AGB stars as tracers to IC 1613 evolution
aa r X i v : . [ a s t r o - ph . GA ] D ec Mem. S.A.It. Vol. 75, 282 c (cid:13) SAIt 2008
Memorie della
AGB stars as tracers to IC 1613 evolution
S. A. Hashemi , , A. Javadi , J. Th. van Loon , Physics Department, Sharif University of Technology, Tehran 1458889694, Iran School of Astronomy, Institute for Research in Fundamental Sciences (IPM), Tehran,19395-5531, Iran Lennard-Jones Laboratories, Keele University, ST5 5BG, UKe-mail: [email protected]
Abstract. we are going to apply AGB stars to find star formation history for IC 1613galaxy; this a new and simple method that works well for nearby galaxies. IC 1613 is aLocal Group dwarf irregular galaxy that is located at distance of 750 kpc, a gas rich andisolated dwarf galaxy that has a low foreground extinction. We use the long period variablestars (LPVs) that represent the very final stage of evolution of stars with low and interme-diate mass at the AGB phase and are very luminous and cool so that they emit maximumbrightness in near–infrared bands. Thus near–infrared photometry with using stellar evo-lutionary models help us to convert brightness to birth mass and age and from this drivestar formation history of the galaxy. We will use the luminosity distribution of the LPVsto reconstruct the star formation history–a method we have successfully applied in otherLocal Group galaxies. Our analysis shows that the IC 1613 has had a nearly constant starformation rate, without any dominant star formation episode.
Key words.
Stars: AGB – Stars: X-AGB – Stars: LPV– Galaxy: dwarf – Galaxy: metallicity
1. Introduction
IC 1613 is an isolated irregular dwarf galaxythat is located in Local Group. We adopt themean distance of 750 kpc (( m − M ) = . ± .
08 mag )) for this dwarf which is deter-mined by Menzies et al. (2015) by fitting aperiod-luminosity relation to the C-rich Miras.Its proximity, low inclination angle (i = ◦ )and low foreground reddening (E(B-V) = ⊙ for tip-RGB stars, ∼ L ⊙ for asymptotic giantbranch (AGB) stars, up to a few 10 L ⊙ forred supergiants (Javadi et al. 2013). Their spec-tral energy distributions (SEDs) peak around1 µ m, so they stand out in the I-band (and red-dening is reduced at long wavelengths). Theyhave low surface gravity causing them to pul-sate radially on timescales of months to years.The most extreme examples among these long-period variables (LPVs) are Mira (AGB) vari-ables, which can reach amplitudes of ten mag-nitudes at visual wavelengths. The variabilityhelps identify these beacons; their luminositiescan be used to reconstruct the star formationhistory; and their amplitudes pertain to the pro-cess by which they lose matter and ultimatelyterminate their evolution. .A. Hashemi: IC 1613 SFH 283 Resolved stellar populations within galax-ies allow us to derive star formation histo-ries on the basis of colour–magnitude dia-gram modelling, rather than from integratedlight. They also allow us to determine distancesbased on the tip of the red giant branch (RGB),based on the period–luminosity relation of rel-atively young populations of Cepheid variablestars, or based on the luminosities of old popu-lations of RR Lyrae variable stars. Distances tounresolved galaxies are highly uncertain, espe-cially in the local Universe where the Hubbleflow does not yet dominate the peculiar mo-tions of galaxies due to local density enhance-ments in the cosmic web. However, stars canbe resolved in all of the Local Group galaxies,down to luminosities < ⊙ (Javadi et al.2011a,b, 2015).The Star Formation History (SFH) is oneof the most important tracers of the galaxiesevolution. We have developed a novel methodto use LPVs to reconstruct the SFH (Javadi etal. 2011b,c, 2016, 2017; RezaeiKh et al. 2014;Golshan et al. 2017). In this paper we willuse this new technique to represent the SFH ofIC 1613.
2. Data
We benefit from a number of published datasets in near–IR and mid–IR wavelengths (seebelow).
Menzies et al. (2015) published JHKs pho-tometry from three years survey of the cen-tral region of the IC 1613 galaxy. Theyused Japanese–South African Infrared SurveyFacility (IRSF) equipped with SIRIUS camera.They identified all objects brighter than RGB–tip (K ∼
18 mag) as supergiants or AGB stars(not foreground stars or background galaxies).Other data source in near–IR is from Sibbonset al. (2015). They used WFCAM camera onUKIRT to obtain JHK photometry of an areaof 0.8 degree squares centered on IC 1613galaxy. This survey is wider and more com-plete than Menzies’s work. From these data theiron abundance of [ Fe / H ] = − . ± .
08 dex has been calculated and their presented cata-logue contains 843 AGB stars within 4.5 kpcof the IC 1613 galactic center.
The Dust in Nearby Galaxies by
Spitzer (DUSTiNGS) is a
Spitzer
Cycle 8 program thatimaged 50 nearby dwarf galaxies in 3.6 and4.5 µ m bands with a wide range in SFH andmetallicity to detect dust–producing AGB stars(Boyer et al. 2015) . The survey discovered, 50new variable AGB candidates in IC 1613, ofwhich 34 are ”extreme” (x–AGB) candidates.The red colors and variability of DUSTiNGSx–AGB candidates support the strong likeli-hood that these stars are true dust–producingAGB stars.
3. Star formation history
The SFH of a galaxy is a measure of therate at which the gas mass was convertedinto stars over a time interval in the past.The most evolved stars with low to interme-diate mass, at the tip of the Asymptotic GiantBranch (AGB) show brightness variations ontimescales of ≈
100 to > ∼ ⊙ , and reach their maximum brightness atnear-infrared wavelengths. Intermediate-massAGB stars may become carbon stars as a resultof the dredge up of carbon synthesized in thehelium thermal pulses; the resulting change inopacity reddens their colours. Since the max-imum luminosity attained on the AGB relatesto the star’s birth mass, we can use the bright-ness distribution function of LPVs to constructthe birth mass function and hence derive theStar Formation Rate (SFR) as a function oftime. In other words, the SFH is the SFR, ξ (inM ⊙ yr − ), as a function of elapsed time, t. Theamount of stellar mass, dM , created during atime interval, dt , is: dM ( t ) = ξ ( t ) dt . (1)
84 S.A. Hashemi: IC 1613 SFH
Fig. 1.
SFR versus look–back time (t) for three choices of metallicity.
Therefore, the number of formed stars are re-lated to this mass by the following equation: dN ( t ) = R maxmin f IMF ( m ) dm R maxmin f IMF ( m ) m dm dM ( t ) , (2)where f IMF is the Initial Mass Function (IMF).We use the IMF defined in Kroupa (2001). Weneed to relate this to the number of stars, N ,which are variable at the present time. If starswith mass between m ( t ) and m ( t + dt ) are LPVsat the present time, then the number of LPVscreated between times t and t + dt is: dn ( t ) = R m ( t + dt ) m ( t ) f IMF ( m ) dm R maxmin f IMF ( m ) dm dN ( t ) . (3)Substituting equation 1 and 2 in equation 3gives: dn ( t ) = R m ( t + dt ) m ( t ) f IMF ( m ) dm R maxmin f IMF ( m ) m dm ξ ( t ) dt . (4) We are considering an age bin of dt , to de-termine ξ ( t ). The number of LPVs observed inthis age bin, dn ′ , depends on the duration ofthe evolutionary stage during which the longperiod variability occurs: dn ′ ( t ) = δ tdt dn ( t ) . (5)Finally, by combining the above equationswe obtain a relation to calculate the SFR basedon LPVs counts: ξ ( t ) = R maxmin f IMF ( m ) m dm R m ( t + dt ) m ( t ) f IMF ( m ) dm dn ′ ( t ) δ t . (6)To obtain the SFR we need to determine theindividual stars’ masses, ages ( t ) and durationof pulsation ( δ t ). For this we rely on theoret-ical models. The most appropriate theoreticalmodels for our purpose are the Padova models(Marigo et al. 2017) .A. Hashemi: IC 1613 SFH 285 Fig. 2.
SFH with considering the metallicity evolution of the galaxy
4. Results
The SFR as a function of look-back time (t)in IC 1613 is shown in figure 1 for di ff erentmetallicites. The horizontal errorbars representthe age bins.As a galaxy ages, the metallicity of theISM and hence that of new generations ofstars changes as a result of nucleosynthesisand feedback from dying stars. So we expectolder stars to have formed in more metal poorenvironments than younger stars have. Figure2 shows the SFH when we consider the e ff ectof chemical evolution of the galaxy.
5. conclusions – Our analysis shows that the SFH of the ob-served field in IC 1613 is consistent withbeing almost constant over the lifetime ofthe galaxy. – Our results were obtained completely inde-pendently, using di ff erent data and a di ff er-ent method, and yet they are corroboratedby previous work (Skillman et al. 2014). References
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