arXiv:astro-ph/0609225v1 8 Sep 2006 orsodneto Correspondence a .Pavlenko V. Ya. h vlto ftelgteh Hne ta.2002). Telescope Space al. Hubble et (Henden on echo based light works the to of According evolution progenitor the massive a of 2005). evolution al. et the around (Munari re- during shells (2004) heavy loss dust through al. arose mass the multiple et shells these in the Alternatively, Loon Mon. origi- Van of ejected V838 2004). discovery and have the Mon al. be may V838 ported et may of Loon or (van envelope dust past 2005) the reflecting in The al. nated et (2005). (Tylenda (2003) al. interstellar al. et et Crause Bond by and detail in studied mag. subsequently 8 echo, by faded the had in January, decline 2003 by gradual the which, a in began Later, flux February. 2002 optical in the 2002; and 2002 al. 2003) in et max- Kimeswenger occurred, al. by 2002; further et subsequently Crause Two variable al. curve et 2002. eruptive light (Munari 6, February in- the an January intense on as on of ima discovery (2002) subject its Brown the since 0825-03833116, been terest has USNO-A2.0 0659-03) 04822- 07015-0346, AAVSO GSC EQJ0704.0-0350, IRAS as 00039, known (also Mon V838 Introduction 1. nrae by increased edopitrqet to requests offprint Send wl eisre yhn later) hand by inserted be (will Astrophysics & Astronomy h itnet 88Mncnb siae from estimated be can Mon V838 to distance The light a of presence the reported (2002) al. et Henden 2 1 4 3 Abstract. 2006 Mar 15 of Version oteosre pcr hwta,i 02Nvme,teeffec the November, 2002 in that, show 2000 spectra H observed the for Next to lists the line using incorporate calculated which were spectra Synthetic November. h o trosre oehrwt 88Mnsosi ob no a be to it words. shows Key Mon V838 with together observed hot the anAtooia bevtr,Aaeyo cecso h U the of Sciences of Academy Observatory, Astronomical Main Hertfords of UK University Research, Astrophysics for Centre eateto srnm,61Cmbl al nvriyo C of University Hall, Campbell 601 Astronomy, Scie of Geographical Department & Physical UK of 5BG, School Group, Astrophysics ± ∼ 0 .Ortertclsetaso oprtvl ekdep weak comparatively a show spectra theoretical Our K. 100 h rpriso 88Mni 02November 2002 in Mon V838 of properties The epeetterslso oeln h 0.45–1 the modelling of results the present We a.Telmnst ekdat peaked The mag. 9 [email protected] : 1 tr:idvda:V3 o–sas topei parameter atmospheric : Mon– V838 individual: stars: , 2 .T.vnLoon van Th. J. , a .Pavlenko V. Ya. : aucitno. manuscript .Foley J. 2 ,TO r,FH O n g,a ela h ADaoi ieli line atomic VALD the as well as MgH, and CO, FeH, CrH, TiO, O, 4 3 .Li W. , .Evans A. , 4 .Smalley B. , V V V 3 band .T Rushton T. M. , ≈ flux 7 20)fudta,frteio ru felements, of group iron the for attempts that, al. sporadic et Kipper found few properties. a atmospheric (2004) star’s only the in derive to resulted by spectrum “L-supergiant” cal giant an M (2003). as May, 2002 cold” al. in by characterized et “very spectrum Evans and infrared was a Its spectrum 2002). to October 2003; the evolved Ashok & in had bands (Banerjee al. et 2002, appeared al. spectrum Kipper mid-April et al. had the et Wisniewski By 2003; Kimeswenger 2005b). TiO several al. of 2002; al. et et Crause in 2002; Rushton 2003; 2004; described al. et al. Kolev et been Osiwa al. la et has (Munari 2002; Mon V838 studies of (2005). al. lution et Munari classi- by and star (2002), B3V al. a et as Wagner fied by discov- confirmed outbursting (2002) the later to star, Munari companion hot & star a Desidera spectroscopically F-type 2002). ered unreddened an al. et of of (Munari colors that integrated looked The progenitor unclear. the are progenitor its . of the ture in star esti- luminous these most If the brightness 2006). maximum was of al. Mon time V838 et the data, at (Sparks then HST correct, kpc are on mates 5.9 2005). based is al. work et distance (Crause recent its kpc to 12 as according high However, as or 2004), (Tylenda aa t itneis distance its data, µ pcrleeg itiuino 88Mnfr2002 for Mon V838 of distribution energy spectral m ie olg ae afil,HrfrsieA1 9AB, AL10 Hertfordshire Hatfield, Lane, College hire, 3 e oe topee fHucid ta.(1999), al. et Hauschildt of atmospheres model Gen n .A Yakovina A. L. and , h opeiyadterrt fV3 o opti- Mon V838 of rarity the and complexity The evo- polarimetric and photometric, spectral, The na- the and Mon V838 of eruption the of cause The ietmeaueo 88Mnwsapproximately was Mon V838 of temperature tive cs el nvriy el tffrsie ST5 Staffordshire, Keele University, Keele nces, rie ooivWos yv17 38 Ukraine 03680 Kyiv-127, Woods, Golosiiv kraine, lfri,Bree,C 42-41 USA 94720-3411, CA Berkeley, alifornia, mlBVdwarf. B3V rmal 3 .M Kaminsky M. B. , tr:evolution stars: – s nec nlog on endence > p Bn ta.20) 8 2003), al. et (Bond kpc 6 2 g rlmnr nlssof analysis Preliminary . 2 .V Filippenko V. A. , t Fits st. ± kpc 2 4 R. , 2 Pavlenko et al.: V838 Mon

[M/H] = −0.4, while abundances of lithium and of low temperatures in M-star atmospheres, and the conse- some s-process elements are clearly enhanced. Later, quent low electron densities, Stark broadening may be ne- Kaminsky & Pavlenko (2005) provided fits to echelle data glected; on the whole the effects of pressure broadening and obtained a similar result, [Fe/H] = −0.3 ± 0.2. These dominate. Computations for synthetic spectra were car- results were obtained using a static LTE model and are ried out with a step of 0.003 µm, for microturbulent ve- −1 therefore very dependent on the model atmosphere and locity vt = 5kms . spectrum synthesis assumptions. There are several reasons to suspect that dust might In this paper we discuss fits to the spectral energy be present in the atmosphere of V838 Mon, see discus- distribution of V838 Mon using data obtained in 2002 sion in Banerjee & Ashok (2002); Evans et al. (2003); November. The observational data used in this paper are Rushton et al. (2005a). However, examination of the re- described in Section 2. Section 3 explains some of the back- sults of preliminary runs of our models without dust show ground to our work and some details of the procedure that the density of condensibles (such as X, Y, Z) at the and input parameters used. The results are discussed in appropriate distance is low, so that precipitation in the Section 5. is extremely unlikely. Hence, we do not in- clude dust in our models. In addition, we do not take account of the effects 2. Observations on the model atmosphere of the presence of a B3 star The spectrum we consider was obtained on 2002 (Munari et al. 2005). Over the wavelength range we are November 6 with the Kast spectograph on the Cassegrain modelling (6000-10 000 A),˚ the SED is dominated by the focus of the Shane 3-m telescope at Lick Observatory. The cool star and the contribution of the B3 star is negligible. resolution of the spectrum is λ/∆λ =3, 000. The observ- There may be irradiation effects to be taken into account ing and data reduction processes are the same as those in if the hot component is part of a V838 Mon binary system, Rushton et al. (2005b) and are not repeated here. but we do not pursue this here. We will, however, include the effects of the B3 star to the overall SED after fitting the red end of the spectrum (see Section 4.4 below). 3. Computation of theoretical spectra The relative importance of the different opacities con- tributing to our synthetic spectra is shown in Fig. 1. Theoretical spectral energy distributions (SEDs) were computed for model atmospheres of giants from the NextGen grid of Hauschildt et al. (1999), with solar 4. Results metallicity (Anders & Grevesse 1989). In our computations we used a number of model at- 4.1. Dependence of fits on input parameters mospheres, with Teff = 2000–2200 K and log g = 0, 0.5. Changes in our input parameters, i.e. Teff , log g, [Fe/H], Computations of the synthetic spectra were carried out us- vt, affect the theoretical spectral distributions (see Fig. 2) ing the program WITA6 (Pavlenko 2000), assuming LTE in different ways, as follows: and hydrostatic equilibrium for a one-dimensional model atmosphere without sources and sinks of energy. (i) The response of the spectrum to changes in log g is The equations of ionization-dissociation equilibrium relatively weak due to the absence of strong atomic were solved for media consisting of atoms, ions, and lines with extended wings. Due to low densities in the molecules. We took into account ∼ 100 components atmosphere of V838 Mon pressure broadening cannot (Pavlenko 2000). The constants for the equations of chem- be significant. Saturated TiO and VO band heads show ical balance were taken from Tsuji (1973). Molecular line rather low sensitivity on log g. data were taken from a variety of sources: (ii) Due to the same reason, changes in the microturbulent velocity mainly affect the heads of strong molecular (i) the TiO line lists of Plez (1998); bands. (ii) CN lines from CDROM 18 (Kurucz 1993); (iii) On the other hand, changes in metallicity reduce the (iii) CrH and FeH lines from Burrows et al. (2002) and molecular absorption over the entire spectral region. Dulick et al. (2003), respectively; (iv) Due to the low temperatures in the atmosphere of 16 (iv) lines of H2 O (Barber et al. 2006); V838 Mon changes in the effective temperature af- (v) absorption by VO, and by a few molecules of (in the fect the slope of the computed spectra in the region case of V838 Mon) lesser importance, was computed 0.6 − 1 µm. This effect can be used to determine the in the JOLA approximation (see Pavlenko 2000); and effective temperature of V838 Mon (see Section 4.3). (vi) atomic line list from VALD (Kupka et al. 1999). Comparison of the computed spectrum with that of The profiles of molecular and atomic lines are de- V838 Mon in 2002 November suggests that the observed termined using the Voigt function H(a, v). Their nat- fluxes at λ >0.9 and λ <0.8 µm are better fitted with a ural (C2) and van der Waals broadening (C6) parame- slightly metal-deficient atmosphere ([Fe/H = −0.5). This ters are taken from Kupka et al. (1999) or, in their ab- is in agreement with the analysis by Kipper et al. (2004) sence, computed following Unsold (1955). Owing to the and Kaminsky & Pavlenko (2004). Pavlenko et al.: V838 Mon 3

10 K I

1 VO AlO Na I 0.1 MgH λ

F MgO 0.01

0.001 TiO 1e-04 Kelu 1 Normalized Flux 1e-05

1e-06 V838 Mon

1e-07 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 Wavelength (micron) 1000 v=2 ∆ v=0 ∆ 100 v=3 v=1 ∆ v=-3 v=-2 --VO, B-X, ∆ , ∆ ∆ γ , , , ε λ 1 1 --CrH, A-X, γ γ F --TiO, v=-1 --TiO, ∆ --TiO, 10 --TiO, , v=2 v=-2-- 1 v=1-- ∆ γ ∆ v=-1-- ∆ , v=1 γ ∆ , ∆ v=0 v=-1-- , φ v=-1-- , v=0 ∆ δ ∆ ∆ γ ∆ , , --TiO, 1 , ε γ γ TiO, --TiO, TiO, VO, B-X, v=0--

1 --TiO, TiO, ∆ , VO, B-X, --TiO, --TiO, δ v=0-- v=0-- v=1-- ∆ Normalized Flux ∆ ∆ , v=1-- v=-3-- TiO, ε ∆ ∆ , , v=2-- v=-2-- δ γ ∆ ∆ v=3-- v=-1-- , , 0.1 TiO, γ δ ∆ ∆ VO, B-X, , , TiO, TiO, VO, B-X, γ δ TiO, TiO, V838 Mon TiO, TiO, JOLA spectrum 0.01 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (micron) Fig. 1. Top: contribution of different molecules to the formation of the spectrum of V838 Mon. The two lower spectra represent observed fluxes of L-dwarf Kelu1 (Leggett et al. 2002) and V838 Mon. Bottom: Identification of the main molecular features in the spectrum of V838 Mon. Data for the figure plotting and color version of the plots are available on ftp://ftp.mao.kiev.ua/%2f/pub/users/yp/Fig.1 4 Pavlenko et al.: V838 Mon

1000 1000

100 N 100 N λ λ F F 10 10

1 1 Normalised Flux Normalised Flux

0.1 0.1 V838 Mon deredd. V838 Mon deredd. 2000/0.0/0 2000/0.0/0, Vt=3 km/s 2200/0.5/0 2200/0.0/0, Vt = 1 km/s 0.01 0.01 0.4 0.5 0.6 0.7 0.8 0.9 1 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (micron) Wavelength (micron) 1000 1000

100 100 N λ λ F F 10 10

1 1 Normalised Flux Normalised Flux

0.1 0.1 V838 Mon deredd V838 Mon deredd. 2000/0.0/0 2000/0.0/0 2000/0.0/-1 2200/0.0/0 0.01 0.01 0.4 0.5 0.6 0.7 0.8 0.9 1 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (micron) Wavelength (micron) Fig. 2. Dependence of computed spectra on the gravity (top left), microturbulent velocity (top right), metallicity (bottom left), and effective temperature (bottom right). The dereddened observed spectrum of V838 Mon is shown by the boldfaced line. Model atmospheres are labeled on the figure as Teff /log/[M/H], i.e 2000/0.0/-1 means Teff = 2000, log g =0.0, [M/H] = -1.

4.2. Comparison with the spectrum of Kelu1 (L2) 4.3. The effective temperature of V838 Mon in 2002

We compare (see Fig. 1) the observed spectrum A comparison of the observed spectrum of V838 Mon with of V838 Mon with that of the brown dwarf Kelu1 theoretical spectra computed using NextGen model atmo- (Leggett et al. 2002). The L2 brown dwarf Kelu1 has ap- spheres is presented in Fig. 3, in which we use a logarith- proximately the same effective temperature as V838 Mon mic scale for the purpose of illustration as the observed but, by definition, its log g is very much higher. There are fluxes cover several orders of magnitude. The instrumen- several significant points to note: tal broadening was modelled by Gaussian profiles set to ˚ (i) Heads of VO bands are clearly seen on at 0.74, 0.79, the resolution of the observed spectra (FHWM = 2 A). − 0.87 and 0.96 µm. The spectrum was dereddened by E(B V )=0.87 mag (ii) The molecular bands of TiO and VO in the spectrum (Munari et al. 2005). of V838 Mon are very intense. Due to the low densities The following are worth noting: these species are practically undepleted in the atmo- sphere of V838 Mon despite the low temperatures. On (i) We can fit the overallslope of the spectrum in the near- the other hand, Ti and V containing species are com- IR/optical region, together with most of the spectral pletely depleted in the atmospheres of L-dwarfs. features. The spectrum is dominated by TiO bands, (iii) The pressure broadened K I and Na I lines are promi- although some bands of VO are also prominent. nent in the optical spectra of L dwarfs Pavlenko (ii) This provides clear evidence that, on 2002 November (2000). Instead of L-dwarfs spectra the alkali lines are 6, C/O < 1 in V838 Mon. absent or very weak in the spectrum of V838 Mon. (iii) The dependence on log g is rather weak, as expected (iv) The electronic bands of CrH and FeH are much weaker (see Section 4.1). Strong atomic lines are practically in the spectrum of V838 Mon than they are in Kelu1. absent in the spectrum. Pavlenko et al.: V838 Mon 5

1000

100 λ F 10

1 Normalised Flux

0.1 V838 Mon obs. V838 Mon dereddened Flux(2000/0.0) Flux+(1.1*λ/.6)4 0.01 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (micron) Fig. 3. Fit of theoretical fluxes to the observed spectrum of V838 Mon. Observed and dereddened spectra are shown by solid thin and thick lines, respectively. The computed V838 Mon spectrum is shown by thin dashed line. The combined spectrum (computed V838 Mon + Rayleigh-Jeans contribution from the B star) is shown by the thick dahsed line. The color version of the plot is available on ftp://ftp.mao.kiev.ua/%2f/pub/users/yp/Fig.3

110 110 V838 Mon V838 Mon 100 19000/4 19000/4 100 90 90 80

λ 70 λ 80 F F

Flux 60 Flux 70

50 60 40 50 30

20 40 3600 3800 4000 4200 4400 4600 4800 5000 3600 3650 3700 3750 3800 3850 3900 3950 4000 Wavelength (A) Wavelength (A)

Fig. 4. Left: Fit of the theoretical spectrum with Teff = 19000 K and log g =4.0 to the hot companion of V838 Mon. Right: The Balmer jump region of the hot companion spectrum. To simplify the picture we exlude the metal line absorption from the accounted opacity sources.

(iv) The dependence on Teff is much stronger. From com- Our value of Teff for 2002 November 6 compares parison of the computed and observed SEDs, we con- favourably with that determined by Tylenda (2005) for clude that the effective temperature of V838 Mon was 2002 day 301 (October 28, the nearest in Tylenda’s com- Teff = 2000 ± 100 K in 2002 November. pilation); he obtained Teff = 2180 K (formally this corre- sponds to a spectral class M8.5). 6 Pavlenko et al.: V838 Mon

4.4. The hot component in the V838 Mon spectrum Fig. 1). Hypothetically, the strong band of CH4 could be formed in a low-temperature regime at these wavelengths. Our observed spectrum of V838 Mon shows a rise in the This band, as well as the bands at 0.619 and 0.890 µm, are flux toward the blue end of the spectrum, at λ ∼< 6500 A,˚ known to be present in the spectra of Jupiter and other which can be attributed to the presence of the “hot” star; gas giants. Possibly the band may be formed in the out- this may be a background or foreground object, or even a ermost layers of the extended envelope of V838 Mon. We member of a V838 Mon binary system (see discussion in plan to provide a more detailed study of the molecular Munari et al. 2005). It is worth noting that all the Balmer features in a forthcoming paper. hydrogen lines, from Hβ to the Balmer jump, are clearly Nonetheless, we have obtained a qualitatively good seen in the observed spectrum. fit of the spectrum of V838 Mon over a wide spectral We first model the contribution of the hot component 4 range (0.4–1 µm) with a model atmosphere having Teff = by a simple Rayleigh-Jeans law Fλ = const/λ and nor- 2000 ± 100 K and C/O < 1, and a hot component consis- malise both fluxes to the same value at 0.67 µm. This com- tent with Teff=18000 - 23000 K and log g =4.0 ± 0.5; this paratively simple procedure allows us to fit to the observed agrees well with previous studies (Tylenda 2005). The spectrum over a wide spectral range, from the blue to the cool component is slightly better reproduced with a mod- near-infrared. erately sub-solar metallicity model. We next computed a set of theoretical spectra using the standard Kurucz model and WITA6, for different val- ues of Teff and log g. A comparison of the observed spec- 6. Acknowledgments trum of the hot companion with theoretical spectra is pre- YP’s work was partially supported by the Royal Society sented in Fig. 4. Unfortunately, our low-resolution spec- and the Leverhulme Trust. The computations were in trum, with its low signal-to-noise ratio in the blue, cannot part performed using the resources of HiPerSPACE com- provide a reliable estimate of the atmospheric parameters puting facility at UCL which is part funded by the (effective temperature, gravity and/or metallicity) of the UK Partical Physics and Astronomy Research Council hot component. From the fit to the observed spectrum we (PPARC). AVF acknowledges support from USA NSF − estimate Teff = 18000 23000 K and log g =4.0. Grant AST-0307894. We thank anonymous referee for the From evolutionary calculation (Schaller et al. 1992) helpful comments. we find the mass of the B star; we then computed the pos- sible range of spectral parallaxes for our hot component. We obtain M =6 M⊙, d = 7.2 kpc for Teff = 18000 and References M =9 M⊙, d = 14.4kpc for Teff = 23000 K, respectively, Anders, E., Grevesse, N., 1989, GeGoAA, 53, 197 i.e only in the case of Teff < 21000 K the hot component Banerjee, D. P. K., Ashok, N. M., 2002, A&A, 395, 161 can be a member of the V838 Mon binary system. Barber R.J., Tennyson J., Harris G.J., Tolchenov R., 2006, MNRAS, 368, 107 5. Discussion and conclusions Bond, H. E., et al., 2003, Nature, 422, 405 Brown, N. J., 2002, IAUC 7785 It seems that the pseudophotosphere of V838 Mon was Burrows, A., Ram, S. R., Bernath, P., 2002, ApJ, 577, 986 evolving sufficiently slowly that its optical spectrum Crause, L.A., et al. 2003, MNRAS, 341, 785 could be approximated by a sequence of classical, i.e. Crause, L.A., Lawson, W.A., Menzies, J.W., Marang, F., static model atmospheres (Kaminsky & Pavlenko 2005); 2005, MNRAS, 358, 1352 we find that the standard computational procedures for Desidera, S., Munari, U., 2002, IAUC7982 SEDs, applied to V838 Mon, give results that are reli- Dulick, M., Bauschlincher, C. W., Burrows, A. 2003, ApJ, able enough to determine the effective temperature. It is 594, 651 worth noting that the presence of P Cyg profiles indicates Evans, A., Geballe, T. R., Rushton, M. T., Smalley, B., 1 expansion velocities up to 200 km s− . However the emis- van Loon, J. Th., Eyres, S. P. S., Tyne, V. H., 2003, sion components of the P Cyg profiles form at the outer MNRAS, 343, 1054 boundary of the envelope, well above the region where the Hauschildt, P. H., Allard, F., Baron, E., 1999, ApJ, 512, observed continuum and absorption lines form. 377 There are distinct differences between the SED of Henden, A., Munari, U., Schwartz, M.B. 2002, IAUC7859 V838 Mon and of late-type dwarfs with similar spectral Kaminsky, B. M., Pavlenko, Y. P., 2005, MNRAS, 357, 38 type, which obviously arise from their differing physi- Kimeswenger, S., Lederle, C., Schmeja, S., Armsdorfer, B., cal nature and the input physical data. In the case of 2002, MNRAS, 336, L43 V838 Mon we have modelled its SED at an advanced stage Kipper, T., Klochkova, V.G., Annuk, K., et al. 2004, A&A, of its evolution and find that its optical spectrum is formed 416, 1107 by absorption of the saturated bands of VO and TiO. Kolev, D., Mikolajevski, M., Tomov, T., et al. 2002, We note that our models do not satisfactorily repro- Collected Papers Physics, Shumen Univers. Press, 147 duce the absorption band located around 0.75 µm; this Kupka, F., Piskunov, N., Ryabchikova, T. A., Stempels, absorption cannot be identified as TiO or VO bands (see H. C., Weiss, W. W. 1999, A&AS, 138, 119 Pavlenko et al.: V838 Mon 7

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