et al. (1989), Blum et al. (1996), Ott et al. (1999), and Clénet et al. (2001). The observations of Clénet et al. (2001) are the most recent of the four references and have been obtained with Adaptive Optics. For this reason, we have preferentially used the values given in Clénet et al. (2001) for the fitting procedure. However, for some sources a better fit was obtained when a different flux calibration was used. This was the case for IRS 7, IRS 9, the stars in the IRS 16 complex as well as IRS 13. IRS 7 and IRS 9 are variable as shown in Ott et al. (1999), Blum et al. (1996), and Tamura et al. (1994, 1996). The relative flux density calibration from Becklin et al. (1978) results in the best fits for our data, since their observations in the K- and L-bands were performed in the same year. The IRS 13 complex consists of at least three bright members, E1, E2, and E3 (Paumard et al. 2001; Maillard et al. 2003). They contain the hottest and most luminous star in the entire region (Najarro et al. 1997). The IRS 13 complex therefore is very likely to be variable in flux density. In this case the fitting results using the K-band fluxes by Blum et al. (1996) were best.
The blackbody temperatures and K-band extinction resulting from the best fits of the spectra are listed in Table 1.
Moreover, most of the spectra are heavily affected by broad absorption features and it is difficult to determine a clear measure of the underlying continuum emission. Also, the assumption of a single temperature blackbody continuum can only be taken as a first approximation. On the other hand, reddened multi-temperature models quickly result in a larger number of not well determined parameters (e.g. temperature and relative flux density contribution for each component).
The resulting best fits of the individual spectra with a single blackbody are shown in Figs. 2 to 4.
We list the resulting parameters of the fits in Table 1 and the spectra with the corresponding blackbody curves in Figs. 2-4. The temperatures and the K-band extinctions derived by the fitting procedure agree well with those found by other authors: The He stars of the IRS 16 complex are best fitted with temperatures that are similar to those found by Najarro et al. (1997). IRS 7 and IRS 9 have typical temperatures of late-type supergiants or giant stars (Chiar et al. 2002; Ott et al. 1999) and the remaining dusty sources are well fitted by blackbody continua of typical temperatures for hot dust (Tanner et al. 2002; Gezari et al. 1996; Genzel et al. 1997; Blum et al. 1996, and others).
The overall shapes we obtain by fitting single-temperature blackbody curves are reasonable and compare favorably to those of Chiar et al. (2002) (especially in the case of the three sources in common, IRS 1W, IRS 3 and IRS 7). In addition, they are supported by an independent procedure to calibrate the line of sight absorption described in the following section (Sect. 3.2). Also, the inclusion of the K-band spectra, which were missing in the work by Chiar et al. (2002) makes it possible to better judge the quality of the fitting procedure by comparison with the K-band continuum fluxes and spectral shapes, especially for the three sources in common (IRS 1W, IRS 3 and IRS 7).
Despite the improvements in the spectral fitting, there are some mismatches we want to comment on:
The fit of the IRS 1W spectrum shown in Fig. 2 does not
match the K-band spectrum of this source and shows a continuum level mismatch in the m to
m region
that has no physical significance. This is certainly due to a non-consistency
between the flux calibrations in the K- and L-spectral bands.
For this reason, we decided to set an upper and a lower limit to the fit by
performing a fit that matched the K-band spectrum perfectly without caring about the adjustment at the red part of the spectrum on the one hand, and another fit compensating the non-physical mismatch at
m.
An absorption at m appears in the spectra of IRS 9 and IRS 29.
This depression in flux is hardly an artefact of the data reduction, since we used the same procedures as for the other sources. We have no scientific explanation for that depression but considered it as real. Also, both in shape and center
wavelength, it does not seem to be consistent with the closest (at
m) strong
absorption line due to the stretching mode resonance of solid CO2 also
observed towards the dust shells of some Young Stellar Objects (de Graauw et al. 1996). The presence of this depression does not affect the results and conclusions in this paper.
Concerning the IRS 16 objects, none of the fits was very satisfactory towards the red part of the L-band spectrum while all of them match well the K-band spectra. It is obvious that this is not due to the fitting procedure itself but to the shape of the L-band spectra of these sources which are very flat (except for the IRS 16 NE case where the shape matches better the overall shape of a Rayleigh-Jeans spectrum). These spectra have been reduced in the same way as all the other spectra with the same flux calibrator stars and thus the shape of the spectra is real.
The origin of this behavior is not clear. It may be due to the fact
that these sources are the least contaminated by local dust emission
features and are likely the most susceptible to variations in the
wavelength dependent line of sight extinction, and to properties of the
associated material. If for instance - compared to the intrinsic,
local absorptions of the dusty sources - the line of sight absorption
is dominated by amorphous H2O ice (Wada et al. 1991), the
corresponding line center and strength of the red wing would be
shifted towards the red. As a result the slope of the reddened
continuum simultaneously fitting the K- and L-band continuum would be
systematically too large.
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Figure 5: Our ISAAC L-band map combined with the spectra of the central sources corrected for the line of sight extinction. |
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The calibration procedure described in this section is supported by
the finding of Scoville et al. (2003) that the extinction shows a
smooth distribution across the central 10 to 20 arcsec with no
indication of concentrations of extinction on scales of about 1
to 2
.
Furthermore Blum et al. (1996) and Clénet et al. (2001)
concluded that the colours of the sources within the central stellar
cluster are due to both intrinsic and foreground reddening.
One of the slit settings we adopted for our observations runs through
a late-type star shown in Fig. 1. This star
shows clear 2.3 m CO absorption band-heads in its K-band spectrum (see Fig. 6); therefore, we hereafter call it "CO-star'' (e.g. Eckart et al. 1995).
It is located at a projected distance of 12.6'' (
0.5 pc)
from the center, is well outside the mini-spiral emission and does not
show excess emission at wavelengths of 3
m or longer (see the L-band spectrum in Fig. 7).
Therefore we can safely assume that this star is largely free of local reddening and that its spectrum is mostly affected by the line of sight extinction along the 8 kpc towards the Galactic Center.
We assume that the L-band spectrum of the CO-star can be represented by a
3600 K (M0-M3 spectral type) blackbody spectrum. If one matches the 4.2
m flux
densities of the measured L-band spectrum to the theoretical
continuum blackbody spectrum then the ratio between the two spectra
provides a measure of the wavelength-dependent extinction in the L-band of the spectrum
due to the interstellar medium along the
line of sight. The corresponding wavelength distribution of the optical depth is shown in
Fig. 8. If we correct this spectrum with
the continuum optical depth value at
m of
(derived by interpolation between the expected L- and M-band
extinctions given by Rieke & Lebofsky 1985) the mean optical depth
would be of the order of 1.64. This result is consistent with the
value of 1.55 obtained by Rieke & Lebofsky (1985) for the L-band
optical depth. We have divided the spectra of all remaining objects
by this extinction spectrum (of which the wavelength distribution is
shown in Fig. 8). The resulting spectra
then represent probably more closely the source spectra as seen at the location of the
Galactic Center - corrected for wavelength-dependent absorption along
the line of sight towards the Galactic Center.
We show the spectra after correction for wavelength-dependent absorption together with our L-band image in Fig. 5.
In Figs. 9 to 11, the spectra corrected for the measured wavelength-dependent line of sight extinction as determined from the CO-star are shown together with the blackbody continuum spectra that were derived by fitting the non-corrected spectra with a single reddened blackbody (Sect. 3.1).
We find that they can be grouped into three classes represented by the three figures.
The comparison of the extinction-corrected spectra (using the CO-star) and the blackbody continuum spectra with temperatures equal to the ones obtained by the fitting procedure (Sect. 3.1 and Table 1) shows that the overall shape of the corrected spectra is consistent with the blackbody curve for the corresponding temperature.
The dusty sources are fitted well with temperatures of the order of 1000 K and the global spectral shapes of the hot
Helium stars in the IRS16 complex closely resemble pure Rayleigh-Jeans
spectra. This implies that the L-band wavelength distribution of the optical depth of
Fig. 8 is consistent with the extinction law
of Martin & Whittet (1990) (see Sect. 3.1) and with
the known absorption value
toward the Galactic
Center (Rieke & Lebofsky 1985). Consequently, the fitting procedure carried out in Sect. 3.1 is reliable and one can use the reddened continua obtained in that section to derive optical depth spectra. The extraction of the optical depth spectra is described in the next section. In the following, we do not make use of the corrected spectra obtained in Sect. 3.2 because small absorption features could still be present in the spectrum of the correction CO-star which may affect the optical depth measurements.
In addition, the good agreement between the two calibration procedures implies that
the overall variation in extinction across the central 0.5 pc of the
Milky Way cannot be much larger than
mag. This is consistent with the results by Scoville et al. (2003) who derived extinction estimates from the P
/6 cm
radio continuum and the P
/H92
line emission over this
central region. These estimates result in a distribution which is
smooth on the
1
scale. This supports the assumption that the
excess extinction seen towards some of the sources must in fact be
associated with the individual objects rather than with the diffuse
ISM (see Blum et al. 1996; Clénet et al. 2001).
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Figure 8: Wavelength distribution of the line of sight extinction towards the Galactic Center. |
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Figure 12:
Optical depths of the Galactic Center sources
normalized to unity at ![]() |
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Figure 13: Mean L-band normalized spectra. The black and grey (or red in the colour version) spectra are obtained by averaging the dusty sources and the stellar (He-stars, IRS 7 and IRS 9) optical depth spectra, respectively. |
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As has been pointed out in the previous section, the optical depth spectra of the Galactic Center sources can be obtained using the reddened blackbody continua of the fitting procedure (Sect. 3.1 and Figs. 2 to 4).
With the results of the blackbody fitting (Table 1),
we have derived the optical depth spectra of the sources using the
equation
where
is the optical
depth, and
and
are the observed and intrinsic
fluxes, respectively. All the spectra are shown in
Fig. 12 where they have been normalised to unity at
m. Mean optical depth spectra for the dust-enshrouded
sources and the non-obscured sources are shown in
Fig. 13. This figure shows very clearly three
absorption features at
m,
m and
m. From
Fig. 12, it is evident that the absorption depths of
the
m,
m and
m features vary from source to source.
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Figure 14: Mean isolated and normalized ice spectra of the dusty sources (in black) and He-stars, IRS 7 and IRS 9 sources (in grey, or red in the colour version). |
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Mean spectra of the hydrocarbon feature at m and the ice
feature at
m have been constructed in the same iterative way
as in Chiar et al. (2002). The optical depth spectrum of IRS 29 shows
the smallest
m absorption in comparison to the
m
feature. A first mean spectrum of the
m feature was obtained
by subtracting the optical depth spectrum of IRS 29 from all other
spectra. Then the average of the residual spectra was
derived. The mean water profile spectrum is then obtained by
subtracting the average spectrum of the
m hydrocarbon feature
multiplied by a free scaling parameter. This parameter is adjusted
such that after subtraction of the average spectrum of the hydrocarbon
feature from the mean source spectrum a featureless spectrum is obtained in the
m region.
Finally, the
spectrum of the
m hydrocarbon feature was deduced by
subtracting the mean
m spectrum from all the spectra and
averaging the residuals. The resulting spectra are shown in Figs. 14 and 15.
The most reliable results were obtained for the dust-enshrouded
sources. As a consequence of the successful continuum fits described
in Sect. 3.1, the absorption spectra level out at
for wavelengths well above and well below the corresponding absorption
features. For the hot He-stars in the IRS16 cluster the situation is
different. Here the continuum fits were less successful (see
discussion in Sect. 3.1) and the derived profiles of the absorption
features are less reliable towards the long wavelength end of the
spectra. This is less severe for the
m feature since in this
case only the continuum longward of
3.5
m is affected.
An examination of the optical depth spectra reveals information on the
origin and physical conditions of the absorbing material in the
central parsec of the GC stellar cluster. We distinguish the following
features:
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Figure 15:
Mean isolated and normalized ![]() ![]() |
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Our mean absorption spectrum (Fig. 14) shows,
however, that in our observations the deepest absorption occurs
m longward of 3.0
m. This behavior is
consistent with the presence of amorphous H2O ice (Wada et al. 1991).
Our mean spectrum of the 3.0 m ice feature appears to be narrower
than the one obtained by Chiar et al. (2002). In their
model, the 3.0
m ice feature is best fitted
by a temperature of 10 K and a maximum mantle thickness of 0.75 to 0.85
m assuming the simple model of core-mantle grains of Bohren
& Huffman (1983) and the hypothesis of a variable mantle thickness of
Smith et al. (1993).
The m absorption features for the dusty sources and
the He-stars are shown in Fig. 15.
For both types of source the short wavelength wing is very
similar. The long wavelength wing of the line feature derived from the
He-stars is affected by the continuum mismatch discussed in Sect. 3.1.
Table 2:
The optical depths of the 4 absorption features at
m,
m and
m.
The optical depths of the 4 absorption features at m,
m and
m are listed in Table 2.
The values are obtained by averaging the optical depth spectra over
the wavelength intervals ]2.95
m,3.05
m[,
]3.35
m,3.45
m[ (thus including the two bands of the feature) and ]3.43
m,3.53
m[, respectively. The
m and
m optical
depths are measured in the spectra from which the
m ice
absorption feature was subtracted (see Sect. 4.1).
The error bars listed in the table take into account only the measuring uncertainties, except for the m feature of IRS 1W where the error bar is taken such that the optical depth value includes the upper and lower limits obtained by fitting upper and lower baselines as described in Sect. 3.1.2.
To estimate the lower limit of the optical depth values of the m absorption feature, we measured the value of the strength of this feature using a linear continuum between
m and
m as baseline. The obtained values are listed in Table 2.
Table 2 shows that the optical depth values span a large interval, suggesting that part of the absorption features arises probably from the local medium and may be associated with the individual sources.
Concerning the optical depth values of the m feature, they agree well with the values obtained by Chiar et al. (2002) for the two sources in common, IRS 1W and IRS 7. The
m feature of these two sources exhibits higher values than those of Chiar et al. (2002). This is probably the result of the derivation of the
isolated ice absorption feature described in Sect. 4.1
which is sensitive to the choice of the subtracted spectrum (here the
spectrum of IRS 29). In particular, in Chiar et al. (2002) the
authors find a negative optical depth value at
m for IRS 7
due to the shape of the ice feature which is narrower than the mean
ice absorption feature. Moreover, their optical depth value of the
m feature of IRS 7 is even smaller than the lower limit obtained in our spectrum.
Concerning the IRS 3 source results, no comparison can be done with Chiar et al. (2002) as the fit of their spectrum was not satisfactory and did not match at all the K-band point flux. This may be due to the problematic relative flux calibration of their L-band spectrum which shows a different slope than ours.
On the other hand, the lower limits of the optical depth values of the hydrocarbon feature for the IRS 3 and IRS 7 sources in common with Sandford et al. (1991, 1995) and Pendelton et al. (1994) agree well with the values obtained by these authors. This is very satifactory as the values provided in these papers were derived using a linear fit similar to the one used here to derive the lower limits.
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Figure 16:
Mean isolated and normalized ![]() |
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The plots of the optical
depths of the absorption features at m versus
m and
m versus
m are shown in Figs. 19
and 18 respectively. In these figures we also plot the data
by Chiar et al. (2001) that were mostly obtained for sources in the outer parts of the central stellar cluster.
We find a good correlation (correlation coefficient of 0.98) between the
m and
m absorption features (Fig. 18) and a vague of correlation (correlation coefficient of
0.43) between the
m and
m features (Fig. 19). In Figs. 18 and 19, the linear regressions are drawn as well; the goodness of fit probabilities are respectively 0.9996 and
.
The first correlation is to be expected since both features (at
m and
m) arise from the same functional groups, as explained in Sect. 4.2.
The possible correlation between the optical depths of
the m and
m features shown in Fig. 19 suggests that the ISM in the central region is a
mixture of diffuse and dense material. On the other hand, if this correlation is real, then in this plot the
discrepancy between the positions of IRS 7 derived by us and by Chiar
et al. (2002) cannot be due only to the variability of the source. In
the case of variability, both positions should follow the overall
trend of the correlation. Actually, the discrepancy between the
positions of the 3 sources (IRS 1W, IRS 3 and IRS 7) in common with
Chiar et al. (2002) is due to the same reasons mentioned in Sect. 4.3.
The fact that the
m to
m
absorption feature "correlation'' shows a significant offset
from the origin of the coordinate system used for display may indicate
that there could be significant
m line absorption even if the
water ice feature were not present. Therefore we may conclude that
a certain amount
of the 3.4
m line absorption is due to the diffuse
ISM on the line of sight to the Galactic Center. Due to the low density of the
mini-spiral gas a substantial amount may, in fact, be closely linked
to the individual sources that we are studying here in detail.
Therefore, the remaining portion of the
m absorption may occur in the central parsec. This is also supported by the finding of Sandford et al. (1995) who derived anomalously high aliphatic CH absorption with respect to visual extinction relative to all the other lines of sight for which data were then available.
The obtained L-band spectra also allow us to investigate the HI line emission
towards selected lines of sight.
The corresponding line strengths, widths and equivalent widths for
Br,
Pf
,
and Pf
are listed
in Tables 4-6.
At the achieved spectral resolution we cannot distinguish
between emission from
the individual objects and from the mini-spiral.
The latter seems to dominate the emission
since sources in or close to the mini-spiral
(IRS 1W, 16NE, 16SW, 21, 13, 13N)
also show the strongest line emission.
Especially for IRS 21 the K-band spectrum by Ott et al. (1999)
suggests that probably all
the hydrogen line emission is due to the mini-spiral.
Using the Br
and P
line strengths published by
Najarro et al. (1997) we can calculate the diagnostic line ratios
Br
/Br
and P
/Br
for three of the IRS 16 sources.
The results are shown in Table 3.
To obtain dereddened line strengths we have considered an absorption
in the L-band wavelength range of AL=1.68, consistent with our optical
depth spectrum of Fig. 8 as well as with the value given
by Rieke & Lebofsky (1985).
Taking the dereddened ratios at face value and omitting possible small
contributions from the He line emission close to the P
and Br
line we can derive first estimates for the temperature and density
of the emitting gas.
Assuming case B recombination, for both sources IRS 16NE and IRS 16C
the line ratios are consistent with emission from gas at low temperatures
of
5000 K and low densities of
102 cm-3.
For IRS 16SW the ratios are more consistent with emission from a gas at
high temperature
20 000 K and high density
104 cm-3.
While it cannot be excluded that the emission towards IRS 16SW contains a
significant contribution
from the hot stellar atmospheres of the He-star IRS 16SW and the other
neighboring hot stars in the IRS 16 complex,
this result reflects the complex density and temperature structure
within the mini-spiral, which consists of a thermal plasma of
cm-3 at
K
(e.g. Brown et al. 1981)
with denser regions of
cm-3 and a few 100 K
(e.g. Jackson et al. 1993).
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Figure 17: L-band intrinsic spectrum (i.e. corrected for extinction along the line of sight as explained in Sect. 3.2) at the position of SgrA* taken through a 0.6'' wide slit using ISAAC on the VLT UT1. The continuum of a blackbody of 22 200 K is also shown. The locations of hydrogen lines are marked. |
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Figure 18:
Optical depth of the hydrocarbon absorption
at the ![]() ![]() |
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Figure 19:
Optical depth of the hydrocarbon absorption at ![]() ![]() |
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Our ISAAC L-band imaging and spectroscopy was part of a
flux density monitoring program of SgrA* (Baganoff et al. 2001, 2003; Eckart et al. 2004)
which has been carried out simultaneously with CHANDRA observations.
Therefore all slit settings were defined such that SgrA* always fell within the slit.
A mean extinction corrected spectrum at that position is shown in
Fig. 17.
Here the weak hydrogen recombination lines
are most likely due to emission from the mini-spiral.
The Rayleigh-Jeans shape of the continuum spectrum is well comparable to that of other hot stars, like the He stars in the central cluster.
In Fig. 17 we show the L-band spectrum of SgrA*
compared with a 22 200 K blackbody spectrum.
In fact, high resolution adaptive optics L-band images were taken
in August 2002 with NAOS/CONICA on the VLT UT4 during the science
verification phase (Clénet et al. 2003; Genzel et al. 2003; Eckart et al. 2004).These images show that at the position of SgrA* the L-band flux
density is dominated by the fast moving star S2 (Schödel et al. 2003).
Our spectrum is consistent with the fact that based on its K-band luminosity
spectrum S2 is likely to be a 15-20 solar mass late O, early B
main-sequence star of age less than 20 Myr
(Gezari et al. 2002; Eckart et al. 1999; Figer et al. 2001; Ghez et al. 2003).
Table 3: Diagnostic line ratios for the three IRS 16 sources in common with Najarro et al. (1997).
Well within the central stellar cluster of the Milky Way, about 0.5
Combined NIR and MIR spectroscopic observations of sources in the
central 0.5 pc parsec of the Milky Way allowed us to obtain a
detailed picture of the absorption features visible in the spectra
towards that region. Our investigation of the central sources (
)
complements the study by Chiar et al. (2002) which mostly
includes sources with larger separations from the center. We find some
evidence that the diffuse ISM in the central 0.5 pc has properties that are
slightly distinct from the ISM at larger distances from the center.
The 3.0 m ice profile usually observed toward the Galactic Center
peaks at 2.96
m, shortward of the ice feature in local molecular
clouds (McFadzean et al. 1989; Tielens et al. 1996; Chiar et al. 2000). It is likely due to cold (15 K) water ices with an
enhanced NH3 abundance (Chiar et al. 2000). Especially processed
ices contribute a substantial portion of the refractory grain
materials that persist when the molecular cloud is dispersed by star
formation, and these products may yield the extinction characteristic
of the diffuse interstellar medium.
However, the profile of the m ice feature obtained in our work peaks
longward of
m and is therefore likely to be associated with
the presence of amorphous H2O ice towards these sources.
Simultaneous fits of our K- and L-band spectra with single reddened blackbody continua allowed us to estimate the extinction towards the individual Galactic Center sources and to determine the approximate continuum shape of the observed spectra. The derived K-band extinctions and blackbody temperatures are consistent with values found for the sources in the central stellar cluster (Chiar et al. 2002; Tanner et al. 2002, 2003).
Using the spectrum of a late-type star assumed to be free from local
extinction, we were able to derive the spectrum of the L-band
extinction along the line of sight toward the Galactic Center. This
spectrum has been used to derive the extinction corrected
spectra of the most luminous sources. The extinction corrected
spectra are consistent with the blackbody temperatures derived from
the previous fitting procedure. The Rayleigh-Jeans continuum spectra
obtained towards all the hot stars indicate that the distribution of the
extinction in the central half parsec is fairly flat and varies in the
K-band only by
mag. This is consistent
with the results of Scoville et al. (2003). Therefore the excess
extinction that we determined after correction for the line of sight
extinction towards some of the sources should be associated with the
individual objects or very clumpy features of the ISM. This is in
support of the findings by Blum et al. (1996) and Clénet et al. (2001) and is also consistent with the large range of the optical depth values derived from the fitting procedure.
The presence of local extinction in the envelopes of dusty sources
is consistent with the bow shock model of Tanner et al. (2002, 2003).
Cotera et al. (1999) had already shown that several of these sources
are indeed offset from nearby local maxima in the extended dust
emission and temperature distribution. Especially for IRS 21, Tanner
et al. (2002) indicate that the extended dust emission of this source
is consistent with a bow shock created by the motion of such a massive
hot star through the dust and gas of the mini-spiral. It is likely
that the bow shock scenario may be applicable to most of the dust-embedded sources in the central stellar cluster.
For IRS 3, however, the bow-shock scenario may not apply.
Gezari et al. (1985) find that IRS 3 is the most compact and
(together with IRS 7) hottest bright source ( K)
in the central cluster.
As suggested in the case of IRS 21 by Tanner et al. (2002)
IRS 3 may be an optically thick dust shell surrounding a
mass-losing source, such as a dusty recently formed WC9 Wolf-Rayet
star.
In addition, a blackbody temperature of 1000 K is found for the
spectrum representing the highly reddened sources located to the North
of IRS 13. While it cannot be excluded that the individual objects
contained in this source complex are lower luminosity analogues of the
class of bow shock objects found by Tanner et al. (2002) and Rigaut et al. (2003), their temperature and luminosity is in good agreement with
the low temperatures of the YSOs classified by Ishii et al. (1998).
Detailed modeling, similar to the studies by Tanner et al. (2003) and Eckart et al. (2004), based on higher angular resolution MIR imaging and spectroscopy (using AO or interferometry) is required to unravel the nature of the highly extincted sources in the IRS 13N association.
Finally, a vague correlation is noticed in the m versus
m optical depths plot. If a real correlation were confirmed between these two features, it would suggest that the ISM along the line of sight toward the Galactic Center is possibly composed of a mixture of diffuse and dense material. Moreover, the plots of the optical depth values suggests that part
of the
m feature arises probably from the foreground ISM and
part of it from the local medium associated with the individual sources.
Acknowledgements
This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG) via grant SFB 494. We are grateful to all members of the ISAAC/VLT and the MPE 3D team.
Table 4:
Continuum and emission line strengths as well as linewidths
and equivalent widths of the Br
line towards the observed sources.
Table 5:
Continuum and emission line strengths as well as linewidths
and equivalent widths of the Pf
line towards the observed sources.
Table 6:
Continuum and emission line strengths as well as linewidths and equivalent widths of the Pf
line towards the observed sources.
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Figure 2:
Best fits of the K- and L-band spectra with single
reddened blackbody continua. When no K-band spectrum was available,
the flux density at ![]() |
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Figure 3: Fitting spectra with single reddened blackbody continua (cont.). |
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Figure 4: Fitting spectra with single reddened blackbody continua (cont.). |
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Figure 6: The H- and K-band spectrum of the late-type CO-star whose L-band spectrum is used for the line of sight absorption correction. |
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Figure 7: The L-band spectrum of the late-type CO-star used for the line of sight absorption correction. |
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Figure 9: L-band spectra corrected for line of sight absorption using the optical depth spectrum shown in Fig. 8. Blackbody continua (not reddened) of temperatures listed in Table 1 are also plotted for comparison. |
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Figure 10: L-band spectra corrected for line of sight absorption (cont.). |
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Figure 11: L-band spectra corrected for line of sight absorption (cont.). |
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