Average Distance Modulus Research Articles

A fresh look at the RR Lyrae population in the Draco dwarf spheroidal galaxy with Gaia

ABSTRACT We present a catalogue of 285 RR Lyrae stars (RRLs) in the Draco dwarf spheroidal (dSph) galaxy, obtained by combining data from a number of different surveys including the Data Release 2 (DR2) of the European Space Agency cornerstone mission Gaia. We have determined individual distances to the RRLs in our sample using for the first time a Gaia G-band luminosity–metallicity relation ($M_{G}\small{--}{\rm [Fe/H]}$) and study the structure of the Draco dSph as traced by its RRL population. We find that the RRLs located in the western/south-western region of Draco appear to be closer to us, which may be a clue of interaction between Draco and the Milky Way. The average distance modulus of Draco measured with the RRLs is $\mu =19.53\pm 0.07$ mag, corresponding to a distance of $80.5\pm 2.6$ kpc, in good agreement with previous determinations in the literature. Based on the pulsation properties of the RRLs, we confirm the Oosterhoff-intermediate nature of Draco. We present an additional sample of 41 candidate RRLs in Draco, which we selected from the Gaia DR2 catalogue based on the uncertainty of their G-band magnitudes. Additional epoch data that will become available in the Gaia Data Release 3 will help to confirm whether these candidates are bona fide Draco RRLs.

PHYSICAL PARAMETER STUDY OF EIGHT W URSAE MAJORIS-TYPE CONTACT BINARIES IN NGC 188

ABSTRACT We used the newly commissioned 50 cm Binocular Network telescope at Qinghai Station of Purple Mountain Observatory (Chinese Academy of Sciences) to observe the old open cluster NGC 188 in V and R as part of a search for variable objects. Our time-series data span a total of 36 days. Radial velocity and proper-motion selection resulted in a sample of 532 genuine cluster members. Isochrone fitting was applied to the cleaned cluster sequence, yielding a distance modulus of and a total foreground reddening of E(V − R) = 0.062 ± 0.002 mag. Light-curve solutions were obtained for eight W Ursae Majoris eclipsing binary systems (W UMas), and their orbital parameters were estimated. Using the latter parameters, we estimate a distance to the W UMas that is independent of the host cluster’s physical properties. Based on combined fits to six of the W UMas (EP Cep, EQ Cep, ES Cep, V369 Cep, and—for the first time—V370 Cep and V782 Cep), we obtain an average distance modulus of , which is comparable to that resulting from our isochrone fits. These six W UMas exhibit an obvious period–luminosity relation. We derive more accurate physical parameters for the W UMa systems and discuss their initial masses and ages. The former show that these W UMa systems have likely undergone angular momentum evolution within a convective envelope (W-type evolution). The ages of the W UMa systems agree well with the cluster’s age.

THE CARNEGIE HUBBLE PROGRAM: THE DISTANCE AND STRUCTURE OF THE SMC AS REVEALED BY MID-INFRARED OBSERVATIONS OF CEPHEIDS

ABSTRACT Using Spitzer observations of classical Cepheids we have measured the true average distance modulus of the Small Magellanic Cloud (SMC) to be 18.96 ± 0.01 stat ± 0.03 sys ?> mag (corresponding to 62 ± 0.3 ?> kpc), which is 0.48 ± 0.01 mag more distant than the LMC. This is in agreement with previous results from Cepheid observations, as well as with measurements from other indicators such as RR Lyrae stars and the tip of the red giant branch. Utilizing the properties of the mid-infrared Leavitt Law we measured precise distances to individual Cepheids in the SMC, and have confirmed that the galaxy is tilted and elongated such that its eastern side is up to 20 kpc closer than its western side. This is in agreement with the results from red clump stars and dynamical simulations of the Magellanic Clouds and Stream.

Strömgren-Hβ Photometry of the Rich Open Cluster NGC 6705 (M 11)

We present CCD uvbyβ, photometry of 1419 stars located in the central region of the rich Galactic open cluster NGC 6705 (M11). Our study utilizes photometric data obtained with the 0.9 m WIYN telescope at the Kitt Peak National Observatory. Based on more than 330 stars in the spectral range B–A3, we derive average color excess E(b - y) = 0.33( ± 0.033 s.d.; 0.004 s.e.), corresponding to E(B - V) = 0.45, and average distance modulus 11.30( ± 0.4 s.d.; 0.04 s.e.), corresponding to 1820 ± 30 s.e. pc. Using the F-type stars of the cluster main sequence, we estimate [Fe/H] = -0.06( ± 0.59 s.d.; 0.05 s.e.). Based on PARSEC isochrones for Z = 0.0167, the age of the cluster is estimated as 250 Myr. Moderate differential reddening at the amount of A V = 0.86 is detected. No evidence of extended star formation is found for this cluster.

ASTROMETRY WITH THEHUBBLE SPACE TELESCOPE: TRIGONOMETRIC PARALLAXES OF SELECTED HYADS

We present absolute parallaxes and proper motions for seven members of the Hyades open cluster, pre-selected to lie in the core of the cluster. Our data come from archival astrometric data from FGS 3, and newer data for 3 Hyads from FGS 1R, both white-light interferometers on the Hubble Space Telescope (HST). We obtain member parallaxes from six individual Fine Guidance Sensor (FGS) fields and use the field containing van Altena 622 and van Altena 627 (= HIP 21138) as an example. Proper motions, spectral classifications and VJHK photometry of the stars comprising the astrometric refer- ence frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each Hyad. The parallax of vA 627 is significantly improved by including a perturbation orbit for this previously known spectroscopic binary, now an astrometric binary. Compared to our original (1997) determina- tions, a combination of new data, updated calibration, and improved analysis lowered the individual parallax errors by an average factor of 4.5. Comparing parallaxes of the four stars contained in the Hipparcos catalog, we obtain an average factor of 11 times improvement with the HST . With these new results, we also have better agreement with Hipparcos for the four stars in common. These new parallaxes provide an average distance for these seven members, < D > = 47.5 pc, for the core a \pm 1 - {\sigma} dispersion depth of 3.6 pc, and a minimum depth from individual components of 16.0 \pm 0.9 pc. Absolute magnitudes for each member are compared to established main sequences, with excellent agreement. We obtain a weighted average distance modulus for the core of the Hyades of m-M=3.376 \pm 0.01, a value close to the previous Hipparcos values, m-M=3.33\pm 0.02.

VLT multi-object spectroscopy of 33 eclipsing binaries in the Small Magellanic Cloud

Aim: Our purpose is to provide reliable stellar parameters for a significant sample of eclipsing binaries, which are representative of a whole dwarf and metal-poor galaxy. We also aim at providing a new estimate of the mean distance to the SMC and of its depth along the line of sight for the observed field of view. Method: We use radial velocity curves obtained with the ESO FLAMES facility at the VLT and light curves from the OGLE-II photometric survey. The radial velocities were obtained by least-squares fits of the observed spectra to synthetic ones, excluding the hydrogen Balmer lines. Results: Our sample contains 23 detached, 9 semi-detached and 1 overcontact systems. Most detached systems have properties consistent with stellar evolution calculations from single-star models at the standard SMC metallicity Z = 0.004, though they tend to be slightly overluminous. The few exceptions are probably due to third light contribution or insufficient signal-to-noise ratio. The mass ratios are consistent with a flat distribution, both for detached and semi-detached/contact binaries. A mass-luminosity relation valid from ~4 to ~18 Msol is derived. The uncertainties are in the +-2 to +-11% range for the masses, in the +-2 to +-5% range for the radii and in the +-1 to +-6% range for the effective temperatures. The average distance modulus is 19.11+-0.03 (66.4+-0.9 kpc). The moduli derived from the V and from the I data are consistent within 0.01 mag. The 2-sigma depth of the SMC is, for our field, of 0.25 mag or 7.6 kpc under the assumption of a gaussian distribution of stars along the line of sight. Three systems show significant apsidal motion, one of them with an apsidal period of 7.6 years, the shortest known to date for a detached system with main sequence stars.

Average luminosity distance in inhomogeneous universes

Using numerical ray tracing, the paper studies how the average distance modulus in an inhomogeneous universe differs from its homogeneous counterpart. The averaging is over all directions from a fixed observer not over all possible observers (cosmic), thus is more directly applicable to our observations. In contrast to previous studies, the averaging is exact, non-perturbative, and includes all non-linear effects. The inhomogeneous universes are represented by Swiss-cheese models containing random and simple cubic lattices of mass-compensated voids. The Earth observer is in the homogeneous cheese which has an Einstein-de Sitter metric. For the first time, the averaging is widened to include the supernovas inside the voids by assuming the probability for supernova emission from any comoving volume is proportional to the rest mass in it. Voids aligned along a certain direction give rise to a distance modulus correction which increases with redshift and is caused by cumulative gravitational lensing. That correction is present even for small voids and depends on their density contrast, not on their radius. Averaging over all directions destroys the cumulative lensing correction even in a non-randomized simple cubic lattice of voids. At low redshifts, the average distance modulus correction does not vanish due to the peculiar velocities, despite the photon flux conservation argument. A formula for the maximal possible average correction as a function of redshift is derived and shown to be in excellent agreement with the numerical results. The formula applies to voids of any size that:(a)have approximately constant densities in their interior and walls; and (b)are not in a deep nonlinear regime. The average correction calculated in random and simple cubic void lattices is severely damped below the predicted maximal one after a single void diameter. That is traced to cancellations between the corrections from the fronts and backs of different voids. The results obtained allow one to readily predict the redshift above which the direction-averaged fluctuation in the Hubble diagram falls below a required precision and suggest a method to extract the background Hubble constant from low redshift data without the need to correct for peculiar velocities.

CCD BV and 2MASS photometric study of the open cluster NGC 1513

We present CCD BV and JHK s 2MASS photometric data for the open cluster NGC 1513. We observed 609 stars in the direction of the cluster up to a limiting magnitude of V∼19 mag. The star-count method showed that the centre of the cluster lies at α 2000=04 h 09 m 36 s , δ 2000=49°28′43″ and its angular size is r=10 arcmin. The optical and near-infrared two-colour diagrams revealed the colour excesses in the direction of the cluster as E(B−V)=0.68±0.06, E(J−H)=0.21±0.02 and E(J−K s )=0.33±0.04 mag. These results are consistent with normal interstellar extinction values. Optical and near-infrared Zero Age Main-Sequences (ZAMS) provided an average distance modulus of (m−M)0=10.80±0.13 mag, which can be translated into a distance of 1440±80 pc. Finally, using Padova isochrones we determined the metallicity and age of the cluster as Z=0.015±0.004 ([M/H]=−0.10±0.10 dex) and log (t/yr)=8.40±0.04, respectively.

Stellar tracers of the Cygnus Arm

Cam OB3 is the only defined OB association believed to belong to the Outer Galactic Arm or Cygnus Arm. Very few members have been observed and the distance modulus to the association is not well known. We attempt a more complete description of the population of Cam OB3 and a better determination of its distance modulus. We present uvby photometry of the area surrounding the O-type stars BD +56 864 and LS I +57 138, finding a clear sequence of early-type stars that define an uncatalogued open cluster, which we call Alicante 1. We also present spectroscopy of stars in this cluster and the surrounding association. From the spectral types for 18 very likely members of the association and UBV photometry found in the literature, we derive individual reddenings, finding a extinction law close to standard and an average distance modulus DM=13.0+-0.4. This value is in excellent agreement with the distance modulus to the new cluster Alicante 1 found by fitting the photometric sequence to the ZAMS. In spite of the presence of several O-type stars, Alicante 1 is a very sparsely populated open cluster, with an almost total absence of early B-type stars. Our results definitely confirm Cam OB3 to be located on the Cygnus Arm and identify the first open cluster known to belong to the association.

A "Pulsational" Distance Determination for the Large Magellanic Cloud

We present results from the theoretical modelling of the observed B,V light curves of 14 RR Lyrae stars in the Large Magellanic Cloud. The sample includes 7 fundamental and 7 first overtone pulsators covering the metallicity range from -2.12 to -0.79 dex in [Fe/H], with average value of -1.54 dex. Masses, intrinsic luminosities, effective temperatures, and reddenings were derived by fitting high accuracy multiband light curves available for these RR Lyrae stars to nonlinear convective pulsation models. Individual distance moduli were determined for each variable, they lead to an average distance modulus for the Large Magellanic Cloud of (m-M)o= 18.54 +/-0.02 (sigm= 0.09 mag, standard deviation of the average), in good agreement with the long astronomical distance scale.

The dispersion in the Cepheid period-luminosity relation and the consequences for the extragalactic distance scale

Using published Hubble Space Telescope (HST) Cepheid data from 25 galaxies, we have found a correlation between the dispersion in the Cepheid period-luminosity (P-L) relation and host galaxy metallicity, which is significant at the 3σ level in the V band. In the I band the correlation is less significant, although the tighter intrinsic dispersion of the P-L relation in I may make it harder to detect such a correlation in the HST sample. One possibility is that low metallicity galaxies have smaller metallicity gradients than high metallicity galaxies; if the Cepheid P-L relation has a significant dependence on metallicity then this might explain the higher P-L dispersion in the higher metallicity galaxies. A second possibility is that the increased P-L dispersion is driven by metallicity dispersion but now due to a relation between metallicity and Cepheid colour rather than luminosity. A third possibility is that the increased P-L dispersion is caused by an increase in the width of the instability strip with metallicity. Whatever the explanation, the high observed dispersions in the HST Cepheid P-L relations have the important consequence that the bias due to incompleteness in the P-L relation at faint magnitudes is more significant than previously thought. Using a maximum likelihood technique which takes into account the effect on the P-L relations of truncation by consistently defined magnitude completeness limits, we rederive the Cepheid distances to the 25 galaxies. In the case of the galaxies with the highest P-L dispersion at the largest distances, we find that the published distance modulus underestimates the true distance modulus by up to 0.5 mag. When both metallicity and magnitude incompleteness corrections are made, a scale error in the published Cepheid distances is seen in the sense that the published distance moduli are increasingly underestimated at larger distances. This results in the average distance modulus to the four galaxies in the Virgo cluster core increasing from (m - M) 0 = 31.2 ± 0.19 to (m - M) 0 = 31.4 ± 0.19 if the γ VI = -0.24 mag dex -1 metallicity correction of Kenni-cutt et al. is assumed. For the 18 HST galaxies with good Tully-Fisher (TF) distances and (m - M) 0 > 29.5 the Cepheid-TF distance modulus average residual increases from 0.44 ± 0.09 to 0.63 ± 0.1 mag with γ VI = -0.24. This indicates a significant scale error in TF distances, which reduces the previous Pierce & Tully TF estimate of H 0 = 85 ± 10 km s -1 Mpc -1 to H 0 = 63 ± 7 km s -1 Mpc -1 , assuming y VI = -0.24 and a still uncertain Virgo infall model. Finally, for the eight HST galaxies with Type Ia supernovae (SNIa), the metallicity and incompleteness corrected Cepheid distances marginally suggest there may be a metallicity dependence of SNIa peak luminosity in the sense that metal-poor hosts have lower luminosity SNIa. Thus, SNIa Hubble diagram estimates of both H 0 and q 0 may therefore also require significant corrections for metallicity, once the exact sizes of the Cepheid metallicity corrections become better established.

FOUR-COLOR PHOTOMETRY FOR THE ASSOCIATION LACERTA OB1.

ABSTRACT We have obtained uvby data for 51 stars in the region of the association Lacerta OB1, near the O9 V star 10 Lac. An analysis of the present data, combined with previous observations by others, leads to an average color excess for the more concentrated of the two subgroups (1b) of (E(b - y)) = (one ) and an average distance modulus of V0 - Mvfi = i (one star). The more dispersed subgroup has a larger average excess and a larger dispersion in both color excess and distance modulus for the stars. Several stars are suggested as nonmembers. Key words: photometry - stellar association - color excess