arXiv subjects

A. Saha

Publications and source records attributed to A. Saha.

157 records · Page 9Linked to original sources

arXiv2000

Deep HST Imaging of IC 1613. I. Variable Stars and Distance

We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyraes and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly-measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] = -1.3 (RR Lyraes) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 +/- 0.20 magnitudes per dex for the RR Lyraes and a shallow slope of -0.07 +/- 0.16 mag/dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We find that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we find that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is mu_0 = 24.31 +/- 0.06, corresponding to a distance of 730 +/- 20 kpc. This distance produces an RR Lyrae absolute magnitude of MV = 0.61 +/- 0.08.

astro-ph↗

arXiv2000

Cepheid Calibration of the Peak Brightness of SNe Ia. X. SN 1991T in NGC 4527

Repeated imaging observations have been made of NGC 4527 with the Hubble Space Telescope between April and June 1999, over an interval of 69 days. Images were obtained on 12 epochs in the F555W band and on five epochs in the F814W band. The galaxy hosted the type Ia supernova SN1991T, which showed relatively unusual behavior by having both an abnormal spectrum near light maximum, and a slower declining light curve than the proto-typical Branch normal SNe Ia. A total of 86 variables that are putative Cepheids have been found, with periods ranging from 7.4 days to over 70 days. From photometry with the DoPHOT program, the de-reddened distance modulus is determined to be (m-M)_0 = 30.67 +/- 0.12 (internal uncertainty) using a subset of the Cepheid data whose reddening and error parameters are secure. A parallel analysis of the Cepheids using photometry with ROMAFOT yields (m -M)_0 =30.82 +/- 0.11. The final adopted modulus is (m -M)_0 =30.74 +/- 0.12 +/- 0.12 (d=14.1 +/- 0.8 +/- 0.8 Mpc). The photometric data for SN1991T are used in combination with the Cepheid distance to NGC 4527 to obtain the absolute magnitude for this supernova of M_V^0(max) = -19.85 +/- 0.29. The relatively large uncertainty is a result of the range in estimates of the reddening to the supernova. Thus SN1991T is seen to be only moderately brighter (by ~ 0.3 mag) than the mean for spectroscopically normal supernovae, although magnitude differences of up to 0.6 mag cannot be ruled out.

astro-ph↗

arXiv2000

Ho from Type Ia Supernovae

The Hubble diagrams in B, V, and I of a complete sample of 35 SNeIa with (B-V) < 0.06 and 1200 < v < 30000 kms^{-1} have a scatter of only 0.1 mag, after small corrections are applied for differences in decline rate Delta m_{15} and color (B-V). The tightness of the Hubble diagrams proves blue SNeIa to be the best ``standard candles'' known. Their absolute magnitudes M_{B,V,I} are calibrated by eight SNeIa with Cepheid distances from HST. Combining this calibration with the appropriate Hubble diagrams yields a large-scale value of H_0 = 58.5+/-6.3 at the 90-percent confidence level. The Hubble diagram of SNeIa has so small scatter that it seems feasable to determine Lambda ``locally'', i.e. within z<0.12, once 100-200 SNeIa with good photometry will be available. Such a local determination would minimize evolutionary effects and K-term corrections.

astro-ph↗

arXiv2000

Supernova type Ia luminosities, their dependence on second parameters, and the value of H_0

A sample of 35 SNe Ia with good to excellent photometry in B and V, minimum internal absorption, and 1200 < v < \approx 30000 km/s is compiled from the literature. As far as their spectra are known they are all Branch-normal. For 29 of the SNe Ia also peak magnitudes in I are known. The SNe Ia have uniform colors at maximum, i.e. =-0.012 mag (sigma=0.051) and =-0.276 mag (sigma=0.078). In the Hubble diagram they define a Hubble line with a scatter of $σ_M$=0.21-0.16 mag, decreasing with wavelength. The scatter is further reduced if the SNe Ia are corrected for differences in decline rate Delta_m_15 or color (B-V). A combined correction reduces the scatter to sigma<=0.13 mag. After the correction no significant dependence remains on Hubble type or galactocentric distance. The Hubble line suggests some curvature which can be differently interpreted. A consistent solution is obtained for a cosmological model with Omega_M=0.3, Omega_Lambda=0.7, which is indicated also by much more distant SNe Ia. Absolute magnitudes are available for eight equally blue (Branch-normal) SNe Ia in spirals, whose Cepheid distances are known. If their well defined mean values of M_B, M_V, and M_I are used to fit the Hubble line to the above sample of SNe Ia one obtains H_0=58.3 km/s/Mpc, or, after adjusting all SNe Ia to the average values of Delta_m_15 and (B-V), H_0=60.9 km/s/Mpc. Various systematic errors are discussed whose elimination tends to decrease H_0. The finally adopted value at the 90-percent level, including random and systematic errors, is H_0=58.5 +/- 6.3 km/s/Mpc. Several higher values of H_0 from SNe Ia, as suggested in the literature, are found to depend on large corrections for variations of the light curve parameter and/or on an unwarranted reduction of the Cepheid distances of the calibrating SNe Ia.

astro-ph↗

arXiv1999

Electron Beam MØller Polarimeter at Jlab Hall a

As part of the spin physics program at Jefferson Laboratory (JLab), a Møller polarimeter was developed to measure the polarization of electron beam of energies 0.8 to 5.0 GeV. A unique signature for Møller scattering is obtained using a series of three quadrupole magnets which provide an angular selection, and a dipole magnet for energy analysis. The design, commissioning, and the first results of the polarization measurements of this polarimeter will be presented as well as future plans to use its small scattering angle capabilities to investigate physics in the very low $Q^2$ regime.

hep-ex↗

arXiv1999

Cepheid Calibration of the Peak Brightness of SNe Ia -- IX. SN 1989B in NGC 3627

(Abridged) Repeated imaging observations have been made of NGC 3627 with the HST in 1997/98, over an interval of 58 days. Images were obtained on 12 epochs in the F555W band and on five epochs in the F814W band. The galaxy hosted the prototypical, `Branch normal', type Ia supernova SN 1989B. A total of 83 variables have been found, of which 68 are definite Cepheid variables with periods ranging from 75 days to 3.85 days. The de-reddened distance modulus is determined to be (m-M)_0= 30.22+/-0.12 (internal uncertainty) using a subset of the Cepheid data whose reddening and error parameters are secure. The photometric data of Wells et al. (1994), combined with the Cepheid data for NGC 3627 give M_B(max)= -19.36+/-0.18 and M_V(max)= -19.34+/-0.16 for SN 1989B. Combined with the previous six calibrations in this program, plus two additional calibrations determined by others gives the mean absolute magnitudes at maximum of = -19.48+/-0.07 and = -19.48 +/-0.07 for `Branch normal' SNe Ia at this interim stage in the calibration program. The second parameter correlations of M(max) of blue SNe Ia with decay rate, color at maximum, and Hubble type are re-investigated. The dependence of on decay rate is non-linear, showing a minimum for decay rates between 1.0< Delta m_15 <1.6. Magnitudes corrected for decay rate show no dependence on Hubble type, but a dependence on color remains. Correcting both the fiducial sample of 34 SNe Ia with decay-rate data and the current 8 calibrating SNe Ia for the correlation with decay rate as well as color gives H_0= 60+/-2 (internal) km/s/Mpc, in both B and V. The same value to within 4% is obtained if only the SNe Ia in spirals (without second parameter corrections) are considered.

astro-ph↗

arXiv1999

The Extragalactic Distance Scale Key Project XVIII. The Discovery of Cepheids and a New Distance to NGC 4535 Using the Hubble Space Telescope

We report on the discovery of Cepheids in the Virgo spiral galaxy NGC 4535, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. NGC 4535 is one of 18 galaxies observed as a part of The HST Key Project on the Extragalactic Distance Scale which aims to measure the Hubble constant to 10% accuracy. NGC 4535 was observed over 13 epochs using the F555W filter, and over 9 epochs using the F814W filter. The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent programs, DoPHOT and DAOPHOT II/ALLFRAME. Period-luminosity relations in the V and I bands were constructed using 39 high-quality Cepheids present in our set of 50 variable candidates. We obtain a distance modulus of 31.02+/-0.26 mag, corresponding to a distance of 16.0+/-1.9 Mpc. Our distance estimate is based on values of mu = 18.50 +/- 0.10 mag and E(V-I) = 0.13 mag for the distance modulus and reddening of the LMC, respectively.

astro-ph↗

arXiv1998

HST observations of the Local Group dwarf galaxy Leo I

We present deep HST F555W (V) and F814W (I) observations of a central field in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting color-magnitude diagram (CMD) reaches I \simeq 26 and reveals the oldest ~10-15 Gyr old turnoffs. Nevertheless, a horizontal branch is not obvious in the CMD. Given the low metallicity of the galaxy, this likely indicates that the first substantial star formation in the galaxy may have been somehow delayed in Leo I in comparison with the other dSph satellites of the Milky Way. The subgiant region is well and uniformly populated from the oldest turnoffs up to the 1 Gyr old turnoff, indicating that star formation has proceeded in a continuous way, with possible variations in intensity but no big gaps between successive bursts, over the galaxy's lifetime. The structure of the red-clump of core He-burning stars is consistent with the large amount of intermediate-age population inferred from the main sequence and the subgiant region. In spite of the lack of gas in Leo I, the CMD clearly shows star formation continuing until 1 Gyr ago and possibly until a few hundred Myrs ago in the central part of the galaxy.

astro-ph↗

arXiv1998

A WFPC2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

The stellar population of the Pegasus dwarf irregular galaxy is investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with WFPC2. These and ground-based data are combined to produce color-magnitude diagrams which show the complex nature of the stellar population in this small galaxy. A young (< 0.5 Gyr) main sequence stellar component is present and clustered in two centrally-located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of A_V = 0.47 mag. The mean color of the well-populated red giant branch is relatively blue, consistent with a moderate metallicity young, or older, metal-poor stellar population. The red giant branch also has significant width in color, implying a range of stellar ages and/or metallicities. A small number of extended asymptotic giant branch stars are found beyond the red giant branch tip. Near the faint limits of our data is a populous red clump superimposed on the red giant branch. Efforts to fit self-consistent stellar population models based on the Geneva stellar evolution tracks yield a revised distance of 760 kpc. Quantitative fits to the stellar population are explored as a means to constrain the star formation history. The numbers of main sequence and core helium-burning blue loop stars require that the star formation rate was higher in the recent past, by a factor of 3-4 about 1 Gyr ago. Unique results cannot be obtained for the star formation history over longer time baselines without better information on stellar metallicities and deeper photometry. The youngest model consistent with the data contains stars with constant metallicity of Z = 0.001 which mainly formed 2-4 Gyr ago. Even at its peak of star forming activity, the Pegasus dwarf most likely remained relatively dim with M_V ~ -14.

astro-ph↗

arXiv1998

WFPC2 Observations of Leo A: A Predominantly Young Galaxy within the Local Group

The unprecedented detail of the WFPC2 colour-magnitude diagrams of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops and the tip of the red giant branch, we obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occured in this galaxy. Leo A provides a nearby laboratory for studying young metal poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.

astro-ph↗

arXiv1997

The HST Key Project on the Extragalactic Distance Scale XIII. The Metallicity Dependence of the Cepheid Distance Scale

Uncertainty in the metal abundance dependence of the Cepheid variable period- luminosity (PL) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and Hubble constant. To test for such a metallicity dependence, we have used the WFPC2 camera on the Hubble Space Telescope (HST) to observe Cepheids in two fields in the nearby spiral galaxy M101, which span a range in oxygen abundance of 0.7+-0.15 dex. A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance, with d(m-M)/d[O/H] = -0.24+-0.16 mag/dex. The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars. External comparisons of Cepheid distances with those derived from three other distance indicators, in particular the tip of the red giant branch method, further constrain the magnitude of any Z-dependence of the PL relation at V and I. The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications, though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10% level.

astro-ph↗

arXiv1996

H_0 from HST

HST has so far provided Cepheid distances to nine galaxies. Although not sufficient yet to determine the distance of the extended Virgo cluster, they are decisive for the distance scale in two ways. (1) Seven of the galaxies contribute to a much improved calibration of the Tully-Fisher relation. Applying this to a complete sample of Virgo spirals one obtains a cluster distance of (m-M)=31.79 +/- 0.09. Other distance indicators support this value. The adopted linear distance of 22.0 +/- 0.8 Mpc combined with the cluster velocity of 1178 +/- 32 km s^-1 (in the CMB frame) gives H_0=54 +/- 2 (internal error). (2) Six of the galaxies have been the site of seven SNe Ia with well observed maxima. Their resulting calibration in absolute magnitudes gives M_B(max)=-19.53 +/- 0.07 and M_V(max)=-19.49 +/- 0.07 with negligible intrinsic scatter. If this calibration is used to determine the distances of all distant SNe Ia with known maxima and with 1100 < v < 30'000 km s^-1, H_0 becomes 56 +/- 3 (internal error). Systematic errors tend to make this an upper limit; in particular the case H_0 >= 70 can be excluded. The conclusion is that the large-scale value of the Hubble constant is H_0=55 +/- 10 (external error).

astro-ph↗

arXiv1995

The Distance to IC 10 from Near-Infrared Observations of Cepheids

We have measured the distance to the Local Group dwarf irregular galaxy IC 10 from near-infrared $JHK$ observations of Cepheids. The reddening-corrected distance modulus is $24.57\pm0.21$ mag or $0.82\pm 0.08$ Mpc. This distance lies at the short end of the range of previous estimates, and is significantly more accurate than all prior determinations. At this distance, IC 10 is comparable in total mass to the Small Magellanic Cloud, but has much more vigorous star formation.

astro-ph↗