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J/A+A/501/563      HKs photometry in the Arches cluster    (Espinoza+, 2009)

The massive star initial mass function of the Arches cluster. Espinoza P., Selman F.J., Melnick J. <Astron. Astrophys. 501, 563 (2009)> =2009A&A...501..563E
ADC_Keywords: Clusters, open ; Photometry, infrared ; Stars, masses Keywords: Galaxy: open clusters and associations: individual: Arches - stars: early-type - instrumentation: adaptive optics - ISM: dust, extinction - stars: luminosity function, mass function Abstract: The massive Arches cluster near the Galactic center should be an ideal laboratory for investigating massive star formation under extreme conditions. But it comes at a high price: the cluster is hidden behind several tens of magnitudes of visual extinction. Severe crowding requires space or AO-assisted instruments to resolve the stellar populations, and even with the best instruments interpreting the data is far from direct. Several investigations using NICMOS and the most advanced AO imagers on the ground revealed an overall top-heavy IMF for the cluster, with a very flat IMF near the center. There are several effects, however, that could potentially bias these results, in particular the strong differential extinction and the problem of transforming the observations into a standard photometric system in the presence of strong reddening. We present new observations obtained with the NAOS-Conica (NACO) AO-imager on the VLT. The problem of photometric transformation is avoided by working in the natural photometric system of NACO, and we use a Bayesian approach to determine masses and reddenings from the broad-band IR colors. A global value of Gamma=-1.1±0.2 for the high-mass end (M>10M) of the IMF is obtained, and we conclude that a power law of Salpeter slope cannot be discarded for the Arches cluster. The flattening of the IMF towards the center is confirmed, but is less severe than previously thought. We find Gamma=-0.88±0.20, which is incompatible with previous determinations. Within 0.4pc we derive a total mass of ∼2.0(±0.6)x104M for the cluster and a central mass density rho=2(±0.4)x105M/pc3 that confirms Arches as the densest known young massive cluster in the Milky Way. Description: The NAOS-CONICA data (ESO Program ID 073.D-0815) were obtained under clear weather conditions with subarcsecond seeing. The detector was an Aladdin 1024x1024 pixel InSb array and the camera had a plate scale of 27.15[mas/pix], giving us a 27x27arcsec2 field of view of the Arches cluster. Total integration times were 1000, 400 and 720[s] in J, H, and Ks respectively, with the telescope moving alternatively to sky positions for a proper background subtraction. To optimize the Adaptive Optics (AO) performance we used the N90C10 dichroic, i.e. 90% of the light was directed to the infrared wavefront sensor. The Strehl ratio of our observations exceeded 27% in Ks, and reached more modest values of 5% in J, and 11% in the H band. Tables 2 and 3 present the DAOPHOT photometry of 427 HKS and 126 JHKS stars in the innermost 10 arcseconds of the Arches cluster. Table 3 is considerably shorter due to the increasing extinction towards bluer wavelengths. Table 5 presents the catalog with all the observed data and physical parameters derived from the Bayesian method and using the Color-magnitude stereogram. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table2.dat 57 427 HKs photometry of stars in the Arches cluster table3.dat 57 126 JHKs photometry of stars in the Arches cluster table5.dat 68 126 Physical parameters of JHKs stars
See also: J/ApJ/581/258 : Infrared photometry in the Arches Cluster (Figer+, 2002) Byte-by-byte Description of file: table[23].dat
Bytes Format Units Label Explanations
1- 5 I5 --- Seq Star unique ID in this work (G1) 7- 12 F6.3 arcsec dRA Right Ascension offset (2) 14- 19 F6.3 arcsec dDE Declination offset (2) 21- 26 F6.3 mag Ksmag NACO Ks band magnitude 28- 32 F5.3 mag e_Ksmag 1σ uncertainty in Ks 34- 39 F6.3 mag H-Ks NACO (H-Ks) color index 41- 45 F5.3 mag e_H-Ks 1σ uncertainty in (H-Ks) 47- 51 F5.3 mag J-H ? NACO (J-H) color index (table3 only) 53- 57 F5.3 mag e_J-H ? 1σ uncertainty in (J-H) (table3 only)
Note (2): Positions with respect to 17:45:50.798-28:49:25.606 (J2000).
Byte-by-byte Description of file: table5.dat
Bytes Format Units Label Explanations
1- 5 I5 --- Seq Star unique ID in this work (G1) 7- 12 F6.3 mag Ksmag NACO Ks band magnitude 13- 18 F6.3 mag H-Ks NACO (H-Ks) color index 20- 24 F5.3 mag J-H NACO (J-H) color index 26- 31 F6.3 mag VMAG Absolute V band magnitude (3) 33- 37 F5.3 [solLum] logL Log of the luminosity (3) 39- 43 F5.3 [K] logTeff Log of the effective temperature (3) 45- 49 F5.3 [N/kg] logg Log of the surface gravity (3) 51- 55 F5.3 mag E(H-Ks) Color excess 57- 62 F6.2 solMass Mini Initial mass (3) 64- 68 F5.2 solMass Mact Present day mass (3)
Note (3): Assuming that the stars in the Arches cluster formed 2.5Myr ago, have solar metallicity, and are located at a distance modulus of 14.52 magnitudes. We compared our observations against the Geneva models with standard mass-loss rates.
Global notes: Note (G1): The star ID is designated by sorting the data in order of increasing Ks magnitude. Only stars inside a radius of 10 arcseconds from the cluster center are included in this catalog.
Acknowledgements: Pablo Espinoza, pespinoza(at)as.arizona.edu
(End) Pablo Espinoza [Univ. of Arizona, USA], Patricia Vannier [CDS] 02-Jun-2009
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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