Download A Revised DDO Abundance Calibration for Population I Red Giants

J. Astrophys. Astr.(1993) 14, 145–165
A Revised DDO Abundance Calibration for Population I Red Giants
A. E. Piatti & J. J. Clariá
Observatorio Astronómico, Universidad National de Córdoba,
Laprida 854, 5000 Córdoba, Argentina
D. Minniti Steward Observatory, University of Arizona, Tucson, Arizona 85721, USA
Received 1993 May 27; accepted 1993 November 10
Abstract. Several arguments that justify establishing a revised abundance
calibration for DDO photometry of population I red giants are presented.
The components of the blanketing vector in the DDO C(45–48) vs C(42–45)
diagram are determined for late-type dwarfs and giants. We have redefined
the DDO cyanogen anomaly and calibrated it against metallicity. The
sample of field giants now available with abundances derived from high
dispersion spectroscopy is substantially larger than previously available,
leading to a more accurate abundance calibration. Iso-abundance lines in
the C(41–42) vs C(42–45) diagram have been determined for population
IG and Κ giants and an iterative method for deriving abundances of these
stars is described. We show that the new DDO abundances are in very
good agreement with those derived from high dispersion spectroscopy.
The new method improves by about 0.1 dex the DDO abundances derived
for early G and/or late Κ giants, with respect to the δCN method of Janes
(1975).
Key words: Technique: photometry—stars: late-type—stars: population I
—stars: general.
1. Introduction
The abundance studies of population I stars of our Galaxy are important in different
contexts. In the particular case of red giants in intermediate-age open clusters and
specially in the older ones, such studies are essential to probe the chemical history
of the galactic disk, to study variations in the metal content with position in the
Galaxy, as well as to examine the age-metallicity relation in different regions.
It is well known that the DDO photometric system offers the possibility of determining abundances, basically iron-to-hydrogen ratios, of population I and II late-type
stars (Janes 1979a; McClure 1979). There exists, however, a substantial difference in
the behaviour of the DDO indices of the stars of one or the other population. The
DDO photometry of population I stars seems to be easier to interpret. With a few
exceptions, the metallicity in this case can be determined unambiguously within the
limitations of the uncertainties of the available calibrations. On the contrary, several
factors conspire to make more difficult the calibrations for the population II stars.
In fact, not only are these stars generally fainter, but more importantly, they appear
145
146
A. Ε. Piatti, J. J. Clariá & D. Minniti
to be more heterogeneous in their chemical composition in comparison with those
of population I (Hesser et al. 1977).
As population I we refer here not only to the younger more metal-rich part of the
Galactic disk, but also to the more metal-poor stars of the old disk having [Fe/H]
between – 0.3 and – 0.8.
There are at least three reasons to justify embarking on a new DDO abundance
calibration for population IG and Κ giants. First of all, the sample of field and open
cluster red giants with abundances derived from high dispersion spectroscopy has
been noticeably increased in latter years.
A second justification comes from the fact that there are some inconsistencies in
the definition and calculation of the cyanogen anomaly. On the one hand, the DDO
metallicity indicator for population I giants, δCN, according to its original definition
by Janes (1975) is calculated from the intrinsic DDO colours of the star without
correcting them for blanketing effect which introduces the first inconsistency. On the
other hand, the cyanogen anomaly should be referred to the G and Κ giants in the
solar neighbourhood (and not to the solar-abundance giants) because these first
ones were used by Janes (1975) to define the δCN index. This introduces a second
inconsistency.
A third argument is that there is up to now no calibration at all of the C (41–42)
vs C (42–45) diagram in terms of the metal content for luminosity class III stars. If
one had this diagram with well-defined iso-abundance lines, it would be possible to
estimate abundances for previously dereddened population IG and Κ giants.
In this paper we present a new DDO abundance calibration for population I red
giants. A preliminary version of this calibration was presented in Ciariá et al. (1992),
while a new DDO calibration for population II giants is given in Ciariá et al. (1993).
In section 2 we determine the components of the blanketing vector in the C (45–48)
vs C (42–45) diagram for late-type dwarfs and giants. In sections 3 and 4 we revise
some previous [Fe/H] vs δCN relations and introduce a new definition of the cyanogen
anomaly. In section 5 the average abundance for G and Κ giants in the solar neighbourhood is determined, while in section 6 the relationship between [Fe/H] and the
new cyanogen anomaly is presented. In section 7 we determine iso-abundance lines
in the C(41–42) vs C(42–45) plane for the luminosity class III stars, we describe a new
procedure for determining DDO abundances in late-type population I giants, and
we show that this approach represents an improvement over the previous technique
of Janes (1975). The main conclusions of this paper are summarized in section 8.
2. Determination of blanketing coefficients
Although the C (42–45) and C (45–48) indices depend mainly on the temperature and
surface gravity, both have a slight dependence on the metallic and/or molecular
blanketing effect. The C (41–42) index is mainly sensitive to the molecular blanketing
in the 4200 Å region as it measures the strength of the CN band at 4216 Å.
Because the DDO calibrations of effective temperature and MK spectral type
recently published by Piatti & Clariá (1992) are based on normal, defined as approximately solar-abundance stars, the blanketing relations must be known so that the
measured DDO colours of non solar-abundance stars may be corrected for blanketing
effect. We have then to determine the α and β coefficients of the following equations:
A revised DDO abundance calibration
147
(1)
(2)
where Cn and Co indicate normal and intrinsic colours, respectively.
These coefficients were first determined by Osborn (1971, 1979). We redetermine
them here using photometric and spectroscopic data published after Osbom’s papers.
We selected G and Κ giants and dwarfs with abundances determined from high
dispersion spectra and for which DDO and Cousins VRI photometry is available.
We added also a few red giants with photometric metallicities derived by Norris et al.
(1985).
The DDO photometric data were taken from several papers, mainly by McClure
(1976), Dean (1981), Hartkopf & Yoss (1982). and Norris et al. (1985), whilst the (R-I)
indices come essentially from Taylor (1986), Cousins (1980), and Norris et al. (1985).
The abundance information was mainly taken from Cayrel de Strobel et al. (1985),
Bell & Gustafsson (1989), and Norris et al. (1985).
In the case of stars with abundances derived from high dispersion spectroscopy,
the E(B-V) colour excesses were determined in two different ways. The first consists
in comparing the observed (B-V) index of the star with the intrinsic (B-V) colour
according to its MK type using the Schmidt-Kaler (1982) calibration. The second
method, valid only for giant stars, is the iterative method of Janes (1977). We adopted
for these stars the average of both determinations. For the stars with photometric
abundances we adopted directly the E(BV) colour excesses derived by Norris et al.
(1985).
Once the E(B-V) colour excesses are known we determined the intrinsic DDO
colours using the colour excess ratios given by McClure (1979), whereas the intrinsic
(R-I) colours were derived using the E(R-I)/E(B-V) ratios given by Taylor (1986)
for G and Κ dwarfs and giants.
The (R-I) colour is essentially free from blanketing effects (Johnson et al. 1968).
In consequence, the deviation in the position of a star from the standard sequence
line in the (R-I) vs C(45–48) diagram, is an indicator of the amount of blanketing
in the C (4548) index, and the same can be said for C(42–45).
In order to determine the blanketing coefficients we proceeded as follows: (1) From
the MK vs (R-I) relation of Straizys (1991) and the MK vs intrinsic DDO colour
relation of Piatti & Clariá (1992), we determined the mean relation between the
intrinsic DDO colours and (R-I) for the luminosity classes III and V. (2) Then, we
computed the normal DDO colours according to their (R-I) indices using the relations
obtained before. (3) We calculated the differences ∆C(45–48) and ∆C(42–45) between
the intrinsic and normal DDO colours and plotted these deviations against [Fe/H]
(Figs. 1 and 2). (4) Finally the α and β coefficients were determined from linear least
squares fits in Figs. 1 and 2.
As shown in Figs. 1 and 2, only a limited range in [Fe/H] has been considered.
This is due to the fact that for metal-poor stars (typically of population II), the
direction of the blanketing vector in the C (45–48) vs C (42–45) diagram is different
from that of population I stars (Clariá et al. 1992). We noted this change in the
behaviour of the blanketing vector, in contrast with Osborn (1979), so that the derived
values are valid only for population I stars.
Table 1 lists the stars used for determining the blanketing coefficients. Its columns
give in succession the identification of the star, the MK type, the source of the (B-V),
148
A. E. Piatti, J. J. Clariá & Minniti
Figure 1. Difference between the intrinsic and normal C(45-48) colours plotted against.
[Fe/H], for luminosity classes III and V. The slope of the broken line is the blanketing coefficient
for the C(45-48) index.
Figure 2. Difference between the intrinsic and normal C(42-45) colours plotted against
[Fe/H], for luminosity classes III and V. The slope of the broken line is the blanketing coefficient
for the C(42–45) index.
A revised DDO abundance calibration
149
Table 1. Stars used for determining the blanketing coefficients.
(Continued)
A. E. Piatti, J. J. Clariá & D. Minniti
150
Table 1. (Continued)
References: 1. Cayrel de Strobel et al. (1985) 2. Jaschek et al. (1972) 3. McClure
(1976) 4. Dean (1981) 5. Hartkopf & Yoss (1982) 6. Janes (1975) 7. Norris et al.
(1985) 8. Flynn & Mermilliod (1991) 9. Taylor (1986) 10. Cousins (1980).
DDO, and (R-I) photometry, the adopted E(B-V) colour excess, and the intrinsic
DDO and (R-I) colours. The MK spectral types were taken from Jaschek et al.
(1972), Houck & Cowley (1975), and Cayrel de Strobel et al. (1985) and when not
available they were derived from the DDO colours using the calibration of Piatti &
Clariá (1992). Table 2 presents the differences between the intrinsic and normal DDO
colours, the adopted abundance, and its reference. Whenever a star appears in two
or more references, the corresponding mean values were adopted. A few values in
brackets have not been used to determine the blanketing coefficients. Table 3 gives
the values found for the α and β blanketing coefficients together with the standard
error of the solution and the spectral range for which they are valid. The resulting
coefficients are very similar to those derived by Osborn (1979).
Figure 3 shows how a decrease in the abundance by a factor of ten would affect
the positions of dwarfs and giants in the C (45–48)vs C (42–45)diagram. The blanketing
vectors run almost parallel to the standard luminosity sequence, as expected since this
diagram discriminates temperatures and surface gravity more than metal abundance.
A revised DDO abundance calibration
Table 2. Metallicities and differences between intrinsic and
normal D D O colours for the stars used to determine the
blanketing vectors.
(Continued)
151
152
A. E. Piatti, J. J. Clariá & D. Minniti
Table 2. (Continued)
* Star used to determine the mean abundance of G and K giants
in the solar neighbourhood (see section 5).
References: 1. Cayrel de Strobel et al. (1985) 2. Bell &
Gustafsson(1989) 3. Norris et al. (1985) 4. Geisler et al. (1991).
Table 3. Derived blanketing coefficients.
3.
Previous calibrations of dCN as a function of [Fe/H]
The cyanogen molecule is a major contributor to the absorption between 3500 and
4200 Å in late-type giants. Their sharp well-defined bands appear towards 4216 Å
and 3883 Å together with the iron lines. The DDO filter 41 lies precisely in the region
of the blue CN band with maximum at 4160 Å. The C(41–42) index measures the
intensity of this band relative to a comparison band situated between the CN band
A. revised DDO abundance calibration
153
Figure 3. The standard sequence lines for dwarfs, giants, and supergiants in the Co (45–48)
vs Co (42–45) diagram. The arrows show how a decrease in the metal abundance by a factor
of ten would affect the positions of giants and dwarfs.
and the G band. This index is sensitive to both the metallicity and the luminosity,
but it is generally used as an abundance indicator.
Janes (1975) has tabulated the average of C(41–42) for G and Κ giants as a function
of C (45–48) and C (42–45). He has shown that the cyanogens anomaly δCN–defined
as the excess (positive δCN) or deficiency of the CN index C(41–42) in magnitudes
over the standard value for a star of the same C(45–48) and C(42–45) indices – is
well correlated with [Fe/H]. From 44 stars with spectroscopically determined values
of [Fe/H], Janes derived the relation:
(3)
valid for [Fe/H] > – 0.8. Since δCN = 0.0 by definition for G and Κ giants of solar
neighbourhood, equation (3) indicates that the Sun is metal-rich by 0.2 dex relative
to the general giant star population.
Deming et al. (1977) while comparing DDO and Strömgren data of binary systems
with early and late-type components found the same slope of equation (3) but changed
the zero point to – 0.07 to obtain a better agreement with the metallicity of the
Hyades. The zero point was changed slightly by other authors also (McClure 1979;
Clariá and Lapasset 1983) following the different [Fe/H] values that were adopted
for the Hyades.
4. A new definition of the cyanogen anomaly
As mentioned in section 1, one of the reasons that justify establishing a new DDO
abundance calibration comes from having detected some inconsistencies in the
calculation of one of the metallicity sensitive DDO indices. The first inconsistency
154
A. E. Piatti, J. J. Clariá & D. Minniti
Figure 4. Difference between the old and new cyanogen anomaly.
was noted in the original definition by Janes (1975) of the cyanogen anomaly and its
later evaluation. In fact, although the two primary DDO indices are recognized
indicators of temperature and luminosity both must be corrected for blanketing effects
before using them to derive the standard C(41–42) index. Therefore we prefer to
redefine the cyanogen anomaly of a star as the difference between the unreddened
C(41–42) and the standard value of this index corresponding to a star with the same
temperature and surface gravity [and not with the same Co(45–48) and Co(42–45)
indices] as the star in question. We call ∆CN the new cyanogen anomaly.
This discussion can be clarified with an example. Let us suppose that the location
of a metal-poor red giant in the C(41–42) vs C(42–45) diagram is indicated by Ρ in
Fig. 4. The point P’ represents the location which the star would have on the standard
luminosity class III sequence traced by Janes (1975) since the PP’ shift follows the
direction of the blanketing vector. According to Janes the cyanogen anomaly δCN
is PQ’ and not QP’ = ∆CN as it should be in order to take the blanketing effect into
account. In practice, first the star must be displaced to the point Q (correcting for
blanketing the temperature index) and then the new cyanogen anomaly ΔCN must
be calculated. A similar consideration pertains to the C(4142) vs C(4548) diagram.
5. Mean abundance of the solar neighbourhood G and K giants
Table 7 of Janes (1975) allows us to obtain standard C(4142) colours as a function
of C(42–45) and C(45–48). This table has been constructed using 1200 G and Κ stars
in the solar vicinity which do not have on the average solar abundance. To use this
table the blanketing correction for a given star should be proportional to the ironto-hydrogen ratio of this star relative to the G and Κ giants of the solar neighbourhood.
155
A revised DDO abundance calibration
That is, the standard DDO indices should be calculated from the equations:
(4)
(5)
where α and β are the blanketing coefficients derived in section 2 and [Fe/H]GK
represents the abundance of the Sun relative to the average abundance of the solar
neighbourhood G and K giants.
The problem now is to estimate the mean abundance of the G and Κ giants in
the solar vicinity relative to the Sun. To do this we gathered from the literature UBV
and DDO data for numerous G and K giants with high dispersion abundances. We
calculated the E(B-V) colour excess as described in section 2. We then derived the
intrinsic (B-V) and DDO colours. Finally, adopting R = Av /E(B-V) = 3.0 we calculated
the spectroscopic parallaxes and hence the distances using the Schmidt-Kaler (1982)
calibration.
The stars selected for estimating the mean abundance of G and Κ giants of the
solar neighbourhood are shown in Tables 4 and 5. Table 4 gives the HD number,
Table 4. Stars used for determining the mean abundance of G and
K giants in the solar neighbourhood.
(Continued)
A. Ε. Piatti, J. J. Clariá & D. Minniti
156
Table 4.
(Continued)
References: 1. Cayrel de Strobel et al. (1985) 2. Geisler et al. (1991)
3. McClure(1976) 4, Hartkopf &Yoss (1982) 5. Flynn & Mermilliod
(1991) 6. Dean (1981) 7. Janes & McClure (1971) 8. Janes (1975)
9. McClure (1970).
157
A revised DDO abundance calibration
Table 5. Distances of stars used, together with those of
Table 4, to determine the mean abundance of G and Κ
giants in the solar neighbourhood. The basic data of these
stars are given in Tables 1 and 2.
Table 6. Stars used for the [Fe/H] vs ∆CN calibration. The UBV
and DDO photometry, MK spectral types, and adopted E(BV)colour
excesses are given in Tables 1 and 4.
(Continued)
Α. Ε. Piatti, J. J. Clariá & D. Minniti
158
Table 6.
(Continued)
A revised DDO abundance calibration
159
the MK spectral type from Jaschek et al. (1972) or Cayrel de Strobel et al. (1985),
the source of the UBV and DDO photometry, the adopted E(B-V) colour excesses,
the distance (in pc), and the metallicity taken from Cayrel de Strobel et al. (1985)
and/or Bell & Gustafsson (1989). Whenever a star appears in two or more references,
the. corresponding mean values were adopted. Table 5 lists the distances of stars
whose basic data are given in Tables 1 and 2.
We find that 82 G and Κ giants located within a radius of 100 pc from the Sun
are on the average more metal-poor than the Sun by 0.10 dex, i.e., 〈 [Fe/H] G K 〉 =
+ 0.10± 0.02 (σn), where ση is the standard deviation from the mean. This value holds
increasing the sample to 100 giants and the distance to 200 pc.
Equations (4) and (5) can now be expressed as follows:
(6)
(7)
where [Fe/H] represents the abundance relative to the Sun.
6. The new DDO abundance calibration
We now have the tools for deriving a new abundance calibration for population I
red giants. For this purpose we selected a sample of 82 red giants – which doubles
the sample used by Janes (1975) – with known UBV and DDO data and accurate
spectroscopic abundances in the range + 0.3 [Fe/H] – 0.8. We corrected for
reddening and blanketing, and then computed for each star the new cyanogen anomaly
∆CN (Table 6).
The correlation found between [Fe/H] and ∆CN is excellent (Fig. 5). A linear least
֒
֒
Figure 5. The correlation between the new cyanogen anomaly ∆CN and [Fe/H] for 82 G
and Κ giants with abundances derived from high dispersion spectroscopy. Typical error bars
are shown.
160
A. E. Piatti, J. J. Clariá & D. Minniti
squares fit leads to:
[Fe/H] = 3.75 ΔCN – 0.16.
(8)
A typical error in ΔCN of ± 0.03 implies an error in the DDO abundance of ± 0.10
dex. However, due to the lack of calibrators with metallicities larger than + 0.30 dex
this error is certainly larger for very metal-rich stars.
7. Iso-abundance lines
The procedure followed to determine the iso-abundance lines in the C(41–42) vs
C(42–45) diagram for luminosity class III stars consists in expressing the intrinsic
DDO colours in terms of standard ones, inverting the equations (6) and (7) and using
the new relation between [Fe/H] and ∆CN given by (8). This leads to the equations:
(9)
(10)
(11)
Knowing the relationship between the three standard DDO colours for G and Κ
giants, one can find the relation between the three intrinsic DDO colours, for different
values of [Fe/H].
Fig. 6 shows the calibration obtained for the luminosity class III stars in the
C(41–42) vs C(42–45) diagram as a function of the metallicity. The arrow gives the
direction of the blanketing vector. The lines of equal abundance (in steps of [Fe/H] = 0.1)
Figure 6. Iso-abundance lines, insteps of [Fe/H] = 0.1, in the C o (41–42) vs Co (42–45)diagram
for the luminosity class III G and Κ stars.
A. revised DDO abundance calibration
161
range from [Fe/H] = + 0.4 to – 0.8 (typically population I stars). We have calibrated
the C(45–48) vs C(42–45) diagram with iso-abundance lines, but in this case these
lines practically overlap because the blanketing vector runs almost parallel to the
luminosity class III mean sequence.
Recently Tripicco & Bell (1991) examined the properties of the DDO system using
colours derived from synthetic spectra for different surface gravity and metallicity
values. Based on these colours physical details which underly the properties of this
system can be examined. It is also possible to see why the DDO system behaves in
the way it does.
Some of the results obtained by Tripicco & Bell are to a certain degree controversial.
In fact, although the direction of the blanketing vector in the C(41–42) vs C(42–45)
diagram follows approximately the one we traced empirically (see Fig. 6), the agreement
does not exist in the iso-abundance lines. Tripicco & Bell recognize this problem which
makes apparently negligible the metallicity effects in the hot giants, in contradiction
with the observations. On the other hand, the models assuming solar-metal abundance
and at the same time carbon depletion and nitrogen enhancement, predict a direction
of the blanketing vector different from what we have found. According to the models,
when CN abundance increases (due to conversion of carbon into nitrogen), the stars
move upward and to the left in the C(41–42) vs C(42–45) diagram. This disagrees
with the direction we have empirically determined (Fig. 6).
We recommend the following procedure for deriving metal abundances from DDO
colours of population I G and Κ giants: (1) Correct the measured DDO indices for
interstellar reddening using the methods described in section 2. (2) Plot the observations
in the C(41–42) vs C(42–45) diagram and interpolate between the iso-abundance
lines to derive an initial estimate of [Fe/H]. (3) Derive the standard DDO colours
from equations (6) and (7) and the new cyanogen anomaly ∆CN using Table 7 of
Janes (1975). (4) Obtain the new value from equation (8). The method can be iterated
until the desired precision is obtained.
Given the nature of the corrections introduced in the new method to derive overall
abundances (see section 4 and Fig. 4), we expect to get an improvement over the
δCN method of Janes (1975) only for metal-poor ([Fe/H] < – 0.3), early G and late
Κ giants (T > 5200 Κ and 3800 < Τ < 4100 Κ).
Recently McWilliam (1990) has published a list with high dispersion spectroscopic
abundances of 671G and Κ field giants, 303 of them having known DDO and
(B-V) data from McClure & Forrester (1981) and other sources (e.g., Hoffleit 1964).
Unfortunately, none of these stars with known photometric data has Τ > 5200 Κ and,
at the same time, lies within the reddening and abundance calibrations of Janes (1975,
1977) and within the range of iso-abundances lines of Fig. 6. On the other hand, 30
stars with 3800 < Τ < 4200 Κ fulfill these conditions and are useful for comparing the
abundances derived from both techniques.
The second column of Table 7 lists the Ε (BV) colour excesses derived for those 30
stars from the iterative procedure of Janes (1977). In the following columns the
cyanogen anomalies and abundances derived from both techniques are presented along
with the number of iterations performed until the [Fe/H] values converge within
0.01. Columns 8 and 9 show the differences (in absolute value): Δ1 =|[Fe/H]HDS –
[Fe/H] Janes | and Δ2 = | [Fe/H]HDS– [Fe/H]new| where [Fe/H]HDS represents the
spectroscopic abundance taken from McWilliam (1990). The resulting mean values
are: 〈Δ1〉 = 0.20 ± 0.10 and 〈Δ2〉 =0.12 ± 0.09.
162
Table 7.
A. E. Piatti, J. J. Clariá & D. Minniti
Comparison between photometric and high dispersion spectroscopy abundances.
As shown in Table 7, the difference ((∆1− ∆2) is always positive, increasing slightly
with decreasing [Fe/H]. The mean value is 〈Δ1 – Δ2〉 = 0.08. From this analysis we
conclude that the improvement in the determination of DDO abundances is of the
order of the 0.1 dex for late Κ giants.
A second test was performed by comparing the mean DDO abundances of several
open clusters with their known high dispersion spectroscopic abundances. For this
purpose we selected 8 open clusters having at least 4 red giant members each (Table 8).
Even though some of these clusters have nearly solar metal content, their members
have typical effective temperatures of early and intermediate G stars. The mean
differences are now: 〈Δ1〉 = 0.13 ±0.08 and áΔ2〉 = 0.06 ± 0.03. Therefore, 〈Δ1 - Δ2〉 =
0.07 ∼ 0.1, i.e., very similar to the mean difference obtained for the field giants.
In summary, the DDO abundances derived from the new calibration reproduce
satisfactorily those obtained from high dispersion spectra. The new DDO abundances
do not differ significantly from those obtained by the technique of Janes (1975). With
the new method, however, the [Fe/H] values of early G and late Κ giants (even those
with nearly solar metal content) are improved by about 0.1 dex.
A. revised DDO abundance calibration
163
Table 8. Mean DDO abundances derived for open clusters with published DDO photometry
and known high dispersion abundances.
References: 1. Canterna et al. (1986) 2. Norris & Hawarden (1978) 3. Osborn (1975) 4 Friel
& Janes (1993) 5. Canterna (1976) 6. Hiltner et al. (1958) 7. Geisler (1984) 8. Gratton (1982)
9. Smith & Suntzeff (1987) 10. Geisler et al. (1991) 11. Wallerstein & Conti (1964) 12. Friel
& Boesgaard (1990).
8. Conclusions
We have presented some arguments to justify establishing a revised DDO abundance
calibration for population I red giants.
We have determined the components of the blanketing vector in the C(45–48) vs
C(42–45) diagram for late-type dwarfs and giants. This supplements the new DDO
effective temperature calibration by Piatti & Clariá (1992). In fact, knowing the
blanketing coefficients one can determine reliable effective temperatures and MK
spectral types.
We have resolved some inconsistencies in the abundance determination via cyanogens
anomaly. For that purpose we have redefined the cyanogen anomaly and in order
to calculate the standard DDO colours, we have taken as a reference the G and Κ
giants of the solar neighbourhood, and not the solar-abundance giants. This implies
a small but not negligible difference. In particular, it was found that [Fe/H] = – 0.10
for the mean abundance of G and Κ giants relative to the Sun.
We have also obtained a new [Fe/H] vs ∆CN relationship. Iso-abundance lines
in the C(41–42) vs C(42–45) diagram were determined for the first time for luminosity
class III stars.
Finally, we have described an iterative procedure for determining DDO abundances
of population I giants. This has been applied to samples of field and open cluster
giants with known abundances derived from high dispersion spectroscopy and we
conclude that the present method yields abundances in very good agreement with those
derived from high dispersion spectroscopy. The new procedure does not invalidate
the δCN technique of Janes (1975) for determining overall abundances. We show,
however, that the present method improves by approximately 0.1 dex the DDO
abundances derived for early G and late Κ giants.
Acknowledgements
We are very grateful to an anonymous referee whose valuable comments and suggestions helped us to improve the manuscript. The assistance of M. A. Bosio in the data
164
A. E. Piatti, J. J. Clariá & Minniti
reduction and of J. E. Laborde in the preparation of figures is acknowledged. This
research was supported by funds provided through CONICET and CONICOR of
Argentina.
References
Bell, R. Α., Gustafsson, B. 1989, Mon. Not. R. astr. Soc., 236, 653.
Canterna , R. 1976, Astr. J., 81, 228.
Canterna, R., Geisler, D., Harris, H. C., Olszewski, E., Schommer, R. 1986, Astr. J., 92, 79.
Cayrel de Strobel, G., Bentolila, C., Hauck, B. Duquennoy, H. 1985, Astr. Astrophys, 59, 145.
Clariá, J. J., Lapasset, E. 1983, J. Astrophys. Astr., 4, 117.
Clariá, J. J., Minniti, D., Piatti, A. E., Lapasset, E. 1992, Bol. Asoc. Argentina Astronomía, 37, 57.
Clariá, J. J., Minniti, D., Piatti, A. E., Lapasset, E. 1993, Mon. Not. R. astr. Soc. (submitted).
Cousins, A. W. J. 1980, Sth. African Astr. Obs. Circ., No 6, 4.
Dean, J. F. 1981, Sth. African Astr. Obs. Circ., No 6, 10.
Deming, D. L., Olson, E. C., Yoss, Κ. Μ. 1977, Astr. Astrophys., 57, 417.
Flynn, C., Mermilliod, J. C. 1991, Astr. Astrophys., 250, 400.
Friel, C, Boesgaard, A. M. 1990, Astrophys. J., 351, 480.
Friel, E. D., Janes, K. A. 1993, Astr. Astrophys., 267, 75.
Geisler, D. 1984, Astrophys. J., 287, L85.
Geisler, D., Clariá, J. J., Minniti, D. 1991, Astr. J., 101, 1836.
Gratton, R. G. 1982, Astrophys. J., 257, 640.
Hartkopf, W. I., Yoss, K. M. 1982, Astr. J., 87, 1679.
Hesser, J. E., Hartwick, F. D. Α., McClure, R. D. 1977, Astrophys. J. Suppl. 33, 417.
Hiltner, W. Α., Iriarte, B., Johnson, H. L. 1958, Astrophys. J., 127, 539.
Hoffleit, D., 1964, Catalogue of Bright Stars, New Haven, Connecticut.
Houck, N., Cowley, A. P. 1975, University of Michigan Catalogue of Two-Dimensional Spectral
Types for the HD Stars, Vol. I.
Janes, K. A. 1975, Astrophys. J. Suppl., 29, 161.
Janes, K. A. 1977, Publ. astr. Soc. Pacific., 89, 576.
Janes, K. A. 1979a, in Problems of Calibrations of Multicolor Photometric Systems, (Dudley
Obs. Rep. No 14) p. 103.
Janes, K. A. McClure, R. D. 1971, Astrophys. J., 165, 561.
Jaschek, C., Hernández, E., Sierra, Α., Gerhardth, A. 1972, Catalogue of Stars Measured
Photoelectrically, Serie Astronómica, Tomo XXXVIII, (National Univ. of La Plata,
Argentina.).
Johnson, H. L., MacArthur, J. W., Mitchell, R. I. 1968, Astrophys. J., 152, 465.
McClure, R. D. 1970, Astr. J., 75, 1.
McClure, R. D. 1976, Astr. J., 81, 182.
McClure, R. D. 1979, in Problems of Calibrations of Multicolor Photometric Systems, (Dudley
Obs. Rep. No 14) p. 83.
McClure, R. D., Forrester, W. T. 1981, Publ. Dom. Astrophys. Obs. Victoria, 15, 439.
McWilliam, A. 1990, Astrophys. J. Suppl., 74, 1075.
Norris, J., Bessell, M. S., Pickles, A. J. 1985, Astrophys. J. Suppl., 58, 463.
Norris, J., Hawarden, T. G. 1978, Astrophys. J., 223, 483.
Osborn, W. 1971, Ph. D. Thesis, Yale University.
Osborn, W. 1975, Mon. Not. R. astr. Soc., 172, 631.
Osborn, W. 1979, in Problems of Calibrations of Multicolor Photometric Systems, (Dudley Obs.
Rep. No 14) p. 115.
Piatti, A. E., Clariá, J. J. 1992, Bol. Asoc. Argentina Astronomía, 37, 181.
Schmidt-Kaler, Th. 1982, Landolt-Börnstein, Numerical Data and Functional Relationships in
Science and Technology, Group VI., 2, 15.
A revised DDO abundance calibration
165
Smith, V. V., Suntzeff, N. B. 1987, Astr. J., 93, 359.
Straizys, V. 1991, Multicolor Stellar Photometry, (Packart Publ. House, Tucson, Arizona).
Taylor, B. J. 1986, Astrophys. J. Suppl., 60, 577.
Tripicco, M. J., Bell, R. A. 1991, Astr. J., 102, 744.
Wallerstein, G., Conti, P. 1964, Astrophys. J., 140, 858.