Time variation of radial gradients in the galactic disk: electron temperatures and abundances

0.14 MB·English

by  W. J. Maciel

Astronomy & Astrophysics manuscript no. 6916 (cid:13)c ESO 2008
February 5, 2008
Letter to the Editor
Time variation of radial gradients in the galactic disk: electron
temperatures and abundances
W. J. Maciel1, C. Quireza2, and R. D. D. Costa1
1
Instituto de Astronomia, Geof´ısica e Ciˆencias Atmosf´ericas (IAG/USP), Universidade de S˜ao Paulo, Rua do Mat˜ao
1226, 05508-900 S˜ao Paulo SP, Brazil
7
e-mail: [email protected], [email protected]
0
2
Observat´orio Nacional, RuaGeneral Jos´e Cristino 77, 20921-400 Rio deJaneiro RJ, Brazil
0
e-mail: [email protected]
2

n Received ..., ...; accepted ..., ...
a
J ABSTRACT

1
1 Aims. We investigate the electron temperature gradient in the galactic disk as measured by young HII regions on the
basis of radio recombination lines and the corresponding gradient in planetary nebulae (PN) based on [OIII] electron
1 temperatures.Themaingoalistoinvestigatethetimeevolutionoftheelectron temperaturegradientandoftheradial
v abundancegradient, which is essentially a mirror image of thetemperature gradient.
7 Methods.TherecentlyderivedelectrontemperaturegradientfromradiorecombinationlinesinHIIregionsiscompared
3 withanewdeterminationofthecorrespondinggradientfromplanetarynebulaeforwhichtheprogenitorstarageshave
3 been determined.
1 Results.ThenewlyderivedelectrontemperaturegradientforPNwithprogenitorstarswithagesinthe4-5Gyrrangeis
0 muchsteeperthanthecorrespondinggradientforHIIregions.Theseelectrontemperaturegradientsareconvertedinto
7 O/H gradients in orderto makecomparisons with previousestimates of theflatteningrateof theabundancegradient.
0 Conclusions. It is concluded that the O/H gradient has flattened out in the past 5 Gyr at a rate of about 0.0094 dex
−1 −1
/ kpc Gyr , in good agreement with our previous estimates.
h
p Key words.abundances– gradients – chemical evolution
-
o
r
t 1. Introduction asareflectionofthe radialabundancegradientofelements
s
such as O/H, S/H, etc. in the galactic disk, since these
a
: Radial abundance gradients in the galactic disk and their elements are effective coolants of the ionized gas (see for
v time variations are among the main constraints of chem- example Shaver et al. 1983).
i ical evolution models for the Milky Way. These gradi-
X Recently, Quireza et al. (2006) presented a detailed
ents can be determined from a variety of objects, such as
study of a large sample containing over a hundred HII
r
a HII regions,cepheid variables,open clusters and planetary regions spanning about 17 kpc in galactocentric distances
nebulae (PN). In a recent series of papers, Maciel et al.
for which accurate electron temperatures were determined
(2003, 2005, 2006) estimated the time variation of the ra-
from radio recombination lines, specifically H91α and
dial abundance gradients taking into account a large sam-
He91α. The observations were made with the 140 Foot
ple of PN for which abundances of O/H, S/H, Ne/H and
telescope of the National Radio Astronomy Observatory
Ar/H have been derived. Based on individual estimates of
(NRAO), and are of unprecedented sensitivity compared
the progenitor star ages, it was concluded that the radial
with previous studies. According to this work, the best es-
gradients are flattening out at an average rate of about
timateofthegradient,obtainedfromasampleof76sources
−1 −1
0.005−0.010dexkpc Gyr for the last 8 Gyr, approxi-
with high quality data, is dTe/dR ≃ 287±46K/kpc, with
mately.AcomparisonofthePNgradientswithresultsfrom
no significantvariationsalongthe galactocentricdistances.
HII regions,OBstarsandassociations,cepheidsand,espe-
A slightly larger gradient(up to 17%) was obtained by ex-
cially, open cluster stars, strongly supports these conclu-
cluding some HII regions which are closer to the galactic
sions.
centre, and may not belong to the disk population.
On the other hand,it has long been knownthat a posi- Regarding planetary nebulae, our earlier work (Maciel
tiveelectrontemperaturegradientofabout250−450K/kpc & Fau´ndez-Abans 1985) based on a sample of PN classi-
is observed in the galactic disk, mainly on the basis of ra- fied according to the Peimbert types (cf. Peimbert 1978)
dio recombination line work on HII regions (see for exam- suggested a positive electron temperature gradient in the
ple Churchwell & Walmsley 1975, Churchwell et al. 1978, range 550−800K/kpc,somewhat steeper than the HII re-
Shaveret al.1983,Wink etal.1983,Afflerbachet al.1996, gion gradients observed at the time.
and Deharveng et al. 2000). Such a gradient is interpreted
In this work, we take into account the recent PN sam-
plesanalyzedbyMacieletal.(2003,2005,2006)andderive
Send offprint requests to: W. J. Maciel the PN electron temperature gradient for a sample of ob-
2 W. J. Maciel et al.: Time variation of radial gradients
Fig.1. Age distribution of the PN progenitor stars in the Fig.2. Galactocentric variationof the [OIII] electrontem-
Basic Sample of Maciel et al. (2006). peratures for PN with progenitor ages of 4–5 Gyr. The
emptycirclesshowsomenebulaehavingextremelyhotcen-
tral stars, not included in the linear regressionanalysis.
jects having similar ages. A comparison of the obtained Te
gradient with the recently derived value by Quireza et al.
(2006) for HII regions gives then an independent estimate tary nebulae. We have preferredour owndata where avail-
of the time variation of the radial abundance gradients in able (Costa et al. 2004, 1996, see a list of references in
the galactic disk. Maciel et al. 2003, 2005, 2006), with additional data by
Henryetal.(2004),Kingsburgh&Barlow(1994),andCahn
etal.(1992).TheresultingTe variationwithgalactocentric
2. The electron temperature gradient in the distance R is shown in Fig. 2, where we adopted the same
galactic disk distancesandsolargalactocentricradiusasinourprevious
work. The total number of objects in Fig. 2 is somewhat
The determination of abundance gradients is a difficult lowerthan shownin the 4–5Gyr bracketof Fig. 1, as for a
task,basicallyforthreemainreasons.First,themagnitudes few nebulae we could not obtain accurate electron temper-
ofthegradientsaresmall,amountingatmosttoafewhun- atures.
dredthsinunitsofdex/kpc,sothatarelativelylargegalac- Itcanbe seenfromFig.2thatthereisacleartendency
tocentric baseline is needed in order to obtain meaningful in the sense that higher electron temperatures are asso-
results. Second, the uncertainties both in the abundances ciated with larger galactocentric distances. The best de-
and in the distances contribute to the observed scattering, rived Te gradient for this sample of PN is dTe/dR≃670±
so that large samples are usually needed. Third, chemical 65K/kpc, with a correlation coefficient of r ≃ 0.76, which
evolution models generally predict some time variation of is similar to the gradient for the ‘selected sample’ of our
thegradients,sothatitisextremelyimportanttotakeinto earlier paper (Maciel & Fau´ndez-Abans 1985). Adopting
account in a given sample only objects with similar ages. instead a homogeneous set of [OIII] electron temperatures
For these reasons, some of the analyses of gradients in the fromHenryetal.(2004),whichisthe largesthomogeneous
literatureproduce relativelyflatgradients(see for example sample available for these nebulae, we obtain essentially
Perinotto & Morbidelli 2006). On the other hand, accu- the same result, namely dTe/dR = 680±140K/kpc and
rate and homogeneousabundances eliminate some ofthese r ≃ 0.65, so that the correlation is real. Our best derived
problems,so thatrelativelysteeper gradientsareobtained, slope is illustrated by the dashed line in Fig. 2.
as in Pottasch & Bernard-Salas (2006). The average uncertainty in the determination of the
In our recent work, we made an attempt to overcome electron temperatures is generally considered to be within
some of these problems, and estimated the individual ages 10% for the brightest nebulae, which corresponds roughly
of the PN progenitor stars using an age-metallicity rela- to 1000 K for most objects (see for example Kingsburgh&
tionshipwhichalsodependsonthegalactocentricdistance. Barlow 1994 and Krabbe & Copetti 2005). For the galac-
As a result, we have obtained the age distribution shown tocentric distances an average uncertainty is more difficult
in Fig. 1, adopting our Basic Sample, which is the largest to establish, as it depends on the adopted distances. Since
and most complete sample we have considered, containing most objects in Fig. 2 are located within about 3 kpc from
234nebulae (see Macielet al.2003,2005,2006for details). the solar galactocentric radius, an average error of 50% in
Itcanbe seenthattheagesarestronglypeakedaround the distances would correspond to a shift in the galacto-
4–5 Gyr, where we have 99 objects. In the present work, centric distances of about 0.03 – 1.0 kpc depending on the
we will consider the objects in this age bracket, in order distance and the direction of the line of sight to the neb-
to make comparisons with the younger HII regions. We ula. As a comparison, for the HII regions in the sample by
havethencollectedthe electrontemperaturesofthe plane- Quireza et al. (2006), average formal uncertainties in the
tary nebulae, selecting only the [OIII] temperatures in or- electrontemperaturesarewithin2%,butsystematicerrors
der to keep our sample as homogenous as possible. These may increase this uncertainty up to 10% for the best data.
temperatures are determined from the ratio of the [OIII] Forspectrophotometricdistances,averageerrorsof15%are
4363/5007˚A lines, which are usually among the brightest quoted, while for kinematic distances non-circular stream-
collisionally excited emission lines in the spectra of plane- ing motions may increase this figure to about 25%. From
W. J. Maciel et al.: Time variation of radial gradients 3
the Te×RplotbyQuirezaetal.(2006),anaveragedisper-
sion of about 2200 K can be obtained, which is about half
the dispersion in Fig. 2.
ItshouldbenotedthatthedispersionobservedinFig.2
is probably real, since the electron temperatures may be
affected by several factors, such as differences in the ef-
fective temperature of the central stars, presence of dust,
opticaldeptheffects,electrondensityandtemperaturefluc-
tuations,etc.,apartfromthemaincauseoftheTe×Rvari-
ation,namely,the radialabundancegradient.Theseeffects
are also partially responsible for the observed dispersion
in HII regions, but for PN the variations in the effective
temperatures of the central stars and the uncertainties in
the distances are larger,so that the observed dispersion in
Fig. 2 is larger than in the case of HII regions. As a con-
sequence, some nebulae appear not to follow the observed
correlationveryclosely.Inparticular,PNhavingextremely Fig.3. The inverse correlationbetween the [OIII] electron
hotcentralstars,withtemperaturesinexcessof105K,gen- temperatures and the O/H abundances for PN in the 4–
erally have higher electron temperatures than expected by 5 Gyr age bracket. The empty circles show some nebulae
their galactocentric distances. Furthermore, these objects having extremely hot central stars.
come from more massive progenitors than most nebulae,
so that their ages may be lower than 4–5 Gyr,as assumed.
conclusions of our recent series of papers are supported by
SomeexamplesincludeM1-57,Me2-1,PB6,NGC6620and
the obtained differences between the electron temperature
a few others, which are plotted in the figure as empty cir-
gradients of PN and HII regions, in view of the fact that
cles. Other objects with hot central stars, such as NGC
thesegradientsessentiallyreflecttheradialabundancegra-
2899, NGC 6302, NGC 6537 and NGC 7008 are not plot-
dients in the galactic disk. In order to make a direct com-
ted in Fig. 2, as their electron temperatures are too high
parisonwiththeestimatedflatteningrateoftheabundance
tofitthescale.Allthesenebulaehavenotbeenincludedin
gradients derived by Maciel et al. (2005), we can convert
the linearregression,sothat our derivedelectrontempera-
the Te gradient into the equivalent O/H gradient. For HII
turegradientappliestostarswithtemperatureslowerthan
105K. In this analysis we have used Zanstra temperatures regions we can use the calibration by Shaver et al. (1983),
according to which the oxygen gradient is related to the
and energy-balance temperatures (see for example Preite-
electron temperature gradient by
Martinez et al. 1991, M´endez et al. 1992, Zhang 1993, and
Stasin´ska et al. 1997). dlog(O/H) −4 dTe
Another interesting object is M1-9, which is the neb- ≃−1.49×10 (1)
dR dR
ula with the largest galactocentric distance in the sample
(R≃12.4kpc).Inview ofits positionontheTe×R plane, where the temperature gradient is in K/kpc and the oxy-
it may single-handedly affect the derived slope. For this gen gradient is in dex/kpc. This relation is also supported
object, our own results suggest an electron temperature of by more recent work on HII regions, such as the analysis
Te ≃11000K(Costaetal.1996),butamoredetailedstudy by Deharveng et al. (2000). The oxygen gradient for HII
byTamura&Shibata(1990)andShibata&Tamura(1985) regions is then dlog(O/H)/dR≃−0.043dex/kpc, which is
gives a larger value, Te ≃ 14800K, which is adopted here. similar to the value obtained by Deharveng et al. (2000)
This object may then alter the derived slope by about 50 based on an entirely different sample. Also, an O/H gradi-
K/kpc, but the main conclusions of this paper are unaf- ent of −0.044 dex/kpc was recently obtained by Esteban
fected. et al. (2005) from oxygen recombination lines, a method
The association of higher electron temperatures with almost independent of the assumed electron temperatures,
lower metallicities can be seen from Fig. 3, where we show andtotallyindependentofthe[OIII]forbiddenlines.Asim-
the inverse correlationbetween the [OIII] electron temper- ilar estimate has been presented by Quireza et al. (2006),
atures and the O/H abundances for the objects with ages also based on the Shaver et al. (1983) calibration.
in the 4–5 Gyr bracket. PN with central stars hotter than For planetary nebulae, we can have an idea of the cor-
105K are also shown as empty circles. Again a relatively responding O/H gradient by inspecting Table 1 of Maciel
large dispersion is observed, but the inverse correlation is et al. (2005) for Group II objects (ages of 4–5 Gyr), from
clear, confirming that oxygen is among the main coolants which we get dlog(O/H)/dR ≃ −0.089± 0.003dex/kpc.
in the photoionized gas within the planetary nebulae. Alternatively, we can compute the O/H gradient directly
forthePNsampleadoptedhere,inwhichcasewegetasimi-
larvalue,dlog(O/H)/dR≃−0.090dex/kpc.Therefore,the
3. Comparison of the PN and HII gradients
flattening rate of the oxygen gradient can be estimated by
From a straigthforward comparison of the electron tem-
perature gradient for PN and HII regions we can already 1 dlog(O/H) dlog(O/H)
χ≃ − (2)
conclude thatthere is someflattening ofthe gradientsdur- ∆T (cid:20) dR HII dR PN(cid:21)
ing the time interval of about 5 Gyr, which is essentially  
 
−1 −1
the difference between the ages of the PN progenitor stars so that χ ≃ 0.0094dexkpc Gyr where we have used
considered in this work and the much younger HII regions ∆t≃5Gyr. In view of our discussion on the electron tem-
in the sample by Quireza et al. (2006). In other words,the peratures of planetary nebulae, we would expect the rate
4 W. J. Maciel et al.: Time variation of radial gradients
to be somewhat smaller than this value, which is in excel- Shibata,K.,&Tamura,S.1985,PASJ,37,325
lent agreement with the range estimated by Maciel et al. Stasin´ska,G.,Go´rny,S.K.,&Tylenda,R.1997, A&A,327,736
(2005), namely χ≃0.005−0.010 dexkpc−1Gyr−1. Tamura,S.,&Shibata,K.1990,PASJ,102,1301
Wink,J.E.,Wilson,T.L.,Bieging,J.H.1983, A&A,127,211
As mentioned in the introduction, abundance gradients
Zhang,C.Y.1993, ApJ,410,239
and their time variation are valuable constraints for chem-
ical evolution models. As an illustration, we have com-
pared our derived flattening rate with the predictions of
some recently published models for the Milky Way.As dis-
cussed by Maciel et al. (2006) there may be large discrep-
ancies between different chemical evolution models, even
whithinthe so-called‘inside-out’classofmodels.Inpartic-
ular,Houetal.(2000)adoptedanexponentiallydecreasing
infall rate for the galactic disk, in which a rapid increase
in the metal abundance at early times in the inner disk
leads to a steep gradient. As times goes on, the star for-
mation migrates to the outer disk and metal abundances
areenhancedin thatregion,with the consequencethatthe
gradients flatten out. A rough estimate for the O/H gradi-
ent variation in these models leads to a steepening rate of
−1 −1
χ ≃ 0.0040−0.0060dexkpc Gyr , which is consistent
with ourpresentresults.Asimilar behaviourhas alsobeen
obtainedbyAlib´esetal.(2001).Ontheotherhand,models
such as those based on two infall episodes by Chiappini et
al. (2001), lead to some steepening of the gradients, even
though the inside-out approach is adopted. Possibly, the
mainreasonforthedifferentpredictionsofthequotedmod-
elsappearstoresideonthedifferentadoptedtimescalesfor
starformationandinfallrate,sothatweexpectourpresent
resultsmaybehelpfulinordertoconstrainthesequantities.
Acknowledgements. This work was partially supported by FAPESP
andCNPq.
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