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Register a new userC32, A Young Star Cluster in IC 1613
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Authors
Ted K. Wyder
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The Local Group irregular galaxy IC 1613 has remained an enigma for many years because of its apparent lack of star clusters. We report the successful search for clusters among several of the candidate objects identified many years ago on photographic plates. We have used a single HST WFPC2 pointing and a series of images obtained with the WIYN telescope under exceptional seeing conditions, examining a total of 23 of the previously published candidates. All but six of these objects were found to be either asterisms or background galaxies. Five of the six remaining candidates possibly are small, sparse clusters and the sixth, C32, is an obvious cluster. It is a compact, young object, with an age of less than 10 million years and a total absolute magnitude of M_V = -5.78+/-0.16 within a radius of 13 pc.
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King, Modjaz, & Li (1999) discovered Nova 1999 in IC1613 at Lick Observatory. Both Fugazza et al. (2000) and Borissova et al. (2000) questioned this classification, because they were able to detect the star on images obtained in previous years. In infrared frames taken on Oct. 15, 1998, the Nova 1999 has (J-K) = 1.14 and K = 14.69 mag. Our light curve study, based primarily on 92 unfiltered Lick images, suggested that the object could be a Mira-type variable with a period of 640.7 days. This period is very close to that obtained by Fugazza et al. (2000) - 631 days. The star is overluminous with respect to the period-luminosity (PL) relation derived by Feast et al. (1989) for Mira variables in the LMC. At longer periods (P > 400 ~ days), many LMC Miras show such behavior and the PL relation appears to break down. It is possible that the situation in IC1613 is similar. An optical spectrum obtained with the Keck-II telescope shows features typical of M3Ie or M3IIIe stars. We conclude that the star is a normal long-period M-type Mira variable, the first such star confirmed in IC1613.
IC 1274 is a faintly luminous nebula lying on the near surface of the Lynds 227 (L227) molecular cloud. Four luminous, early-type (B0-B5) stars are located within a spherical volume ~5 in diameter that appears to be clear of heavy obscuration. Approximately centered in the cleared region is the B0 V star HD 166033, which is thought to be largely responsible for the cavitys excavation. Over 80 H-alpha emission sources brighter than V~21 have been identified in the region. More than half of these are concentrated in IC 1274 and are presumably members of a faint T Tauri star population. Chandra Advanced CCD Imaging Spectrometer (ACIS) imaging of a nearby suspected pulsar and time-variable gamma-ray source (GeV J1809-2327) detected 21 X-ray sources in the cluster vicinity, some of which are coincident with the early-type stars and H-alpha emitters in IC 1274. Deep (V~22) optical BVRI photometry has been obtained for the cluster region. A distance of 1.82 +/- 0.3 kpc and a mean extinction of Av ~1.21 +/- 0.2 mag follow from photometry of the early-type stars. Using pre-main-sequence evolutionary models, we derive a median age for the H-alpha emitters and X-ray sources of ~1 Myr; however, a significant dispersion is present. Our interpretation of the structure of IC 1274 is that the early-type stars formed recently and are in the process of dispersing the molecular gas on the near surface of L227. The displaced material was driven against what remains of the molecular cloud to the east, enabling the formation of the substantial number of T Tauri stars found there. We identify a V~21.5 star very near the position of X-ray source 5, the assumed gamma-ray source and young pulsar candidate. The lack of distinctive characteristics for this source, however, coupled with the density of faint stars in this region suggest that this may be a random superposition.
We present sub-arcsecond optical images and low- to moderate-resolution spectra of the compact, X-ray and optically bright supernova remnant known as S8 in the nearby dwarf galaxy IC 1613. Deep H$alpha$ images of the remnant show a sharply defined crescent shaped nebula, while narrow passband images reveal a coincident and unexpectedly bright continuum nebulosity exhibiting a size and morphology like that seen for the remnants line emissions. Low-dispersion spectra covering 3600 - 9000 A show numerous low-ionization line emissions such as [O I] and [Fe II], along with higher-ionization emission lines including He II and optical coronal lines [Fe VII], [Fe X], [Fe XI], and [Fe XIV]. This suggests the presence of a wide range of shock velocities from $sim$ 50 to over 350 km s$^{-1}$, corresponding to preshock densities of $sim1 - 30$ cm$^{-3}$. Higher resolution spectra indicate an expansion velocity around 180 km s$^{-1}$ with a $sim45$ km s$^{-1}$ wide central cavity. H$alpha$ emission spans rest frame velocities of $+120$ to $-240$ km s$^{-1}$ and we estimate a total nebula mass of $119 pm 34$ M$_{odot}$. We conclude S8 is a relatively young supernova remnant ($simeq2700 - 4400$ yr) exhibiting properties remarkably like those seen in the young LMC remnant N49, including age, physical size, shock velocities, filament densities, optical line strengths, X-ray and optical luminosities, and coronal line and continuum emissions.
We present multi-object spectroscopy of young, massive stars in the Local Group galaxy IC 1613. We provide the spectral classification and a detailed spectral catalog for 54 OBA stars in this galaxy. The majority of the photometrically selected sample is composed of B- and A-type supergiants. The remaining stars include early O-type dwarfs and the only Wolf-Rayet star known in this galaxy. Among the early B stars we have serendipitously uncovered 6 Be stars, the largest spectroscopically confirmed sample of this class of objects beyond the Magellanic Clouds. We measure chemical abundances for 9 early-B supergiants, and find a mean oxygen abundance of 12+log(O/H)=7.90 +/- 0.08. This value is consistent with the result we obtain for two HII regions in which we detect the temperature-sensitive [OIII]4363 auroral line.
Determining the star formation history (SFH) is key to understand the formation and evolution of dwarf galaxies. Recovering the SFH in resolved galaxies is mostly based on deep colour--magnitude diagrams (CMDs), which trace the signatures of multiple evolutionary stages of their stellar populations. In distant and unresolved galaxies, the integrated light of the galaxy can be decomposed, albeit made difficult by an age--metallicity degeneracy. Another solution to determine the SFH of resolved galaxies is based on evolved stars; these luminous stars are the most accessible tracers of the underlying stellar populations and can trace the entire SFH. Here we present a novel method based on long period variable (LPV) evolved asymptotic giant branch (AGB) stars and red supergiants (RSGs). We applied this method to reconstruct the SFH for IC 1613, an irregular dwarf galaxy at a distance of 750 kpc. Our results provide an independent confirmation that no major episode of star formation occurred in IC 1613 over the past 5 Gyr.
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