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Massive stars in the galactic center
New Astronomy Reviews 44 (2000) 213–220 www.elsevier.nl / locate / newar
Massive stars in the galactic center F. Najarro Intituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
Abstract The relatively brief history of infrared observations and quantitative analysis of massive stars in the Galactic Center is reviewed. Current observational and the theoretical status is also reported: A new generation of NLTE wind blanketed models, together with high S / N spectra of the hot star population are allowing one, for the first time, to perform metal abundance determinations (Fe, Si, Mg, Na, etc). Metallicity studies of hot stars in the IR will provide major constraints not only on the theory of evolution of massive stars but also on our efforts to solve the puzzle of the central parsecs of the Galaxy. Preliminary results are presented. 2000 Published by Elsevier Science B.V. Keywords: Hot stars; Stellar Winds; Galatic Center; Infrared
1. Introduction The dramatic continuous progress in infrared detectors since the mid-eighties has finally opened the central parsecs of our galaxy to stellar spectroscopic studies. One of the first targets was the IRS16 object (presently resolved into four sources), which was known for a long time to be an unusual infrared source nearly coincident with the non-thermal radio source Sgr A*. First measurements of the He I 2.058 mm and Ba lines (Geballe et al., 1984, 1987) seemed to point towards the presence of strong winds and the possibility that this object could dominate the ionization of the central parsec. On the other hand an ‘unusual’ Ba source (Forrest et al., 1987) was also detected 100 away from Sgr A* and K-Band mid-resolution spectra (Allen et al., 1990) pointed towards an early type nature of this object, presently known as the AF star. However, it was not until the beginning of this decade, that the detection of a whole He I emission line cluster (Krabbe et al., 1991) definitely posed the question about the physˆ on the ical nature of these objects and their role energetics of the central parsec. This discovery
triggered a substantial improvement in the atmospheric models for hot stars in the near-infrared, where theory was clearly lagging behind. In a significant first step the codes were tuned, and then used as diagnostic tools to extract the astrophysical information present in the H and He I infrared lines, e.g. (Nahar, 1995; Najarro et al., 1994; Crowther & Smith, 1996). Reliable values for luminosities, temperatures, mass-loss rates, helium abundances and ionizing photons were obtained for the brightest members of the He I cluster (Najarro et al., 1994, 1997). With these values, the above questions could be confidently answered. Nevertheless, though important constraints were placed on the theory of the evolution of massive stars the metal content of these objects could not be determined. Indeed, the metallicity issue in the galactic center is still a source of controversy. Based on measurements of the gasphase, Shields & Ferland (1994) obtained twice solar metallicity from Argon and Nitrogen emission lines while a solar abundance was derived for Neon. For the cool stars, and based on LTE-differential analysis with other cool supergiants, Carr et al., (1996, 1999) have obtained strong indications for a solar Fe
1387-6473 / 00 / $ – see front matter 2000 Published by Elsevier Science B.V. PII: S1387-6473( 00 )00053-1
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F. Najarro / New Astronomy Reviews 44 (2000) 213 – 220
abundance. It is, therefore, crucial to obtain metallicity estimates from direct analysis of the hot stars to see if the values are compatible with those obtained from the cool-star and gas-phase analyses. It is also of importance since the metallicity has a substantial influence on stellar evolution. To tackle this problem, two conditions were required: Observations with sufficiently high S / N and spectral resolution to detect with confidence the metal lines, and models for hot stars accounting for metals such as Fe, Mg, Ca, etc. and their effects on the photospheric and wind structure (line blanketing). In this paper, we report impressive recent progress in both observations and theoretical models for hot stars. First steps towards metallicity determinations will also be presented.
VLT (Eckart et al., 1999). Of concern is the presence in Fig. 1 of fairly well isolated lines of Si II, Mg II and Fe II, a invaluable gift for an infrared spectroscopist willing to determine metal abundances and to whom the UV and optical spectra of the star have been swallowed by extinction. Now that the lines are there, the appropriate tool is ‘just’ required: A unified photosphere-wind NLTE-model, accounting for the presence of metals and correctly treating line blanketing. Impressive efforts have been made within the last years in this field by the Munich-group, Hillier & collaborators, and Schmutz & collaborators (see Pauldrach et al., 1997; Hillier, 1997; Schmutz, 1998, for excellent reviews in this topic). In the next section we will present the new generation of line blanketed models developed by John Hillier and compare them to the formerly used standard model.
2. Improved observations The advances in ground based infrared astronomy in this decade are striking. The introduction of very efficient spectrographs suitable for mid–high resolution observations have opened the near-infrared sky to quantitative spectroscopy. In the field of hot stars, high quality IR-spectra have recently appeared in the literature (Krabbe et al., 1991; Hanson et al., 1996; Figer et al., 1997; Fullerton & Najarro, 1997; Blum et al., 1997). Except for Fullerton & Najarro (1997)’s sample (R|10 000), all these spectra were obtained with low–mid (R|500–2000) resolution, which is enough to classify the stars but insufficient in most of the cases to perform accurate quantitative spectroscopic studies. Examples of mid–high resolution observations of early type stars with strong winds are shown in Fig. 1 the H-band of the Pistol Star obtained with UKIRT-CGS4 with a resolution of R|5000 is displayed. The spectra shown in Fig. 1 demonstrate the necessity of high resolution and S / N. We note how the same spectral region observed with R|1000 can blur the presence of the important diagnostic lines. The number of new observational constraints provided by the new spectroscopic data is striking when compared to the previous use of only some H and He lines. The availability of high-quality IR spectroscopic data is currently being substantially improved with NIRSPEC on the Keck II and ISAAC on the
3. New unified models versus standard model The standard model which we used to analyse the He I stars in the central cluster (Najarro et al., 1994, 1997) is basically an iterative, non-LTE method presented by Hillier (1987, 1990) to solve the radiative transfer equation for the expanding atmospheres of early-type stars in spherical geometry, subject to the constraints of statistical and radiative equilibrium. Steady state is assumed, and the density structure is set by the mass-loss rate and the velocity field via the equation of continuity. The transfer equation is solved using either the Sobolev approximation, corrected for the diffuse radiation field, or by means of an accurate solution in the comovingframe. The atmosphere was considered to consist of H, He, N and C, though the latter two species were used basically to better account for cooling processes. The standard model was then prescribed by the stellar radius, R * , the stellar luminosity, L* , the ~ the element abundances, and the mass-loss rate M, velocity field, v(r). The new model (Hillier & Miller, 1998) incorporates new species: O, Mg, Ca, Si, Na, Al and Fe, and can still treat lines and continuum separately using the Sobolev approximation or comoving frame solution, as in the standard old model, or can be switched to ‘blanketing’. In this case, no distinction between
F. Najarro / New Astronomy Reviews 44 (2000) 213 – 220
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Fig. 1. Potential of high S / N and mid–high spectral resolution for early type stars. Top: UKIRT-CGS4 (R | 5000) H-band of the Pistol Star (Geballe priv. comm.). Bottom: Zoom of the region around 1.74 mm with spectra degraded to R|1000.
lines and continuum frequencies is made and the transfer (moment) equations are solved consecutively from the highest to lowest frequency. Hence, the effect of continua on lines and lines on the continua as well as overlapping lines is automatically handled. In addition, the new code accounts for X-ray ionization, clumping, level dissolution, and a smooth transition to the ‘exact’ photospheric stratification based on the method by Santolaya-Rey et al. (1997). Below we compare a set of models with stellar
parameters close to those of the Pistol Star (e.g. Figer et al., 1998) to illustrate the differences between the old standard model and the new code. We consider the old standard model (‘Old’), the new model were Fe is included but no blanketing is used (‘New NBL’) and the new model with Fe and blanketing (‘New BL’). The two classical and related effects of line blanketing: Blocking and backwarming, are clearly shown in Fig. 2 where the flux distribution and the
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Fig. 2. Effects of the inclusion of Fe and line blanketing on the flux distribution (top) temperature structure (bottom). Flux blocking and redistribution as well as backwarming are produced by blanketing. See discussion in text.
temperature structure are displayed for the three models under consideration. In Fig. 2 we see that in order to preserve the luminosity the temperature gradient (and hence the temperature) has to be increased in the inner photosphere to drive out the photons blocked in the UV. The blocked UV flux emerges in the visual and infrared giving rise to a flux excess over the standard model. We note that these are indeed blanketing effects as in the inner parts, log n e . 12, the ‘New NBL’ model shows the same behavior as the ‘Old’ one. In the wind, log n e ,
12, the inclusion of metals and Fe (and not the blanketing) are responsible for the different temperature structure. The effects on the ionization structure of H and Fe are shown in Fig. 3. For these two reference species, we note again the importance of line blanketing, which can significantly change the ionization structure and hence affect the line profiles. The inclusion of metals without blanketing makes only minor changes to the ionization structure and the line profiles. Finally, in Fig. 4 we consider both NLTE and
F. Najarro / New Astronomy Reviews 44 (2000) 213 – 220
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Fig. 3. Standard Old versus New Blanketed models: Ionization structure. (left:) For Hydrogen significant changes are only introduced when blanketing is incorporated (New LB). The Fe ionization structure is also altered by blanketing.
blanketing effects on two important diagnostic metal lines present in the K-band spectra of the Pistol Star. These are the Fe II 2.089 mm line and the Mg II 2.13 / 4 mm doublet. The Fe II 2.089 mm (z 4 F 3 / 2 – c 4 F 3 / 2 ) transition, despite its rather low gf value ( gf ¯6 3 10 25 ), can be tremendously amplified due to very efficient UV continuum fluorescence pumping. This is the case when the line is formed in the outer regions of the wind, where the population of the upper level can exceed its LTE value by factors up to 10 000 (see Fig. 4). Such conditions are met by stars such as the Pistol Star with very dense winds. There, the Fe II 2.089 mm line is formed in the region with 10 6.5 #n e #10 9 . Similar fluorescent mechanism operates on the much more stronger Mg II 5s 2 S 1 / 2 – 5p 2 P1 / 2 / 5s 2 S 1 / 2 –5p 2 P3 / 2 2.1432 / 2.1368 mm doublet ( gf ¯1.2 and 2.4 respectively). These lines, though formed closer to the photosphere (10 9.5 #n e #10 12 ) are already affected by NLTE effects as the departure coefficient of the upper level takes off in this region (see Fig. 4). It is also important to note that,
though these lines are driven by a NLTE mechanism, i.e., UV pumping, their absolute strengths (i.e., exact run of the departure coefficients) are intimately coupled to blanketing effects as shown in Fig. 4.
4. Metallicity determinations: Ongoing work In this section we present preliminary results from quantitative spectroscopic studies using line blanketed models for the Pistol Star from which, for the first time, abundances for Fe and Mg are obtained. We chose the galactic P Cygni type star HDE 316 285 as a calibrator for our method, to check for consistency between optical and IR analysis before proceeding to investigate GC Center sources for which only IR-spectra are available. We started our analysis of HDE 316 285 using the stellar parameters we have derived recently from a spectroscopic investigation using the ‘Old’ model (Hillier et al., 1998).
Fig. 4. NLTE and blanketing effects on the formation of metal lines in the K-band. Departure coefficients for the upper and lower levels of the Left: Fe II 2.089 mm and right Mg II 2.13 / 4 mm doublet lines. Note the very efficient pumping to the upper levels through UV continuum fluorescence. See further discussion in text.
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First results show only small corrections to the stellar parameters derived by Hillier & Miller (1998) and will be discussed elsewhere. Excellent agreement was found between the synthetic spectra of the new line blanketed models and the observations, both in the optical and the infrared K-band. This is a very important result as it confirms the consistency of the results between optical and IR studies, and hence provides confidence to study Galactic Center sources. Further consistency of our model with recently obtained ISO-SWS observations of HDE 316 285 is also found (Najarro et al., in preparation). The derived magnesium abundance for HDE 316 285 is twice solar, while for iron an abundance four times solar is obtained. The latter result is somewhat surprising as similar enhancements should be expected for Mg and Fe (Langer private communication) for this kind of star. Looking for possible sources which could lead to this discrepancy we first investigated the quality of the atomic data. While we are confident about the quality of the much simpler Mg II model atom, the Iron Project atomic data for Fe II (Nahar, 1995) show discrepancies with Kurucz semi-empirical values, in some cases up to factor of 5 or more. Detailed comparison of model results with the observations seem to favor Kurucz gf values and not those from the Iron Project. We are currently combining both data sets in oder to obtain an optimized Fe II atom. However, the uncertainties in some gf values are not enough to remove the factor of two in the derived Fe abundance. Recent work suggests that the discrepancy may be due to the neglect of the Fe 21 1H↔Fe 1 1H 1 charge exchange reaction. This reaction is exothermic in the left to right direction and requires a non negligible amount of H I to work efficiently. This the case in the outer wind of HDE 316 285 and the Pistol Star where the recombination of hydrogen (see Fig. 3) provides enough H I. Thus, as Fe III strongly recombines into Fe II in this zone, the strength of the Fe II lines formed there, such as Fe II 2.089 mm are strongly increased and a lower Fe abundance is required. First results show that when charge exchange reactions are included the models can reproduce the observations with a Fe abundance twice solar, consistent with the results obtained for Mg.
The resulting lower Fe abundance is strongly supported by new observational data with high S / N and resolution, in which other sort of Fe II lines with strong ( gf |1) values connecting higher lying levels as e 6 F–5p 6 D have been detected. These lines form much closer to the photosphere than the so-called semi-forbidden ( gf |10 25 ) lines like FeII z 4 F 9 / 2 – c 4 F 7 / 2 16 787.177 mm and FeII z 4 F 3 / 2 –c 4 F 3 / 2 20 888.098 mm which form in the outer stellar wind. It turns out that the e 6 F–5p 6 D transitions tend to favor a lower iron abundance than that derived from the the z 4 F–c 4 F transitions. Preliminary results for the Pistol Star show similar trends as for HDE 316 285. While a Mg abundance of roughly 3 to 4 times solar is required to match the observed spectra, we obtain a Fe abundance about 8 times oversolar when charge exchange reactions are neglected. Fig. 5 shows the best current model fits to the K- and H-band spectra of the Pistol Star. The new models including Fe and Mg favor the low temperature model versus the high temperature one proposed by Figer et al. (1998), solving the dichotomy on the Pistol Stars luminosity. From Fig. 5 we see, that although the K-band is fairly well reproduced by our models, some discrepancies appear in the H-band (Si II lines at 1.6907 mm and 1.6907 mm are missing as Si was not included in our models). The He I 1.700 mm line in our models appears severely contaminated by Fe II lines. The strength of these lines is overestimated by our models while that of the strong Fe II 1.868 mm line is underestimated. Though part of these discrepancies are removed if the Iron Project oscillator strengths are replaced by Kurucz semi-empirical values, there is still a non-negligible difference on the derived Fe abundance depending on the sort of Fe II lines used. Therefore, we are currently optimizing our Fe II model atom and investigating in detail the effects of charge exchange reactions on the Pistol Star in order to attain consistency between both Fe II diagnostics and hence see how our estimates reconcile with the solar metallicity value derived from the cool stars (Carr et al., 1996, 1999). The Pistol Star is not the only Galactic Center object showing the presence of Fe and other metal lines. Metal lines also appear in the low–mid resolution spectra of IRS16NE (Tamblyn et al., 1996) and in spectra of the recently discovered second
F. Najarro / New Astronomy Reviews 44 (2000) 213 – 220
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Fig. 5. Comparison of the blanketed model (solid) and the observed (grey dashed) spectrum of the Pistol Star. The K-band (Top) is well reproduced. Missing lines in the fit to the H-band (Bottom) are from Si II (neglected in the code) and discrepancies in the Fe II lines are due to uncertainties in the Fe II model atom. See also discussion in text.
LBV candidate in the quintuplet (Figer et al., 1999; Geballe et al., 2000). ISAAC on the VLT (Eckart et al., 1999) and especially NIRSPEC (with R|25 000) on the Keck II telescope are starting to provide new high quality infrared spectra of hot stars in the Galactic Center, for which reliable abundance determinations will be made. It is evident that a new, fascinating research window has opened — the field of quantitative infrared spectroscopy of early type stars.
Acknowledgements F.N. acknowledges grants of the DGYCIT under PB96-0883 and ESP98-1351.
References Allen, D.A., Hyland, A.R. & Hillier, D.J., 1990, MNRAS, 244, 706.
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Blum, R.D. et al., 1997, AJ, 113, 1855. Carr, J.S., Sellgren, K. & Balachandran, S.C., 1996, in: R. Gredel (Ed.), The Galactic Center, 4th ESO / CTIO Workshop, Astron. Soc. Pacific Conf. Series, Vol. 102, p. 212. Carr, J.S., Sellgren, K. & Balachandran, S.C., 1999, ApJ, in press. Crowther, P.A. & Smith, L.J., 1996, A&A, 305, 541. Eckart, A., Ott, T. & Genzel, R., 1999, A&A, in press. Figer, D.F., McLean, I.S. & Najarro, F., 1997, ApJ, 486, 420. Figer, D.F. et al., 1998, ApJ, 384, 506. Figer, D.F., McLean, I.S. & Morris, M., 1999, ApJ, in press. Forrest, W.J. et al., 1987, in: D.C. Backer (Ed.), The Galactic Center, AIP 155, 153. Fullerton, A.W. & Najarro, F., 1997, in: I. Howarth (Ed.), Properties of Hot Luminous Stars, Astron. Soc. Pacific Conf. Series, Vol. 131, p. 47. Geballe, T.R. et al., 1984, ApJ, 284, 118. Geballe, T.R. et al., 1987, ApJ, 287, 2118. Geballe, T.R., Najarro, F. & Figer, D.F., 2000, ApJ, in press. Hanson, M.M., Conti, P.S. & Rieke, M.J., 1996, ApJSS, 107, 281. Hillier, D.J., 1987, ApJSS, 63, 947.
Hillier, D.J., 1990, A&A, 231, 116. Hillier, D.J. 1997, in: T.R. Bedding, A.J. Booth & J. Davis (Eds.), IAU Symp. 189, Fundamental Stellar Properties: the Interaction between Observation and Theory, Kluwer, pp. 209–216. Hillier, D.J., Miller, D.L., 1998, ApJ, 496, 407. Hillier, D.J. et al., 1998, A&A, 340, 483. Krabbe, A. et al., 1991, ApJ, 382, L19. Nahar, S.N., 1995, A&A, 293, 967. Najarro, F. et al., 1994, A&A, 285, 573. Najarro, F. et al., 1997, A&A, 325, 700. Pauldrach, A.W.A. et al., 1997, in: I. Howarth (Ed.), Properties of Hot Luminous Stars, Astron. Soc. Pacific Conf. Series, Vol. 131, p. 258. Santolaya-Rey, A.E., Puls, J. & Herrero, A., 1997, A&A, 323, 488. Schmutz, W., 1998, in: I. Howarth (Ed.), Properties of Hot, Luminous Stars, ASP Conf. Series, 131, 119. Shields, J.C., Ferland, G.J., 1994, ApJ, 430, 236. Tamblyn, P. et al., 1996, ApJ, 456, 206.
Massive stars in the galactic center F. Najarro Intituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
Abstract The relatively brief history of infrared observations and quantitative analysis of massive stars in the Galactic Center is reviewed. Current observational and the theoretical status is also reported: A new generation of NLTE wind blanketed models, together with high S / N spectra of the hot star population are allowing one, for the first time, to perform metal abundance determinations (Fe, Si, Mg, Na, etc). Metallicity studies of hot stars in the IR will provide major constraints not only on the theory of evolution of massive stars but also on our efforts to solve the puzzle of the central parsecs of the Galaxy. Preliminary results are presented. 2000 Published by Elsevier Science B.V. Keywords: Hot stars; Stellar Winds; Galatic Center; Infrared
1. Introduction The dramatic continuous progress in infrared detectors since the mid-eighties has finally opened the central parsecs of our galaxy to stellar spectroscopic studies. One of the first targets was the IRS16 object (presently resolved into four sources), which was known for a long time to be an unusual infrared source nearly coincident with the non-thermal radio source Sgr A*. First measurements of the He I 2.058 mm and Ba lines (Geballe et al., 1984, 1987) seemed to point towards the presence of strong winds and the possibility that this object could dominate the ionization of the central parsec. On the other hand an ‘unusual’ Ba source (Forrest et al., 1987) was also detected 100 away from Sgr A* and K-Band mid-resolution spectra (Allen et al., 1990) pointed towards an early type nature of this object, presently known as the AF star. However, it was not until the beginning of this decade, that the detection of a whole He I emission line cluster (Krabbe et al., 1991) definitely posed the question about the physˆ on the ical nature of these objects and their role energetics of the central parsec. This discovery
triggered a substantial improvement in the atmospheric models for hot stars in the near-infrared, where theory was clearly lagging behind. In a significant first step the codes were tuned, and then used as diagnostic tools to extract the astrophysical information present in the H and He I infrared lines, e.g. (Nahar, 1995; Najarro et al., 1994; Crowther & Smith, 1996). Reliable values for luminosities, temperatures, mass-loss rates, helium abundances and ionizing photons were obtained for the brightest members of the He I cluster (Najarro et al., 1994, 1997). With these values, the above questions could be confidently answered. Nevertheless, though important constraints were placed on the theory of the evolution of massive stars the metal content of these objects could not be determined. Indeed, the metallicity issue in the galactic center is still a source of controversy. Based on measurements of the gasphase, Shields & Ferland (1994) obtained twice solar metallicity from Argon and Nitrogen emission lines while a solar abundance was derived for Neon. For the cool stars, and based on LTE-differential analysis with other cool supergiants, Carr et al., (1996, 1999) have obtained strong indications for a solar Fe
1387-6473 / 00 / $ – see front matter 2000 Published by Elsevier Science B.V. PII: S1387-6473( 00 )00053-1
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F. Najarro / New Astronomy Reviews 44 (2000) 213 – 220
abundance. It is, therefore, crucial to obtain metallicity estimates from direct analysis of the hot stars to see if the values are compatible with those obtained from the cool-star and gas-phase analyses. It is also of importance since the metallicity has a substantial influence on stellar evolution. To tackle this problem, two conditions were required: Observations with sufficiently high S / N and spectral resolution to detect with confidence the metal lines, and models for hot stars accounting for metals such as Fe, Mg, Ca, etc. and their effects on the photospheric and wind structure (line blanketing). In this paper, we report impressive recent progress in both observations and theoretical models for hot stars. First steps towards metallicity determinations will also be presented.
VLT (Eckart et al., 1999). Of concern is the presence in Fig. 1 of fairly well isolated lines of Si II, Mg II and Fe II, a invaluable gift for an infrared spectroscopist willing to determine metal abundances and to whom the UV and optical spectra of the star have been swallowed by extinction. Now that the lines are there, the appropriate tool is ‘just’ required: A unified photosphere-wind NLTE-model, accounting for the presence of metals and correctly treating line blanketing. Impressive efforts have been made within the last years in this field by the Munich-group, Hillier & collaborators, and Schmutz & collaborators (see Pauldrach et al., 1997; Hillier, 1997; Schmutz, 1998, for excellent reviews in this topic). In the next section we will present the new generation of line blanketed models developed by John Hillier and compare them to the formerly used standard model.
2. Improved observations The advances in ground based infrared astronomy in this decade are striking. The introduction of very efficient spectrographs suitable for mid–high resolution observations have opened the near-infrared sky to quantitative spectroscopy. In the field of hot stars, high quality IR-spectra have recently appeared in the literature (Krabbe et al., 1991; Hanson et al., 1996; Figer et al., 1997; Fullerton & Najarro, 1997; Blum et al., 1997). Except for Fullerton & Najarro (1997)’s sample (R|10 000), all these spectra were obtained with low–mid (R|500–2000) resolution, which is enough to classify the stars but insufficient in most of the cases to perform accurate quantitative spectroscopic studies. Examples of mid–high resolution observations of early type stars with strong winds are shown in Fig. 1 the H-band of the Pistol Star obtained with UKIRT-CGS4 with a resolution of R|5000 is displayed. The spectra shown in Fig. 1 demonstrate the necessity of high resolution and S / N. We note how the same spectral region observed with R|1000 can blur the presence of the important diagnostic lines. The number of new observational constraints provided by the new spectroscopic data is striking when compared to the previous use of only some H and He lines. The availability of high-quality IR spectroscopic data is currently being substantially improved with NIRSPEC on the Keck II and ISAAC on the
3. New unified models versus standard model The standard model which we used to analyse the He I stars in the central cluster (Najarro et al., 1994, 1997) is basically an iterative, non-LTE method presented by Hillier (1987, 1990) to solve the radiative transfer equation for the expanding atmospheres of early-type stars in spherical geometry, subject to the constraints of statistical and radiative equilibrium. Steady state is assumed, and the density structure is set by the mass-loss rate and the velocity field via the equation of continuity. The transfer equation is solved using either the Sobolev approximation, corrected for the diffuse radiation field, or by means of an accurate solution in the comovingframe. The atmosphere was considered to consist of H, He, N and C, though the latter two species were used basically to better account for cooling processes. The standard model was then prescribed by the stellar radius, R * , the stellar luminosity, L* , the ~ the element abundances, and the mass-loss rate M, velocity field, v(r). The new model (Hillier & Miller, 1998) incorporates new species: O, Mg, Ca, Si, Na, Al and Fe, and can still treat lines and continuum separately using the Sobolev approximation or comoving frame solution, as in the standard old model, or can be switched to ‘blanketing’. In this case, no distinction between
F. Najarro / New Astronomy Reviews 44 (2000) 213 – 220
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Fig. 1. Potential of high S / N and mid–high spectral resolution for early type stars. Top: UKIRT-CGS4 (R | 5000) H-band of the Pistol Star (Geballe priv. comm.). Bottom: Zoom of the region around 1.74 mm with spectra degraded to R|1000.
lines and continuum frequencies is made and the transfer (moment) equations are solved consecutively from the highest to lowest frequency. Hence, the effect of continua on lines and lines on the continua as well as overlapping lines is automatically handled. In addition, the new code accounts for X-ray ionization, clumping, level dissolution, and a smooth transition to the ‘exact’ photospheric stratification based on the method by Santolaya-Rey et al. (1997). Below we compare a set of models with stellar
parameters close to those of the Pistol Star (e.g. Figer et al., 1998) to illustrate the differences between the old standard model and the new code. We consider the old standard model (‘Old’), the new model were Fe is included but no blanketing is used (‘New NBL’) and the new model with Fe and blanketing (‘New BL’). The two classical and related effects of line blanketing: Blocking and backwarming, are clearly shown in Fig. 2 where the flux distribution and the
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Fig. 2. Effects of the inclusion of Fe and line blanketing on the flux distribution (top) temperature structure (bottom). Flux blocking and redistribution as well as backwarming are produced by blanketing. See discussion in text.
temperature structure are displayed for the three models under consideration. In Fig. 2 we see that in order to preserve the luminosity the temperature gradient (and hence the temperature) has to be increased in the inner photosphere to drive out the photons blocked in the UV. The blocked UV flux emerges in the visual and infrared giving rise to a flux excess over the standard model. We note that these are indeed blanketing effects as in the inner parts, log n e . 12, the ‘New NBL’ model shows the same behavior as the ‘Old’ one. In the wind, log n e ,
12, the inclusion of metals and Fe (and not the blanketing) are responsible for the different temperature structure. The effects on the ionization structure of H and Fe are shown in Fig. 3. For these two reference species, we note again the importance of line blanketing, which can significantly change the ionization structure and hence affect the line profiles. The inclusion of metals without blanketing makes only minor changes to the ionization structure and the line profiles. Finally, in Fig. 4 we consider both NLTE and
F. Najarro / New Astronomy Reviews 44 (2000) 213 – 220
217
Fig. 3. Standard Old versus New Blanketed models: Ionization structure. (left:) For Hydrogen significant changes are only introduced when blanketing is incorporated (New LB). The Fe ionization structure is also altered by blanketing.
blanketing effects on two important diagnostic metal lines present in the K-band spectra of the Pistol Star. These are the Fe II 2.089 mm line and the Mg II 2.13 / 4 mm doublet. The Fe II 2.089 mm (z 4 F 3 / 2 – c 4 F 3 / 2 ) transition, despite its rather low gf value ( gf ¯6 3 10 25 ), can be tremendously amplified due to very efficient UV continuum fluorescence pumping. This is the case when the line is formed in the outer regions of the wind, where the population of the upper level can exceed its LTE value by factors up to 10 000 (see Fig. 4). Such conditions are met by stars such as the Pistol Star with very dense winds. There, the Fe II 2.089 mm line is formed in the region with 10 6.5 #n e #10 9 . Similar fluorescent mechanism operates on the much more stronger Mg II 5s 2 S 1 / 2 – 5p 2 P1 / 2 / 5s 2 S 1 / 2 –5p 2 P3 / 2 2.1432 / 2.1368 mm doublet ( gf ¯1.2 and 2.4 respectively). These lines, though formed closer to the photosphere (10 9.5 #n e #10 12 ) are already affected by NLTE effects as the departure coefficient of the upper level takes off in this region (see Fig. 4). It is also important to note that,
though these lines are driven by a NLTE mechanism, i.e., UV pumping, their absolute strengths (i.e., exact run of the departure coefficients) are intimately coupled to blanketing effects as shown in Fig. 4.
4. Metallicity determinations: Ongoing work In this section we present preliminary results from quantitative spectroscopic studies using line blanketed models for the Pistol Star from which, for the first time, abundances for Fe and Mg are obtained. We chose the galactic P Cygni type star HDE 316 285 as a calibrator for our method, to check for consistency between optical and IR analysis before proceeding to investigate GC Center sources for which only IR-spectra are available. We started our analysis of HDE 316 285 using the stellar parameters we have derived recently from a spectroscopic investigation using the ‘Old’ model (Hillier et al., 1998).
Fig. 4. NLTE and blanketing effects on the formation of metal lines in the K-band. Departure coefficients for the upper and lower levels of the Left: Fe II 2.089 mm and right Mg II 2.13 / 4 mm doublet lines. Note the very efficient pumping to the upper levels through UV continuum fluorescence. See further discussion in text.
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First results show only small corrections to the stellar parameters derived by Hillier & Miller (1998) and will be discussed elsewhere. Excellent agreement was found between the synthetic spectra of the new line blanketed models and the observations, both in the optical and the infrared K-band. This is a very important result as it confirms the consistency of the results between optical and IR studies, and hence provides confidence to study Galactic Center sources. Further consistency of our model with recently obtained ISO-SWS observations of HDE 316 285 is also found (Najarro et al., in preparation). The derived magnesium abundance for HDE 316 285 is twice solar, while for iron an abundance four times solar is obtained. The latter result is somewhat surprising as similar enhancements should be expected for Mg and Fe (Langer private communication) for this kind of star. Looking for possible sources which could lead to this discrepancy we first investigated the quality of the atomic data. While we are confident about the quality of the much simpler Mg II model atom, the Iron Project atomic data for Fe II (Nahar, 1995) show discrepancies with Kurucz semi-empirical values, in some cases up to factor of 5 or more. Detailed comparison of model results with the observations seem to favor Kurucz gf values and not those from the Iron Project. We are currently combining both data sets in oder to obtain an optimized Fe II atom. However, the uncertainties in some gf values are not enough to remove the factor of two in the derived Fe abundance. Recent work suggests that the discrepancy may be due to the neglect of the Fe 21 1H↔Fe 1 1H 1 charge exchange reaction. This reaction is exothermic in the left to right direction and requires a non negligible amount of H I to work efficiently. This the case in the outer wind of HDE 316 285 and the Pistol Star where the recombination of hydrogen (see Fig. 3) provides enough H I. Thus, as Fe III strongly recombines into Fe II in this zone, the strength of the Fe II lines formed there, such as Fe II 2.089 mm are strongly increased and a lower Fe abundance is required. First results show that when charge exchange reactions are included the models can reproduce the observations with a Fe abundance twice solar, consistent with the results obtained for Mg.
The resulting lower Fe abundance is strongly supported by new observational data with high S / N and resolution, in which other sort of Fe II lines with strong ( gf |1) values connecting higher lying levels as e 6 F–5p 6 D have been detected. These lines form much closer to the photosphere than the so-called semi-forbidden ( gf |10 25 ) lines like FeII z 4 F 9 / 2 – c 4 F 7 / 2 16 787.177 mm and FeII z 4 F 3 / 2 –c 4 F 3 / 2 20 888.098 mm which form in the outer stellar wind. It turns out that the e 6 F–5p 6 D transitions tend to favor a lower iron abundance than that derived from the the z 4 F–c 4 F transitions. Preliminary results for the Pistol Star show similar trends as for HDE 316 285. While a Mg abundance of roughly 3 to 4 times solar is required to match the observed spectra, we obtain a Fe abundance about 8 times oversolar when charge exchange reactions are neglected. Fig. 5 shows the best current model fits to the K- and H-band spectra of the Pistol Star. The new models including Fe and Mg favor the low temperature model versus the high temperature one proposed by Figer et al. (1998), solving the dichotomy on the Pistol Stars luminosity. From Fig. 5 we see, that although the K-band is fairly well reproduced by our models, some discrepancies appear in the H-band (Si II lines at 1.6907 mm and 1.6907 mm are missing as Si was not included in our models). The He I 1.700 mm line in our models appears severely contaminated by Fe II lines. The strength of these lines is overestimated by our models while that of the strong Fe II 1.868 mm line is underestimated. Though part of these discrepancies are removed if the Iron Project oscillator strengths are replaced by Kurucz semi-empirical values, there is still a non-negligible difference on the derived Fe abundance depending on the sort of Fe II lines used. Therefore, we are currently optimizing our Fe II model atom and investigating in detail the effects of charge exchange reactions on the Pistol Star in order to attain consistency between both Fe II diagnostics and hence see how our estimates reconcile with the solar metallicity value derived from the cool stars (Carr et al., 1996, 1999). The Pistol Star is not the only Galactic Center object showing the presence of Fe and other metal lines. Metal lines also appear in the low–mid resolution spectra of IRS16NE (Tamblyn et al., 1996) and in spectra of the recently discovered second
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Fig. 5. Comparison of the blanketed model (solid) and the observed (grey dashed) spectrum of the Pistol Star. The K-band (Top) is well reproduced. Missing lines in the fit to the H-band (Bottom) are from Si II (neglected in the code) and discrepancies in the Fe II lines are due to uncertainties in the Fe II model atom. See also discussion in text.
LBV candidate in the quintuplet (Figer et al., 1999; Geballe et al., 2000). ISAAC on the VLT (Eckart et al., 1999) and especially NIRSPEC (with R|25 000) on the Keck II telescope are starting to provide new high quality infrared spectra of hot stars in the Galactic Center, for which reliable abundance determinations will be made. It is evident that a new, fascinating research window has opened — the field of quantitative infrared spectroscopy of early type stars.
Acknowledgements F.N. acknowledges grants of the DGYCIT under PB96-0883 and ESP98-1351.
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