J/A+A/523/A23 Properties of OB associations in IC 1613 (Garcia+, 2010)
The young stellar population of IC 1613.
II. Physical properties of OB associations.
Garcia M., Herrero A., Castro N., Corral L., Rosenberg A.
<Astron. Astrophys., 523, A23-23 (2010)>
=2010A&A...523A..23G 2010A&A...523A..23G
ADC_Keywords: Galaxies, nearby ; Associations, stellar ; Stars, OB
Keywords: galaxies: individual: IC 1613 - stars: early-type -
galaxies: stellar content - galaxies: star clusters: general
Abstract:
To understand the structure and evolution of massive stars, systematic
surveys of the Local Group galaxies have been undertaken, to find
these objects in environments of different chemical abundances. We
focus on the metal-poor irregular galaxy IC 1613 to analyze the
stellar and wind structure of its low-metallicity massive stars. We
ultimately aim to study the metallicity-dependent driving mechanism of
the winds of blue massive stars and use metal-poor massive stars of
the Local Volume as a proxy for the stars in the early Universe. In a
previous paper we produced a list of OB associations in IC 1613. Their
properties are not only a powerful aid towards finding the most
interesting candidate massive stars, but also reveal the structure and
recent star formation history of the galaxy. We characterize these OB
associations and study their connection with the galactic global
properties. The reddening-free Q parameter is a powerful tool in the
photometric analysis of young populations of massive stars, since it
exhibits a smaller degree of degeneracy with OB spectral types than
the B-V color. The color-magnitude diagram (Q vs. V) of the OB
associations in IC 1613 is studied to determine their age and mass,
and confirm the population of young massive stars. We identified more
than 10 stars with M≥50M☉. Spectral classification available
for some of them confirm their massive nature, yet we find the common
discrepancy with the spectroscopically derived masses. There is a
general increasing trend of the mass of the most massive member with
the number of members of each association, but not with the stellar
density. The average diameter of the associations of this catalog is
40pc, half the historically considered typical size of OB
associations. Size increases with the association population. The
distribution of the groups strongly correlates with that of neutral
and ionized hydrogen. We find the largest dispersion of association
ages in the bubble region of the galaxy where hydrogen is abundant,
implying that recent star formation has proceeded over a longer period
of time than in the rest of the galaxy, and is still ongoing. Very
young associations are found at the west of the galaxy far from the
bubble region, traditionally considered the sole locus of star
formation, but still rich in neutral hydrogen. The contrast in the
stellar properties derived from photometry and spectroscopy (when the
latter is available) shows that the Q pseudo-color is very useful for
estimating the parameters of OB stars when only photometric
observations exist. This work helped define an extensive pool of
candidate OB stars for subsequent spectroscopic analyses designed to
study the structure and winds of metal-poor massive stars.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 110 166 Properties of OB associations
notes.dat 80 79 Individual notes
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See also:
J/A+A/502/1015 : OB associations in IC 1613 (Garcia+, 2009)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- ID OB association identification number
5- 13 F9.6 deg RAdeg Right ascension (J2000) (1)
15- 22 F8.6 deg DEdeg Declination (J2000) (1)
24- 25 I2 --- NOB [0/69] Number of OB members (NOB) used in age
and mass determination of the association (2)
27- 31 F5.2 arcsec Rad Radius estimate (3)
33- 40 E8.3 pc-2 Sigma Stellar surface density (Sigma=NOB/(πR2)
42- 45 F4.2 arcsec Distance of each OB member to its closest
neighbor, averaged over the association
47- 49 F3.1 [yr] logA0 ?=- Lower value of logAge interval (4)
50 A1 --- --- [-]
51- 53 F3.1 [yr] logA1 ? Upper value of logAge interval (4)
55- 57 I3 Msun Mup0 [7/77]?=-99 Initial stellar mass (5)
58 A1 --- --- [/]
59- 61 I3 Msun Mup [7/77]?=-99 Current stellar mass (5)
63 A1 --- n_ID [n] n indicates a note in notes.dat file
65- 70 A6 --- f_ID [?l*± ] Dubious association (6)
72- 77 F6.3 mag E(B-V) ?=99.99 Average value of extinction towards
the bona-fide members of the association (7)
79- 84 F6.3 mag e_E(B-V) ?=99.99 Standard deviation of the extinction
towards the members of the association (8)
86- 94 E9.3 Msun Mtot ?=-1.00E+02 Total mass of the association (9)
96-110 A15 --- SpType Known spectral types for members,
from Bresolin et al. (2007ApJ...671.2028B 2007ApJ...671.2028B)
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Note (1): Right ascension and declination of the center determined from the
average of the positions of the OB members.
Note (2): this figure includes the blue members listed in Garcia et al.
(2009A&A...502.1015G 2009A&A...502.1015G, Cat. J/A+A/502/1015, Paper I) except for blends found
from inspection of images and objects with Q-color bluer than the youngest
considered isochrone beyond the errors.
Note (3): defined as half the maximum distance between two members of the
association (non-OB stars are not considered in the calculation).
Note (4): determined from the analysis of the association's Q vs. Vcorr diagram.
When two values are provided, they represent the age interval enclosing the
association.
Note (5): Mass of the most massive star of the association, determined from
the position of the star on the Q vs. Vcorr diagram and comparison with
theoretical isochrones. The provided value is the weighted average of the
closest isochrone-point masses to the star. Stars showing blends were not
used for Mup determination.
Note (6): Notes as follows:
? = dubious association (associations with a small number of members, with a
significant fraction under suspicion of being blends or not blue stars)
l = at least one member was discarded for age determination because of very
blue Q (←1.0)
* = sequence of objects with Q smaller than the youngest isochrone (in
disagreement with the remaining members) but separated from it by a
smaller distance than the errors; we checked that these points are
neither galaxies nor blends, but were not used for age determination
+ = the association shows two or more age trends.
Note (7): estimated from their colors using Eq. (3) of Paper I
(Garcia et al. (2009A&A...502.1015G 2009A&A...502.1015G, Cat. J/A+A/502/A1015).
The derived mean value for extinction when averaged over all the OB
associations is 0.16.
Note (8): if only one member is available to calculate the average extinction,
this value is set to 99.99.
Note (9): calculated using a regular Salpeter initial mass function (IMF) down
to 0.5M☉ (see Sect. 3.4) and maximum mass MupI. A minimum of two valid
members (NOB) are required to calculate the association mass, otherwise the
value provided here is -1.00E+02.
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Byte-by-byte Description of file: notes.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- ID OB association identification number
5- 80 A76 --- Note Text of the note
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History:
From electronic version of the journal
References:
Garcia et al., Paper I 2009A&A...502.1015G 2009A&A...502.1015G, Cat. J/A+A/502/A1015
Garcia & Herrero, Paper III 2013A&A...551A..74G 2013A&A...551A..74G
(End) Patricia Vannier [CDS] 20-Jun-2014