Bulgarian Chemical Communications, Volume 47, Special Issue B (pp. 364–367) 2015

Extinction and massive stellar population in Andromeda galaxyP. L. Nedialkov∗

Department of Astronomy, Faculty of Physics, St. Kliment Ohridski University of Sofia,5 James Bourchier Blvd., BG-1164 Sofia, Bulgaria

We probed to derive a new information on the massive star population and the extinction in the disc of Andormeda galaxy from aground based UBV stellar photometry. Classical dereddening procedure was applied to evaluate both the extinction of starlight causedby the dust and the intrinsic properties (luminosity, color) of stars themselves. Thus, we were able draw conclusions about the spatialdistribution of the dust, and – with the use of isocrones – to estimate the age of each successfully dereddened star. The last parameterallowed us to perform a search for sites of coherent star fomation on different time and size scales. We paid a special attention tothe spiral arm S4 where we confirm that evidences exist in support to the density wave theory. Some hidden star formation may bedetectable with the available catalog data along the lanes of highest concentration of dust and gas.

Key words: spiral galaxies, interstellar medium in external galaxies, stellar content and populations, spiral arms, stellar characteristicsand properties

INTRODUCTION

Due to its large angular extend on the sky the spi-ral galaxy Andormeda (hereafter M31) is not an easytarget for complete surveys and photometry of indi-vidual stars. Even nowadays when HST is equippedwith the wide field WFPC3 and ACS detectors only1/3 of disc will be covered by a survey like PHAT [2].When it comes to ground based telescopes, UBV pho-tometry is crucial for deriving the intertwined extinc-tion and physical characteristics of massive stars. Inthis paper we used the UBV RI catalog of [1], contain-ing about two hundred thousands of stars with thatappropriate photometry.

DATA REDUCTION

We started with the construction of the classicaltwo-color diagram in terms of (U −B) vs. (B−V )colors. Dereddening was performed by the means ofconstant slope κ of the reddening vector, equal to 0.72and total-to-selective- extinction ratio AU/EB−V =4.85 [3]. These commonly accepted parameters aretypical for the diffuse component of the ISM of Milkyway rather than for the dense clouds in which somenewborn stars could be found. Adopting of the abovevalues is only a first step approximation allowing usto acquire physically meaningful extinction, luminos-ity, true color and age to some of stars. On the otherhand, the usage of these values simply means that weexpect the extinction by the diffuse ISM to predom-

∗ To whom all correspondence should be sent:japet@phys.uni-sofia.bg

inate the extinction by the dense ISM which is rea-sonable for a highly inclined system as M31 disk. Allstars with normal Q-index earlier than B6V (AOIa)were slide backward to the zero reddening line for themain sequence stars and their true color and absolutemagnitude were calculated.

Later on we put them of color magnitude-diagramto figure out whether they belong to the main se-quence or they are blue supergiants candidates. TheHertzsprung-Russell diagram (see Fig. 1) was con-structed using the synthetic UBV photometry fromthe Database of Geneva stellar evolution tracks andisochrones [4] based on the grids of 5 to 120 M�tracks of Geneva’s stellar evolution models for solarinitial composition and standard mass loss of Mdot =10−6M�/yr and corresponding 46 isochrones withages in the range of 103 yr to 108 yr in time steps of∆ log t = 0.05 dex. We have excluded of further con-sideration both stars with negative extinction on twocolor diagram or bluer than the left end of the main se-quence (zero age curve). Since the fraction of ground-based sources that turned out to be multiple sources inHST is qute high [2] we also put a conservative limitfor the absolute magnitude MU >−10 mag. The rightend of the main sequence was defined as a curve thatconnects the turn off points of all the tracks within thestudied mass range. The stars falling between thesetwo curves and between the tracks of the most mas-sive and the lowest mass star are actually the mainsequence candidates which will be hereafter calleddwarfs for simplicity. Finally we have∼ 34000 dwarfcandidates and ∼ 45000 blue supergiants candidatesin total area of 8 170 �′ of the M31 disc.

364 © 2015 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria

P. L. Nedialkov: Extinction and massive stellar population in Andromeda galaxy

Since [1] did not provide additional photometryuseful for estimation of the foreground contaminationwe selected several fields with a total area of 522 �′

in the outskirts of M31 disc well beyond the stan-dard isophote at µB = 25m/�′′ and as far as possible– away from the major axis. Thus we compare a pho-tometry of the same quality and origin but the contam-ination estimates should be taken as upper limits be-cause some star formation could be presented even atthese remote places. Taking into account the area cor-rection the corresponding percentage of the expectedforeground contamination is 2.5% for the dwarfs sam-ple and 9.7 % for the blue supegians candidates sam-ple.

Following error propagation technique we calcu-lated the error of the derived physical quantities foreach successfully dereddened star. In the case of ex-tinction, luminosity and true color its application isstraightforward. The acquried age is the mean of agesof all isochrones weighted by the inverse square of theratio of two distances measured on HRD. The first oneis the distance star-isochrone, and the second is 1σ

radius of the ellipse of errors towards each isochrone.The error of the age is just the weighted standard de-viation from the weighed mean.

RESULTS AND DISCUSSION

A preliminary analysis of the intial mass function(IMF) which is based on counts of stars between theevolutionary tracks like those on Fig. 1 showed thatone could relay on sample of M31 massive dwarfswhich is completed down to 20 M�. Moreover, theobtained slope of the IMF Γ =−1.5±0.1 is closeto the empirical Salpeter slope [5] and to the theo-retical slope [6]. Practically, it coincides with the

Fig. 1. Hertzsprung-Russell diagram for the massive stel-lar population in M31 galaxy. Stellar tracks of different ini-tial masses (yellow solid lines) and isochrones (violet solidlines) of different ages are indicated on the plot. Stars be-tween the two black solid lines and the stellar tracks of themost massive star and less massive star are Main sequencestars, which spent the bulk of their life burning hydrogenin their cores. The empirical edge where stars become un-stable or Humphrey-Davidson limit is plotted (blue dashedline) on the right upper corner.

result of [7] where some high resolution HST datawas also involved.

We consider the azymuthally averaged distribu-tion of the mean extinction. It is expected in a mag-nitude limited sample as ours the Malquist bias to be

Fig. 2. Radial distribution of the mean extinction versus galactocentric distance as derived for all main sequence stars(blue triangles) and stars with individual extinctional error σE(B−V ) < 0.1 mag, plotted with red circles (main sequence)and squares (blue supergians). The error bars represent the standard deviation of the mean.

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presented. When a highly inclined disc and a largevertical optical thickness are combined the maximumextinction, including the mean, will depend on theabsolute magnitude of the dereddened star. Surpris-ingly, that is not the case with the studied distributionwhich have been found almost independent from theimposed different absolute magnitude limits. As seenon Fig 2, there exists a constancy of of the mean ex-tinction EB−Vw0.5m with the galactocentric distancewhich may indicate optically thick case, indeed. Thestandard deviation of the mean extinction is of thesame order as the extinction itself which reflects thelarge variance rather than big errors of the derivedquantity. Some information about the optical thick-ness of the disk may be locked in the distribution ofEB−V at a certain galactocentric distance or spiral armfragment which demands Monte-Carlo simulations,analogous to those perfomed in [8].

In Fig. 3 we show the spatial distribution in theview plane of the NE part of M31 disc. The dered-dened dwarfs were divided into three subsamples ofdifferent age: oldest stars: log t > 7.0, intermediate

Fig. 3. Spatial distribution in the view plane of the NEpart of M31 disc of three subsamples of main sequencestars wiith different age: oldest stars: log t > 7.0 (red cir-cles), intermediate age stars: 6.0 < log t < 7.0 (green cir-cles) and youngest stars: log t > 6.0 (blue circles). In orderto distinguish between to first two samples only stars withindividual logarithmic age error σlog t < 0.3 were consid-ered. Note the blurring of the stars with the age hintingthat the young groups do dissipate with time. The locationof of 3 prominent van den Bergh’s associations outlined by

solid yellow line are shown as well.age stars: 6.0 < log t < 7.0 and youngest stars:log t > 6.0 (blue circles). By taking only stars withindividual logarithmic age error σlog t < 0.3 in the firsttwo samples we aimed at distinguishing them confi-dently. The idea here is to compare the clustering ofthe samples. We confirm the finding of [9] that theyounger stars are being systematically more stronglyclustered than the older, which are more dispersed.Most clearly it can be seen in the large complexesOB102 and OB54 [10], though there are ’clusters’like OB7 less populated by young stars. The faintstars in OB7 are real older stars rather than heav-ily obscured young stars which is proved here by themeans of proper dereddening. The resolution of ourdata for certain goes down to size scales of few 10 pc,characteristic of unbound stellar aggregates and OBassociations.

Finally, on Fig. 4 we superimposed the starsyounger than 106 yr over all dereddened dwarfs tocheck for signs of density wave. It was done on therectified plane the M31 which corresponds to a faceon orientation of the disc. The only fragment of spi-ral arm where the youngest stars are located along its

Fig. 4. Spatial distribution in the rectified plane the M31disk of all main sequence stars with individual logarithmicage error σlog t < 0.5 (green circles for the fragment of spi-ral arm S4 in the area of association OB78 and red circleselsewhere) with a subsample containing only stars youngerthan log t = 6.0 (blue circles) superimposed. The maximaof gas and dust emission were used to approximate the S4fragment with a logarithmic spiral (yellow solid line). Theage gradient across the spiral arm fragment is shown on the

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inset in the upper right corner.inner edge is S4. At the region of association OB78the age gradient across the spiral arm fragment ismost prominent.

CONCLUSIONS

By the means of a standard dereddening procedurewe derive extinction estimates along ∼ 80000 sight-lines towards M31 galaxy and obtained meaningfulphysical characteristics (luminosity, color, age) of thesame number of stars. Their masses varies form 5 to120 M� with a completeness limit of ∼ 20M�.

The azymuthally averaged mean extinction isnearly constant with the galactocetric distance indi-cating thick disk in the optical: AVw1.5m. We assignthis to the high inclination of the M31 disc, but moresophisticated approach is needed to derive its true ver-tical thickness. Highly variable extinction from star tostar even within the boundaries of one and the samestellar group is the main cause for the high dispersionof the mean extinction.

The younger the stars are, the stronger they areclustered pointing that the groups with mean size-scales like those of the OB associations (∼80 pc)are gravitationally unbound and will be dissolved intodisc background after a certain time.

Spiral arm S4 is still the only place where somenew evidences can be raised in support to the den-sity wave theory. They come from the age gradient ofstars across the arm which is most prominent in theregion of association OB78.

REFERENCES

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EКСТИНКЦИЯ И МАСИВНО ЗВЕЗДНО НАСЕЛЕНИЕ В ГАЛАКТИКАТА АНДРОМЕДА

П. Недялков

Катедра Астрономия, Софийски университет “Св. Климент Охридски”, бул. “Дж. Баучер”№5, 1164 София, България

(Резюме)

Изследвахме потенциала на фотометрията от звездния каталог на галактиката М31 (Massey et al. 2006) да служи като из-точник на нова информация относно звездобразуването и екстинкцията в нейния диск. За целта разработихме процедура заосвобождаване от почервеняване, причинено от праховата компонента на междузведната среда. Тя позволява да бъдат напра-вени изводи както за количеството и пространственото разпределение на праха, така и за физическите параметри (светимост,температура, възраст) на населението от масивни звезди. Потърсени са области на кохерентно звездообразуване на различ-ни времеви скали и размери. Намерени са както доказателства в подкрепа на теорията на спиралната вълна на плътността заръкава S4, така и свидетелства за скрито звездобразуване, близо до областите с най-висока концентрация на газ и прах.

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