# 2003A&A...407..303M

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 Query : 2003A&A...407..303M

2003A&A...407..303M - Astronomy and Astrophysics, volume 407, 303-309 (2003/8-3)

Why hot horizontal branch stars can appear redder than red giants.

MOMANY Y., CASSISI S., PIOTTO G., BEDIN L.R., ORTOLANI S., CASTELLI F. and RECIO-BLANCO A.

Abstract (from CDS):

In this paper we report on a curious feature in the V, (U-B) color-magnitude diagrams of globular clusters. In our database, we find that a considerable fraction of blue horizontal branch stars, hotter than the instability strip and cooler than the Grundahl et al. (1999ApJ...524..242G) jump (i.e., 6000≲Teff(K)≲10000), have (U-B) colors redder than their red giant progenitors. This red incursion is not expected on theoretical grounds, as horizontal branch stars (whose convective regions are less extended than in red giant structures) should not appear'' cooler than a red giant. Analyzing data from different telescopes we show that: 1) the horizontal branch red incursion is strongly dependent on the shape of the adopted U filter and to a lesser extent, on the B filter; 2) the photometry done with U filters that do not encompass the Balmer jump shows the blue horizontal branch red incursion; 3) the occurrence of this feature is also due to the peculiar dependence of the U and B magnitudes on star effective temperature, gravity, and metallicity; 4) theoretical tracks can reproduce the observed horizontal branch morphology, provided that the appropriate (i.e. exactly responding to the filters effectively used in the observations) transmission curve efficiencies are used for deriving color-Teff transformations; 5) the red incursion extent depends on metallicity.

Journal keyword(s): stars: imaging - stars: evolution - stars: Hertzsprung-Russell diagram (HR) and C-M diagrams - stars: horizontal-branch - ultraviolet: stars

Full paper

 Number of rows : 9
N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2023
#notes
1 NGC 1841 GlC 04 45 23 -83 59.8   12.23 11.43     ~ 180 0
2 M 79 GlC 05 24 10.59 -24 31 27.3     8.16     ~ 782 0
3 M 68 GlC 12 39 27.98 -26 44 38.6     7.96     ~ 956 0
4 NGC 4833 GlC 12 59 33.92 -70 52 35.4           ~ 400 0
5 NGC 5139 GlC 13 26 47.28 -47 28 46.1           ~ 3315 0
6 M 13 GlC 16 41 41.634 +36 27 40.75     5.8     ~ 2136 0
7 M 15 GlC 21 29 58.33 +12 10 01.2           ~ 3073 0
8 M 2 GlC 21 33 27.02 -00 49 23.7     6.25     ~ 991 1
9 M 30 GlC 21 40 22.12 -23 10 47.5     7.10     ~ 1003 0

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2023.03.30-07:58:57