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the kinematics of the large amplitude , oxygen - rich agb variables known as o - miras have long been known to give valuable clues to the ages and evolution of agb stars as well as being important for galactic structure studies ( e.g. feast 1963 ; feast & whitelock 2000 ) .
until recently the galactic carbon - rich miras ( c - miras ) have been much less intensively studied .
the present paper summarizes the results of a kinematic and photometric study of galactic c - miras leading to a determination of their absolute magnitudes and ages .
it then goes on to discuss what c- and o - miras tell us about galactic structure and composition .
the paper is based on three recently published papers ( whitelock et al .
2006 ; menzies , feast , & whitelock 2006 ; feast , whitelock , & menzies 2006 ) .
it has been known for a long time that an optically selected sample of c - miras in the lmc shows a well defined @xmath0 - logp relation ( feast et al .
it is now possible to extend this relation to longer periods using data for obscured c - miras ( whitelock et al .
the relation which extends from about 160 to 1000 days has the form @xmath3 where the distance modulus of the lmc has been taken to be 18.50mag .
the scatter about this relation is only 0.17mag .
one aim of the present work was to derive the zero - point of this relation for galactic c - miras .
multi - epoch jhkl observations were obtained of 239 c - rich variables allowing classification and period determination .
mean bolometric magnitudes were obtained by combining these data with iras or msx observations ( details in whitelock et al .
2006 ) . combining these results with data in the literature and
some new optical radial velocities we have available for analysis 177 c - miras with known bolometric magnitudes and radial velocities from optical or millimetre ( co ) observations ( details in menzies et al .
2006 ) . assuming a pl relation of the form given in eq . 1
, we found clear evidence of differential galactic rotation .
the zero - point of this relation was then adjusted until , using a standard formulation of differential rotation , we recovered the oort constant @xmath4 derived from cepheid proper motions ( feast & whitelock 1997 ) . in this way we obtained a galactic zero - point of @xmath5 , not significantly different from that for lmc c - miras ( eq.1 ) . in the kinematic analysis
, we used only stars within @xmath6 of the solar distance from the galactic centre .
the data are not sufficient to allow us to derive an independent pl slope .
cccccc group & period & @xmath7 & age & mass & n + & [ days ] & [ @xmath8 & [ gyr ] & @xmath9 & + all & 521 & @xmath10 & @xmath11 & @xmath12 & 149 + 1 & 373 & @xmath13 & @xmath14 & @xmath15 & 49 + 2 & 521 & @xmath16 & @xmath17 & @xmath18 & 50 + 3 & 655 & @xmath19 & @xmath20 & @xmath21 & 50 +    the residual velocities from differential rotation were then used to derive velocity dispersions .
table 1 shows @xmath7 , the velocity dispersion radial from the galactic centre , for the whole sample and the sample divided by period , as well as the numbers ( n ) of stars involved .
the ages and initial masses were derived using the relation between the kinematics and age of local dwarfs ( nordstrm et al .
2004 ) based on padova models ( girardi et al .
there is some evidence of a trend of age with period , the longer period stars being younger .
this is qualitatively similar to the trend of age with period shown by o - miras ( e.g. feast & whitelock 2000 ) .
the results for the c - miras contrast with the younger ages and higher initial masses sometimes suggested for the bulk of c - miras .
in recent years there has been a considerable amount of work based on the hypothesis that the relative frequency of carbon to oxygen rich ( m - type ) cool stars is a simple measure of the overall metallicity ( normally characterized by [ fe / h ] ) of the stellar system or part of the system in which they are found .
the results of the last section allow one to test this in the limited case of the c- and o - miras and to extend the discussion to cool stars generally .
ccc period & @xmath7 & n + @xmath22 $ ] & [ @xmath8 & + 175 & @xmath23 & 17 + 228 & @xmath24 & 24 + 272 & @xmath25 & 26 + 324 & @xmath26 & 40 + 383 & @xmath27 & 32 + 453 & @xmath28 & 15 +    table 2 shows the velocity dispersion radial from the galactic centre for local o - miras as a function of period . the data are derived from feast & whitelock ( 2000 ) and show the dependence of @xmath7 ( and hence age ) on period for these stars .
a comparison of tables 1 and 2 shows that there appears to be no significant populations of c - miras with @xmath7 in the range 40 to @xmath29 whilst there are strong o - mira populations in this range .
since age increases with @xmath7 , this implies that there are o - miras in an ( old ) age range where there is not a significant c - mira population .
thus the ratio of c- to o - miras will depend on the age distribution of any population considered and not simply on [ fe / h ] .
this result may be seen in a somewhat different way for miras in the solar neighbourhood . for an optically selected sample wood & cahn ( 1977 ) found that the ratio of the number of o - miras to c - miras was about 14 . however , in a local sample of dust enshrouded miras which is biased to longer period ( and hence younger ) stars this ratio is about unity ( olivier , whitelock , & marang 2001 ) .
it is useful to consider in this connection the stellar population of the galactic bulge .
with one possible exception , this contains no c - miras or any normal carbon stars .
however , it does contain o - miras with a wide range of periods .
these range up to 700 days in the sgri window ( glass et al .
1995 ) and to even longer periods near the centre ( wood , habing , & mcgregor 1998 ) .
it has long been known that this implies a considerable age range in the bulge .
the age range implied overlaps with that expected for c - miras ( table 1 ) .
this suggests that the absence of c - miras from the bulge is not simply an age effect .    in fact , in the past it was usually assumed that the absence of carbon stars in general from the bulge was due to a high metal abundance .
it is , however , now known that bulge k giants have a mean [ fe / h ] of @xmath30 and m giants @xmath31 ( fullbright , mcwilliam , & rich 2005 ; rich & origlia 2005 ) .
these values are close to the mean value for the local thin disc , [ fe / h ] = @xmath32 ( nordstrm et al .
thus neither age nor metallicity can apparently explain the absence of carbon stars from the bulge .
however , it has been know for some while ( glass et al .
1995 ) that the jhk colours of o - miras in the bulge differed from those in the lmc and this could be explained by a high oxygen abundance ( feast 1996 ) .
recent work ( rich & origlia 2005 ) now shows that in bulge m giants oxygen is overabundant with respect to comparable local stars ( @xmath33 \sim + 0.3 $ ] ) whilst several workers find @xmath34 \leq 0 $ ] in relevant lmc populations .
this suggests that oxygen abundance may be a controlling factor in the ratio of carbon to oxygen giants in stellar systems .
the above does not rule out the possible dependence of the relative frequency of carbon stars on [ fe / h ] at a given [ o / fe ] .
if that were the case we would expect that in a system with a spread in [ fe / h ] the carbon stars would evolve preferentially from the lower metallicity stars ( [ o / fe ] assumed constant ) .
however , in the galactic disc the available work on carbon stars ( e.g. abia et al .
2001 ) suggests that their [ fe / h ] is near that of the mean of local disc stars and not skewed to lower abundances . but
further work on this is desirable .
two alternative interpretations of the kinematics of local disc stars are currently under discussion .
nordstrm et al .
( 2004 ) found that the velocity dispersion of these stars increases steadily with age from 1 to 10 gyr ( see their fig 31 where their @xmath35 corresponds to the @xmath7 used in the present paper ) . on the other hand quillen & garnett(2000 , 2001 ) ( see also freeman & bland - hawthorn fig 5 where their @xmath36 corresponds to the @xmath7 of the present paper ) suggest that the velocity dispersion remains constant from 2 to 10 gyr at about @xmath37 and then jumps suddenly to about @xmath38 , the two regimes constituting the thin and thick discs .
this latter interpretation apparently leaves no room for galactic components with @xmath7 in the range 35 to @xmath39 where , as table 2 shows there is a significant o - mira population . since o - miras are rare objects we expect them to be tracers of a large main sequence population .
such a population is accounted for naturally in the nordstrm et al .
interpretation of galactic kinematics .
it is , however , difficult to see how it could be accommodated in the quillen and garnett model .
\1 . assuming the @xmath0 - period relation for galactic c - miras has the same slope as for those in the lmc , the zero - points in the two systems are the same within the uncertainties + 2 .
galactic c - miras have ages of @xmath40 gyr and initial masses of @xmath2 .
there is some evidence that the ages of c - miras decrease with increasing period .
the ratio of c- to o - miras in a population depends on the age distribution of that population .
the lack of carbon stars in the galactic bulge is connected to an oxygen overabundance .
the galactic o - miras favour the interpretation of galactic kinematics of nordstrm et al .
this paper is based on three papers mentioned in the text and i am grateful to my colleagues , patricia whitelock , john menzies , freddy marang and m.a.t .
groenewegen for their contributions to this work .
in particular , i am especially indebted to patricia whitelock and john menzies with whom the data were analysed and the conclusions formulated .
abia , c. et al .
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this paper summarizes the conclusions of an extensive investigation of variable carbon agb stars in our galaxy . 
 the zero - point of the period- @xmath0 relation for galactic c - miras is found to be close to that in the lmc . 
 the mean age of galactic c - miras is @xmath1 1.8 gyr and their initial masses @xmath2 with some evidence that age decreases and initial mass increases with increasing pulsation period . 
 kinematic studies of galactic c- and o - miras show that the relative frequency of these two miras classes depends on the age of the parent population . 
 the lack of carbon stars in the galactic bulge seems to be due to a high oxygen abundance . 
 the velocity dispersion of o - miras allows one to distinguish between two possible models of galactic kinematics .