spheric lithium content, being about 0.3 dex
more abundant than its B companion (Figure
2). As lithium is most likely destroyed when it
reaches the high temperatures of the stellar
interior by means of stellar convection, this
great difference between two similar coeval
stars is surprising and unexpected.
In addition, we also found that star A is en-
hanced in refractory elements when com-
pared to its stellar companion; here we
identify as “refractory” and “volatile” those
species with condensation temperatures (T C )
> 900 K and < 900 K, respectively. To obtain
this result, we plotted the abundance differ-
ence (B – A) between both stars versus T C for
each chemical species. Figure 3 also shows
weighted linear fits (long dashed lines) to all
species and only to the refractory compo-
nents. A similar refractory to volatile content
between both stars would correspond to
null slopes in this plot. However, the nega-
tive slopes in Figure 3 point notably toward
a higher content of refractory species in star
A than in its stellar companion.
tis stars, δ Scuti stars, and blue strag-
glers. The so-called λ Bootis stars are a
small group (only 2%) of population I
stars, that show moderate to extreme
surface underabundances of iron (Fe)-
peak elements, but with solar abun-
dances of carbon, nitrogen, and oxy-
gen. In contrast, both stars A and B in
the HAT-P-4 system are metal rich, and
their effective temperatures are lower
than those of the λ Bootis group; this
allowed us to discard them as belong-
ing to the λ Bootis class. δ Scuti stars
are regularly pulsating variables with
~A6–F6 spectral types, located in the
instability strip of the Hertzsprung-Russell
diagram. While most of these stars present
overabundances of heavy elements (be-
tween 0.5 – 1.0 dex), both the effective tem-
perature and mass of stars A and B are not
only lower than those of the δ Scuti group,
but they also lie out of the instability strip
boundaries. Furthermore, no stellar pulsa-
tions have been reported in either compo-
nent A or B; so we also abandoned a possible
δ Scuti classification. Finally, blue stragglers
are stars significantly bluer than those
placed on the main-sequence turnoff of the
cluster (or population) to which they belong.
Figure 2.
GRACES spectra near
the 6,707.8 Å lithium
line for star A in HAT-
P-4 (blue dotted line)
and its B companion
(black continuous line).
The lithium content is
notably enhanced in
star A.
Figure 3.
Differential abundances
(B - A) vs. condensation
temperature T C . Long
dashed lines are
weighted linear fits
to all species and to
refractory species.
The solar-twins trend
of Meléndez et al.
(2009) is shown with
a continuous line for
comparison.
Chemically Peculiar Stars?
Our attempts at classifying the HAT-P-4
components to other stars with peculiar
chemical patterns also brought surprises.
We looked at three cases in particular: λ Boo-
January 2018 / 2017 Year in Review
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