arXiv subjects

J. Borissova

Publications and source records attributed to J. Borissova.

102 records · Page 6Linked to original sources

arXiv2001

The First Known Mira-type Variable Star in IC 1613

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.

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arXiv2001

The influence of massive stars in the interstellar medium of IC 1613: the supernova remnant S8 and the nebula S3 associated with a WO star

We present a detailed kinematical analysis of two selected nebulae in the Local Group irregular galaxy IC 1613. The nebulae are: S8, the only known supernova remnant in this galaxy, and S3, a Wolf-Rayet nebula associated with the only WO star in this galaxy. For S8, we have obtained and analyzed its radial velocity field, where we found complex profiles which can be fitted by several velocity components. These profiles also show the presence of high velocity, low density gas. From this, we have obtained the expansion velocity, estimated the preshock density and calculated the basic kinematical parameters of this SNR. We suggest that in S8 we are seing a SNR partially hidden by dust. This suggestion comes from the fact that the SNR is located between two superbubbles where a ridge of obscured material unveils the existence of dust. Moreover, we show that this hypothesis prevails when energetic arguments are taken into account. In the case of S3, this nebula shows bipolar structure. By means of its kinematics, we have analyzed its two lobes, the ``waist'', as well as its relation with the nearest superbubbles. For the first time we are able to see closed the NW lobe, showing a clover leaf shape. This fact allows a better quantitative knowledge of the nebula as a whole. Furthermore, we found evidence of an expansion motion in the NW lobe. In the light of our results, we can express that these nebulae are the product of very massive stellar evolution. It is surprising the influence these stars still have in shaping their surrounding gas, and on the energy liberation towards the interstellar medium of this galaxy.

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arXiv2001

IC 10: More evidence that it is a blue compact dwarf

We present optical spectroscopy of H {\sc ii} regions in the Local Group galaxy IC 10 and $UBVR$ photometry of foreground stars in three fields towards this galaxy. From these data, we find that the foreground reddening due to the Milky Way is $E(B-V)=0.77\pm 0.07 \mathrm{mag}$. We find that IC 10 contains considerable internal dust, which qualitatively explains the variety of reddening values found by studies of its different stellar populations. Based upon our foreground reddening, IC 10 has intrinsic photometric properties like those of a blue compact dwarf galaxy, and not those of a dwarf irregular. This result is consistent with much evidence that IC 10 is in the throes of a starburst that began at least $10 \mathrm{Myr}$ ago. We also report the discovery of a new WR star in the H {\sc ii} region HL111c.

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arXiv2001

Romano's star in M33 - LBV candidate or LBV?

We present the light curve of Luminous Blue Variable candidate star GR290 (Romano's star) in M33. The photographic photometry was made in photographic plates taken in B band of the M33 galaxy and cover an eight year period, 1982 - 1990. Twenty five plates, separated in seven groups, have been used. CCD B magnitude of the star is also presented. The analysis of our data together with the Romano's magnitudes (1978) shows "normal" eruptions with amplitude of more than 1 mag and timescale of about 20 years and smaller oscillations with amplitude 0.5 mag and a period of about 320 days. This is a typical photometrical behavior for LBVs.

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arXiv2000

Infrared photometry of the Local Group Dwarf Irregular Galaxy IC 10

We present near infrared JHK photometry for the central area of the irregular dwarf galaxy IC10. Comparing the red supergiants of IC10 and IC1613 we have determined the reddening of E(B-V)=1.05+-0.10 and the dereddened distance modulus (m-M)_0=23.86+-0.12 mag of Population I stars in IC10. Detection of six Brgamma emission structures clearly outline two star forming regions in the central area of IC10. There is no H_2 emissions down to 4.65 $10^{-16}$ $\rm erg s^{-1} cm^{-2} arcsec^{-2}$ indicating that the dominant excitation mechanism of the molecular gas is from UV radiation from hot young stars. From the comparison of Brgamma and Halpha fluxes, we derive average extinction toward th e star forming regions E(B-V)_ Halpha= 1.8+-0.2. The summarized SFR derived from six Halpha and Brgamma emission structures in our field of view is 0.6 $M_\odot {\rm year^{-1}}$.

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arXiv2000

Kinematics of the ionized gas in the Local Group irregular galaxy IC 1613

We present Halpha and [SII] observations for the Local Group irregular galaxy IC 1613 using the PUMA scanning Fabry-Perot interferometer. Our goal is to analyze the kinematics of the ionized gas in the complex sample of superbubbles located in the whole extension of our field (10 arcmin), which includes most of the optical emission of this galaxy, and to study the inter-relationship between young stellar associations and nebulae based on a previous study that we have made on the stellar associations of the central region of this galaxy. The ionized gas in this galaxy is distributed in classical HII regions and in a series of superbubbles (also called giant shells) covering a large fraction of the optical extent of the galaxy. We present a catalog of kinematical properties of both the HII regions of this galaxy and the superbubbles. We have also compared the kinematics of the ionized gas in HII regions to search for possible dynamic differences between neutral and ionized gas.

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arXiv2000

The Red Stellar Contents of Three Selected Fields of The Local Group Dwarf Irregular Galaxy IC1613

We present a moderately-deep JK photometry for three selected areas of the dwarf irregular galaxy IC1613. The color-magnitude diagrams contain a mixture of red supergiants, asymptotic giant branch stars and the brightest red giant stars. The red supergiants are massive (20 - 25 M_\odot) and young -with ages between 8 and 25 Myr. The most important result is the evidence of the decreasing density of the intermediate age AGB population in the vicinity of the HII regions in the galaxy. We also find age differences between AGB stars in the main body of the galaxy and those near the HII regions in the North-East. The former span a range in ages between 1 and 10 Gyr, while the latter are younger than 1 Gyr. Using the period-luminosity relation derived by Madore & Freedman (1991) and JK magnitudes of the Cepheid variable V20, we calculated (m-M)_K = 24.37+-0.2. The recently discovered Nova (King et al. 1999) was identified in Field III. Its presence of our images and its brightness questioned its classification as a nova.

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arXiv2000

The outer-halo globular cluster NGC 6229. IV. Variable stars

We present time-series B, V photometry of NGC 6229, obtained with the purpose of providing the first extensive CCD variability study of this cluster. As a result, we were able to obtain periods for all NGC 6229 variables, with the exception of five stars from the Borissova et al. (1997) candidate list located very close to the cluster center. Two variable stars suspected by Carney, Fullton, & Trammell (1991) are first-overtone RR Lyrae (RRc) stars, whereas seven of the 12 candidates from Borissova et al. (1997) are confirmed variables -- three of them being fundamental RR Lyrae (RRab) pulsators, two first-overtone pulsators, one eclipsing binary (most likely an Algol system) and one bright star whose variability status could not be satisfactorily determined. A new image subtraction method (ISM) suggested by Alard (1999) has been employed which, together with the Welch & Stetson (1993) technique, has allowed us to discover twelve new RR Lyrae variables in the cluster, for which ephemerides are provided. Ten of these are RRab's, whereas the other two are RRc's. As originally suggested by Mayer (1961), NGC 6229 is clearly an Oosterhoff type I globular cluster. We also confirm that V8 is a Population II Cepheid of the W Virginis type, and suspect that this is the case for V22 as well. The physical properties of the NGC 6229 RR Lyrae population are compared against those of M3 (NGC 5272) using several different methods, including a standard period-shift analysis. Possible differences between these two clusters are discussed.

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arXiv1998

The outer-halo globular cluster NGC 6229. III. Deep CCD photometry

Deep BV photometry for a large field covering the outer-halo globular cluster NGC 6229 is presented. For the first time, a colour-magnitude diagram (CMD) reaching below the main sequence turnoff has been obtained for this cluster. Previous results regarding the overall morphology of the horizontal and giant branches are confirmed. In addition, nine possible extreme horizontal-branch stars have been identified in our deep images, as well as thirty-three candidate blue stragglers. We also find the latter to be more centrally concentrated than subgiant branch stars covering the same range in V magnitude. A comparison of the cluster CMD with the M5 (NGC 5904) ridgeline from Sandquist et al. (1996) reveals that: i) NGC 6229 and M5 have essentially identical metallicities; ii) NGC 6229 and M5 have the same ages within the errors in spite of their different horizontal-branch morphologies.

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arXiv1997

Three contributions to the "A Half Century of Stellar Pulsation Interpretations" proceedings

Paper I: "RR Lyrae Variables in Globular Clusters and the Second-Parameter Phenomenon" Authors: M. Catelan, A. V. Sweigart, J. Borissova Abstract: We propose to use RR Lyrae variables in globular clusters to probe the origin of the second-parameter phenomenon. Paper II: "New Possible Variables in the Outer-Halo Globular Cluster Palomar 3" Authors: J. Borissova, N. Spassova, M. Catelan, V. D. Ivanov Abstract: We present a list of ten possible variable stars in the globular cluster Pal 3. Seven are new suspected variables. The variability of the RR Lyr candidate reported by Burbidge & Sandage (1958), as well as of the Pop. II Cepheid and RR Lyr from Gratton & Ortolani (1984), are confirmed. Paper III: "On the Production of Bright RR Lyrae Variables in Metal-Rich Globular Clusters" Authors: A. V. Sweigart, M. Catelan Abstract: See astro-ph/9708174.

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arXiv1997

Bimodality and Gaps on Globular Cluster Horizontal Branches. II. The Cases of NGC 6229, NGC 1851 and NGC 2808

The outer-halo globular cluster NGC 6229 has a peculiar horizontal-branch (HB) morphology, with clear indications of a bimodal HB and a ``gap" on the blue HB. In this paper, we present extensive synthetic HB simulations to determine whether peculiar distributions in the underlying physical parameters are needed to explain the observed HB morphology. We find that a unimodal mass distribution along the HB can satisfactorily account for the observed HB bimodality, *provided* the mass dispersion is substantially larger than usually inferred for the Galactic globular clusters. In this case, NGC 6229 should have a well-populated, extended blue tail. A truly bimodal distribution in HB masses can also satisfactorily account for the observed HB morphology, although in this case the existence of an extended blue tail is not necessarily implied. The other two well-known bimodal-HB clusters, NGC 1851 and NGC 2808, are briefly analyzed. While the HB morphology of NGC 1851 can also be reproduced with a unimodal mass distribution assuming a large mass dispersion, the same is not true of NGC 2808, for which a bimodal, and possibly multimodal, mass distribution seems definitely required. The problem of gaps on the blue HB is also discussed. Applying the standard Hawarden (1971) and Newell (1973) chi-squared test, we find that the NGC 6229 gap is significant at the 99.7% level. However, in a set of 1,000 simulations, blue-HB gaps comparable to the observed one are present in ~ 6 - 9% of all cases. We employ a new and simple formalism, based on the binomial distribution, to explain the origin of this discrepancy, and conclude that Hawarden's method, in general, substantially overestimates the statistical significance of gaps.

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arXiv1996

Stellar Photometry of the Globular Cluster NGC 6229. I. Data Reduction and Morphology of the Brighter Part of the CMD

BV CCD photometry of the central (1.5 arcmin x 2.0 arcmin) part of the mildly concentrated outer-halo globular cluster NGC 6229 is presented. The data reduction in such a crowded field was based on a wavelet transform analysis. Our larger dataset extends the previous results by Carney et al. (1991, AJ, 101, 1699) for the outer and less crowded fields of the cluster, and confirms that NGC 6229 has a peculiar color-magnitude diagram for its position in the Galaxy. In particular, NGC 6229's horizontal branch (HB) presents several interesting features, among which stand out: a well populated and very extended blue tail; a rather blue overall morphology, with (B-R)/(B+V+R) = 0.24+/-0.02; a bimodal color distribution, resembling those found for NGC 1851 and NGC 2808; and gaps on the blue HB. NGC 6229 is the first bimodal-HB cluster to be identified in the Galactic outer halo. A low value of the R parameter is confirmed, suggestive of a low helium abundance or of the presence of a quite substantial population of extreme HB stars fainter than our photometric limit (~ 2.5 mag below the RR Lyrae level in V). Twelve new possible variable stars were found in the central part of the cluster. The morphology of the red giant branch (RGB) also seems to be peculiar. In particular, the RGB luminosity function ``bump'' is not a prominent feature and has only been tentatively identified, on the basis of a comparison with a previously reported detection for M3 (NGC 5272). Finally, we compare the properties of NGC 6229 with those for other outer-halo globular clusters, and call attention to what appears to be a bimodal HB distribution for the outer-halo cluster population, where objects with very red or very blue HB types are much more frequently found than clusters with intermediate HB types.

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