Issue |
A&A
Volume 503, Number 3, September I 2009
|
|
---|---|---|
Page(s) | 985 - 990 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200912631 | |
Published online | 16 July 2009 |
A FEROS spectroscopic study of the extreme O supergiant
He 3-759
(Research Note)
P. A. Crowther1 - C. J. Evans2
1 - Department of Physics & Astronomy, Hicks Building, University of Sheffield,
Hounsfield Road, Sheffield, S3 7RH, UK
2 -
UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
Received 4 June 2009 / Accepted 10 July 2009
Abstract
We present a study of the extreme O-type supergiant He 3-759 using
new high-resolution FEROS data, revealing that it is a near
spectroscopic twin of HD 151804 (O8 Iaf). We investigate the extinction
towards He 3-759 using a variety of methods, revealing
.
If we assume He 3-759 has an identical absolute K-band
magnitude to HD 151804 we find that it lies in the Sagittarius-Carina
spiral arm at a distance of
6.5 kpc. We derive the physical and wind
properties for He 3-759, revealing
kK,
and
yr-1 for
a clumped wind
whose terminal velocity is estimated at 1000 km s-1. The atmosphere of He 3-759
is enriched in helium (
%) and nitrogen
(
%). A reanalysis of HD 151804 and HD 152408 (WN9ha) reveals similar
parameters except that the WN9ha star possesses a stronger wind and
reduced surface hydrogen content. HD 151804 and HD 152408 lie within the
Sco OB1 association, with initial masses of
and ages
2.7 Myr, consistent with NGC 6231 cluster members using standard
Geneva isochrones. Improved agreement with observed surface
abundances are obtained for similar initial masses with more recent Geneva group predictions
from which higher ages of
3.75 Myr are obtained. No young,
massive star cluster is known to be associated with He 3-759.
Key words: stars: early-type - stars: fundamental parameters - stars: individual: He 3-759, HD 151804, HD 152408
1 Introduction
In normal star-forming galaxies, massive O-type stars dominate both the
Lyman continuum ionizing budget and the feedback of mechanical energy
through their intense stellar winds and, ultimately, as core-collapse
supernovae. The bulk of their short (3-10 Myr) lives is spent on the
main sequence as an unevolved dwarf or giant, before rapidly shedding
their hydrogen envelope during either the Red Supergiant, Luminous Blue
Variable or Wolf-Rayet phase. O-type supergiants represent the transition
between these stages for the highest mass stars, with characteristic
emission lines of He II 4686 and H
due to their
relatively strong stellar winds. N III
4634-41 emission
is also commonly seen in such stars, with a corresponding Of nomenclature.
Surveys of emission-line O-type stars in the Small and Large Magellanic
Clouds are relatively complete due to the low interstellar extinction
towards their sight-lines. Comparable Milky Way surveys remain highly
incomplete with the exception of known OB associations in the solar
neighbourhood (e.g. Humphreys 1978), although H
surveys such as
IPHAS (Witham et al. 2008; Drew et al. 2005) and VPHAS+ (Arnaboldi et al. 2007) are in the process
of remedying this deficit, at least for sources detected optically.
Still, many sources from the extensive Michigan-Mt Wilson southern
H
survey (Henize 1976) remain largely neglected. He 3-759 is one
such source, and is the focus of the present study. This was first
reported in the catalogue of Galactic Wolf-Rayet (WR) stars by
Roberts (1962, Star )41# due to broad H
emission.
The intensity and sharpness of H
,
He II
4686 and N
III
4634-41 emission led Carlson & Henize (1979) to favour an Of
classification. However, ultraviolet spectroscopy from Shore et al. (1990)
suggested a contradictory early B-type classification, providing the
motivation for the present study. Subsequently, de Winter et al. (2001) included
He 3-759 in their photometric catalogue of southern emission line
sources, assigning it a (classical) Be spectral type (see
also Thé et al. 1994).
New observations are reported in Sect. 2, with estimates of the distance and extinction towards He 3-759 obtained in Sect. 3. A spectroscopic analysis is performed in Sect. 4, including a comparison to HD 151804 (O8 Iaf) and HD 152408 (WN9ha), and conclusions are drawn in Sect. 5.
2 Observations
Previously unpublished spectroscopy of He 3-759 was obtained with the Double Beam Spectrograph (DBS) mounted at the Australian National University (ANU) 2.3 m telescope in April 1996. Subsequent high-resolution spectroscopy was obtained with the Fibre-fed Extended Range Optical Spectrograph (FEROS) at the 2.2-m Max Planck Gesellschaft (MPG)/European Southern Observatory (ESO) telescope in March 2009.
2.1 Coordinates of He 3-759
The new observations highlighted discrepancies in previously published
coordinates of He 3-759. These are summarised in
Table 1, with coordinates precessed to J2000 equinox using
the STARLINK COCO package where necessary. Note that the
coordinates listed by the SIMBAD
database from Thé et al. (1994) are
incorrect. The positions published by de Winter et al. (2001) are presumably rounded
values from Thé et al.
FEROS observations of He 3-759 were initially attempted on 2009 March 18
using the SIMBAD coordinates. However the resulting spectrum was,
surprisingly, of a cool M-type star, namely the long period variable IRAS
12094-6212 from Caldwell et al. (1991). We subsequently inspected the fits header
information from the ANU/DBS spectroscopy, which were consistent with
Carlson & Henize (1979) values, although 5
45 away from the Thé et al. (1994)
position. Accurate astrometry of He 3-759 from Tycho-2 is included in
Table 1.
Table 1: Published coordinates of He 3-759, including astrometry from Tycho-2.
![]() |
Figure 1:
Blue region FEROS spectrum of He 3-759, compared with the AAT-UCLES
spectra of HD 151804, HD 152408 and HDE 313846 from Crowther & Bohannan (1997).
Spectral lines identified in HD 151804 are He I
|
Open with DEXTER |
![]() |
Figure 2:
Green-red region FEROS spectrum of He 3-759, compared with the AAT-UCLES
spectra of HD 151804, HD 152408 and HDE 313846 from Crowther & Bohannan (1997).
Emission lines identified in HDE 313846 are C III |
Open with DEXTER |
2.2 ANU/DBS spectroscopy
We used DBS at the ANU 2.3m telescope to obtain blue, yellow and
red spectroscopy of He 3-759 on 1996 Apr 1-3. The detectors
for both arms of DBS were 1752532 pix SITE CCDs with
blue and red 1200 l mm-1 gratings providing a dispersion of
0.5 Å pix-1. The blue DBS arm was used on 1 Apr to obtain
1 Å (2 pix) resolution spectroscopy of
3980-4975, with
the red DBS arm used on Apr. 2 and 3 to obtain 1 Å resolution
spectroscopy of the
5695-6700 and
4955-5955 regions,
respectively. A standard CCD reduction was followed, enabling
He 3-759 to be confirmed as an Of star. However, the modest S/N achieved
(
10-40) was inadequate for a quantitative analysis. A relative
flux calibration was also achieved, which was absolutely calibrated
using the Tycho-2 filter magnitudes, from which
is estimated.
2.3 MPG-ESO 2.2 m/FEROS spectroscopy
Two 1800s exposures of He 3-759 were obtained with FEROS on the nights
of 2009 March 19 & 20. FEROS is a cross-dispersed, fixed configuration
instrument (Kaufer et al. 1999), which delivers R=48 000 on the 2.2-m, with
continuous spectral coverage of 3600-9200 Å. The spectra
presented here are from the reduction pipeline that runs at the
telescope; subsequent checks with reduction routines tailored for FEROS
(as used by Sana et al. 2009) yielded indistinguishable final spectra.
2.4 Spectral classification
The blue and green-red spectral regions of He 3-759 are presented in Figs. 1 and 2, respectively. Also shown are high-resolution spectra from Crowther & Bohannan (1997) of HD 151804, HD 152408, and HDE 313846 from the Anglo-Australian Telescope (AAT) using the University College London Echelle Spectrograph (UCLES). HD 151804 is a `normal' Of star, classified as O8 Iaf (Walborn 1972; Conti & Alschuler 1971), with the more extreme sources HD 152408 and HDE 313846 (WR108) reflected by their classification of WN9ha (Crowther & Bohannan 1997; Bohannan & Crowther 1999).
These spectra illustrate an elegant morphological sequence in terms of
increasing emission-line intensities. The N III, He II and
H
emission in He 3-759 is slightly stronger than in HD 151804,
but otherwise their blue (stellar) spectra are very similar
(Fig. 1), including the diagnostic
He I
4471/He II
4542 ratio
and we therefore adopt an O8 Iaf classification for
He 3-759. Its appearance confirms the description of Carlson & Henize (1979),
with He 3-759 and HD 151804 among the rare subset of O stars in which
H
is observed in emission, signifying extreme mass-loss properties.
The green-red region (Fig. 2) reinforces the notion that
He 3-759 is a slightly more extreme Of star than HD 151804, with
stronger H
and He I 5876 emission. However, its
emission features
are not as pronounced as in HD 152408 and
HDE 313846, and a normal Of supergiant classification is sufficient.
Also, note the increasing intensities of the Si IV
6667,
6701 and He I
6678 emission in the sequence.
Table 2:
Visual (Tycho-2 B
and V
in parenthesis)
and near-IR photometry for the O8 Iaf stars He 3-759 and HD 151804, including a distance
estimate to He 3-759.
3 Reddening and distance
Reliable photometry of He 3-759 is somewhat sparse in the literature
so we consider three different approaches to estimate its reddening
drawn from (i) infrared photometry from the Two Micron All Sky Survey
(2MASS, Skrutskie et al. 2006); (ii)
ultraviolet International Ultraviolet Explorer (IUE) spectroscopy
from Shore et al. (1990); (iii) the strength of Diffuse Interstellar Band (DIBs)
observed in the FEROS spectroscopy. An estimate of the distance to
He 3-759 then follows from comparison with HD 151804 which is
a member of the Sco OB1 association (distance modulus 11.4,
Humphreys 1978).
thence its distance, for quantitative analysis.
3.1 Photometry
A summary of visible and near-IR photometry for He 3-759 is presented in Table 2.
Optical measurements are rather heterogeneous, including the Tycho-2
(
),
2nd USNO CCD (10.94) and USNO-B1.0 catalogues (B1 = 12.35, R1 = 10.85,
I = 10.38). We include visual and IR photometry of HD 151804 (O8 Iaf) drawn from
Leitherer & Wolf (1984), Crowther & Bohannan (1997) and references therein. Intrinsic near-IR colours
are obtained from our analysis of He 3-759 (Sect. 4) from
which
-band extinctions,
,
may be obtained using the extinction
relations from Indebetouw et al. (2005). Our derived extinction of
for He 3-759
corresponds to
EB-V=1.65, assuming a standard Galactic extinction law.
A similar approach for HD 151804 reveals
,
or
,
rather higher than results derived previously,
such as
from Crowther & Bohannan (1997).
3.2 Archive ultraviolet spectroscopy
Alternatively, we can exploit archival IUE ultraviolet spectroscopy of
He 3-759 published by Shore et al. (1990). We have downloaded low
dispersion, large aperture datasets SWP 36664 and LWP 15903 (obtained
on 12 Jul 1989) from the IUE Newly Extracted Spectra
archive. We have reddened
the spectral energy distribution of our He 3-759 model from Sect. 4
and obtain an optimum fit to the combined UV spectrophotometry and IR photometry
with
EB-V=1.4 using a standard
= 3.2
extinction law. This is presented in Fig. 3, for which overall
agreement is satisfactory, including the comparison with
Spitzer GLIMPSE (Benjamin et al. 2005) at mid-IR wavelengths. Intrinsic colours from our He 3-759 model
include (
and (
.
![]() |
Figure 3:
Reddened model spectral energy distribution of He 3-759
(
EB-V=1.4,
|
Open with DEXTER |
3.3 Diffuse interstellar bands
We may also exploit the strong DIB features in the visual
spectrum of He 3-759 with respect to other moderately reddened
stars in Figs. 1-2, notably 4428,
5780 and 6613. The DIB at
4428 is particularly strong, with an
equivalent width of 2.5
0.1 Å, arguing for EB-V in excess
of 1.0 according to Snow et al. (2002).
Weaker DIB lines are seen to correlate reasonably well with
EB-V, in particular 5780, 5797 (Herbig 1993) and
8620
(Munari et al. 2008). Precise measurement of these lines can be complicated,
e.g.,
5780 is blended with the broad
5778 feature
(cf. Table A1, Herbig 1993). Continuum placement also introduces
uncertainties. Equivalent width (
)
estimates for these three
DIB features
are given in Table 3, with uncertainties of
10% (sufficient
for the purposes of the current investigation). The average of these
three estimates is
.
Finally, our FEROS spectroscopy
confirms the claim from Carlson & Henize (1979) that the Ca II H line is broadened,
albeit owing to stellar H
,
rather than being of interstellar
origin.
Table 3:
Equivalent widths (
)
of selected diffuse interstellar bands (DIBs)
and the resulting estimates of EB-V.
3.4 Distance to He 3-759
The three methods outlined above provide the following estimates
of
.
IR photometry results in
= 5.1 (for
= 3.1), UV spectrophotometry
implies
= 4.5 and the line strengths of DIB features also
suggest
= 4.5, yielding
4.7
or
= 0.53
.
If we assume that He 3-759 has a similar absolute
-band
magnitude to HD 151804 (O8 Iaf) we may estimate its distance.
HD 151804 is a member of Sco OB1 (distance 1.9 kpc, Humphreys 1978) from
which
is obtained (Table 2), giving
a distance modulus of
or
distance of
6.5+1.6-1.3 kpc for
He 3-759. For an adopted Solar
galactocentric distance of 8.0 kpc (Reid 1993), He 3-759 would lie in the
Sagittarius-Carina arm, close to the Solar circle
7.5
+0.6-0.4 kpc from the Galactic Centre.
4 Physical and wind parameters
We have derived the physical and wind properties of He 3-759 using CMFGEN (Hillier & Miller 1998), and re-analysed optical spectroscopy of HD 151804 (O8 Iaf) and HD 152408 (WN9ha) from Crowther & Bohannan (1997) for comparison.
4.1 Method
CMFGEN solves the radiative transfer equation in the co-moving frame,
under the additional constraint of statistical equilibrium. The
temperature structure is determined by radiative equilibrium. Since CMFGEN
does not solve the momentum equation, a density or velocity
structure is required. For the supersonic part, the velocity is
parameterized with a classical -type law, with an exponent derived
from fits to H
.
This is connected to a hydrostatic density structure at
depth, such that the velocity and velocity gradient match at the
interface. The subsonic density structure is set by a corresponding
fully line-blanketed plane-parallel TLUSTY model (v.200, see Lanz & Hubeny 2003).
The atomic model is similar to that adopted by Crowther et al. (2002),
including ions from H, He, C, N, O, Si, P, S and Fe.
![]() |
Figure 4: Spectroscopic fits (dotted, red) to FEROS observations (solid, black) of He 3-759. |
Open with DEXTER |
Table 4:
Physical and wind properties of He 3-759 with respect
to HD 151804 and HD 152408, allowing for an uncertainty in absolute
magnitude of 0.5
.
Clumped mass-loss rates are quoted
here for volume filling factors of f=0.1.
We have assumed a depth-independent Doppler profile for all lines when
solving for the atmospheric structure in the co-moving frame, while in the
final calculation of the emergent spectrum in the observer's frame, we
have adopted a uniform turbulence of 50 km s-1. Incoherent electron
scattering and Stark broadening for hydrogen and helium lines are adopted.
Finally, we convolve our synthetic spectrum with a rotational broadening
profile for which
km s-1.
Clumping is incorporated using a volume filling
factor, f, as described in Hillier et al. (2003), with a typical value of f=0.1
resulting in a reduction in mass-loss rate by a factor of
.
4.2 Results for He 3-759 and other extreme supergiants
We derive the stellar temperature of He 3-759 using diagnostic He
I 4471,
5876, He II
4542,
5411 lines, together
with H
and H
for the mass-loss rate and velocity structure.
We have estimated a terminal wind velocity of
km s-1
based upon low-resolution IUE observations of C IV
1548-51
using the method of Prinja (1994), while a (slow) velocity law of
exponent
is used for the supersonic velocity structure
since this provides an excellent fit to the H
profile.
Regarding wind clumping in Of supergiants, either He II
4686 or H
is suitable for determination of the volume
filling factor f, if the velocity law is known. However, since
H
is used to estimate the
velocity law and the peak emission of He II
4686 is
very poorly reproduced, an independent determination of f is not
achievable.
Spectroscopic fits to FEROS observations are presented in
Fig. 4, with a summary of physical and wind
parameters presented in Table 4.
Overall, the fits are satisfactory, with the exception of He II
4686 that is predicted significantly too strongly in emission.
In addition, P Cygni absorption for He I
5876 is also
predicted too strong, and the singlet He I
4143,
4387
lines are predicted to be in emission, yet they are observed in
absorption. Najarro et al. (2006) discuss problems relating to the use of singlet
He I lines in O stars, such that triplets (e.g.
4471,
5876)
are favoured. We obtain a helium enriched atmosphere with He/H = 0.25 by
number or
= 49% by mass. The prominent N
III
4097-4103 and
4634-41 features of He 3-759 are well
matched using a mass fraction
of
= 0.3%, corresponding to an enrichment of 4 times the
solar value. However, N III
4379 is predicted to be
too strong and N III
5320-24 is too weak, such that we admit a
factor of two uncertainty in the nitrogen abundance. Turning to
other elements, both C III
5696 and C
IV
5801-12 favour a high carbon abundance while
4647-51 requires a low abundance. The model presented in
Fig. 4 was obtained for an intermediate abundance of
= 0.2% (0.7 times the solar case), although large
uncertainties are admitted. For oxygen, solely O
III
5592 is observed, from which we estimate
=
0.2% (0.5 times the solar value). For silicon, sulphur and iron we adopt
solar values.
We have also reanalysed two of the reference stars - HD 151804 (O8 Iaf)
and HD 152408 (WN9ha) - based upon our AAT UCLES datasets presented in
Figs 1, 2 and the method outlined above. A TLUSTY
model at depth was adopted for HD 152408 since
models were not available for
T2/3 = 32.5 kK. For current
stellar masses of
(see Sect. 4.3), surface
gravities are
,
while effective gravities, corrected for radiation
pressure, are
.
Fits are of comparable quality to those presented here for He 3-759,
also failing to reproduce He II 4686 emission,
with their physical and wind properties also provided in
Table 4. As expected, the physical parameters and chemical
composition of the three stars are very similar, with the more advanced
spectral type of WN9ha for HD 152408 attributable to a somewhat higher
mass-loss rate - see Bohannan & Crowther (1999) for a general discussion of this
subject. In addition, the hydrogen contents of HD 151804 and He 3-759
are similar, with a significantly lower hydrogen mass fraction for
HD 152408. Subtle differences between the present study and Crowther & Bohannan (1997)
follow from the improved metal line blanketing (primarily Fe), TLUSTY
structure at depth and allowance for wind clumping.
4.3 Comparison with evolutionary model predictions
A comparison between the physical properties of He 3-759 and
non-rotating, solar metallicity Geneva models from Meynet et al. (1994; see also
Lejeune & Schaerer 2001) suggests an age of 2.7 Myr and
initial mass of
.
Similar results are obtained for
HD 151804 and HD 152408, in good agreement with the age of the NGC 6231
cluster within Sco OB1, as derived by Crowther et al. (2006) using the same set of
isochrones. However, these standard evolutionary models are well known not
to predict the observed helium enrichment at such phases.
In contrast, comparisons with the
evolutionary models of Meynet & Maeder (2000) allowing for rotation and
contemporary mass-loss rate prescriptions enable
reasonable matches to both the surface hydrogen abundance (40%)
and location in the H-R diagram. For a distance of 6.5 kpc to He 3-759,
initial 60
models rotating at 300 km s-1suggest a greater age of 3.9 Myr, while a slightly lower age of 3.6 Myr is obtained
for a non-rotating 60
model. At these ages, current
stellar masses lie in
the range 35-45
,
from which we adopt 40
for surface gravity
estimates. Lower mass evolutionary
models from Meynet & Maeder (2000) fail to predict the combination of surface hydrogen content and
its position in the H-R diagram, favouring our preferred distance to He 3-759.
In summary, He 3-759 appears to be a very high mass star
at a relatively young age, but unlike HD 151804 and HD 152408 it does
not reside within a known cluster or OB association. According to
Larson (2003), the most massive star of a cluster (of mass
)
scales with cluster mass according to 1.2
suggesting
a lower limit of
for its birth cluster. He 3-759
does not possess a high radial velocity so it would be expected to be
located close to its natal cluster. Alternatively, Parker & Goodwin (2007) have
proposed that some massive stars may form in relatively low mass clusters.
Such clusters would not necessarily be easily identified at large
distances, as is the case for He 3-759.
5 Summary
We have presented a high quality FEROS spectrum of the poorly
studied, early-type emission line supergiant He 3-759, from which an
O8 Iaf classification is obtained, and clarified its coordinates.
We have used three methods to estimate its high interstellar extinction,
namely fitting a stellar model to its IUE ultraviolet spectrophotometry
and 2MASS and GLIMPSE photometry; obtaining its near-IR extinction from
comparison with intrinsic colours; deriving its visual extinction from
measured strengths of DIBs. Combining these approaches implies
or
.
If we
assume that
He 3-759 has a similar absolute
-band magnitude to HD 151804
(O8 Iaf)
its distance is estimated as 6.5 kpc, within the Sagittarius-Carina arm.
The presence of such a high-mass (
)
star in isolation
is curious given the lack of a nearby cluster, which would be expected to
be relatively massive (
6000
).
No doubt, many other emission-line OB supergiants await discovery, in view of large optical surveys such as IPHAS and VPHAS+. Alternatively, visibly obscured extreme early-type supergiants may be identified by their infrared free-free excess following the approach of Hadfield et al. (2007).
Acknowledgements
We thank John Hillier for his development of CMFGEN, Hugues Sana for his reprocessing of the data, and Martin Cordiner and Keith T. Smith for helpful discussion regarding the interstellar features. This publication is based in part upon INES data from the IUE satellite, 2MASS which is a joint project of the University of Massachusetts and the IPAC/CalTech, funded by the NASA and the NSF, and Spitzer datasets from NASA/IPAC Infrared Science Archive (IRSA). IRSA is operated by JPL, CalTech under contract with NASA.
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- Thé, P. S., de Winter, D., & Perez, M. R. 1994, A&AS, 104, 315 [NASA ADS] (In the text)
- Walborn, N. R. 1972, AJ, 77, 312 [NASA ADS] [CrossRef]
- Witham, A. R., Knigge, C., Drew, J. E., et al. 2008, MNRAS, 384, 1277 [NASA ADS] [CrossRef]
Footnotes
- ...
He 3-759
- Based on observations made with ESO telescopes at the La Silla observatory under program ID 082-D.0136.
- ...
database
- http://simbad.u-strasbg.fr/simbad/
- ... ratio
- We measure
= 0.19 from our FEROS dataset for He 3-759 versus 0.22 from our UCLES observations of HD 151804.
- ...
archive
- http://sdc.laeff.inta.es/ines/
- ...
- The Eddington
parameter - the ratio of radiation pressure to gravity -
is
0.35 for He 3-759 and HD 151804.
All Tables
Table 1: Published coordinates of He 3-759, including astrometry from Tycho-2.
Table 2:
Visual (Tycho-2 B
and V
in parenthesis)
and near-IR photometry for the O8 Iaf stars He 3-759 and HD 151804, including a distance
estimate to He 3-759.
Table 3:
Equivalent widths (
)
of selected diffuse interstellar bands (DIBs)
and the resulting estimates of EB-V.
Table 4:
Physical and wind properties of He 3-759 with respect
to HD 151804 and HD 152408, allowing for an uncertainty in absolute
magnitude of 0.5
.
Clumped mass-loss rates are quoted
here for volume filling factors of f=0.1.
All Figures
![]() |
Figure 1:
Blue region FEROS spectrum of He 3-759, compared with the AAT-UCLES
spectra of HD 151804, HD 152408 and HDE 313846 from Crowther & Bohannan (1997).
Spectral lines identified in HD 151804 are He I
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Green-red region FEROS spectrum of He 3-759, compared with the AAT-UCLES
spectra of HD 151804, HD 152408 and HDE 313846 from Crowther & Bohannan (1997).
Emission lines identified in HDE 313846 are C III |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Reddened model spectral energy distribution of He 3-759
(
EB-V=1.4,
|
Open with DEXTER | |
In the text |
![]() |
Figure 4: Spectroscopic fits (dotted, red) to FEROS observations (solid, black) of He 3-759. |
Open with DEXTER | |
In the text |
Copyright ESO 2009
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