Reddening Values Research Articles

Integrated spectral properties of four previously unstudied small angular size galactic open clusters

AbstractWe have derived fundamental parameters for the previously unstudied concentrated Galactic open clusters Berkeley 77, NGC2587, BH58 and ESO324‐SC15 by means of integrated spectroscopy in the range 3800–6800 Å. The cluster integrated spectra are compared with those of solar metallicity template spectra of Galactic open clusters with known parameters. The spectra were obtained at CASLEO (San Juan, Argentina) with a resolution of 14 Å.We estimated simultaneously age and foreground interstellar reddening by comparing the continuum distribution and line strengths of the cluster spectra with those of template spectra. Berkeley 77 was found to be an old cluster of 3–4 Gyr, while the remaining three clusters are between 400 and 1000 Myr old. The derived foreground reddening values range from E (B – V) = 0.0 in NGC2587 and ESO324‐SC15 to approximately 0.30 in Berkeley 77. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Old open clusters as key tracers of Galactic chemical evolution

Aims. To constrain the formation and chemical evolution of the Galactic disk, we surveyed open clusters of different ages, metal contents, and distances from the Galactic centre. We employed FLAMES on VLT-UT2 to collect UVES spectra of five to ten giant stars in each of the selected clusters and used them to derive the iron abundance and the detailed chemical composition. Methods. Equivalent widths were measured and abundance analysis was performed using the MOOG code and Kurucz model atmospheres on all stars considered as cluster members on the basis of their radial velocity. Results. We derived the atmospheric parameters and the Fe abundance for NGC 2324 and NGC 2477 (average [Fe/H] = -0.17 with rms 0.05 dex, and +0.07 with rms 0.03 dex, respectively), two clusters never analyzed using high resolution spectra. We also derived the abundances of Mg, Al, Ca, Si, Ti, Cr, Ni, and Ba for these two clusters and for NGC 2660, NGC 3960, and Berkeley 32, whose atmospheric parameters and metallicities were measured in a previous paper. We determined the reddening values for the five clusters, based on the spectroscopic temperatures, literature photometry, and a colour-temperature relation. Conclusions. All clusters show solar-scaled abundances for α- and Fe-peak elements, while [Na/Fe] appears slightly enhanced and [Ba/Fe] significantly enhanced. Our findings were compared to thin-disk stars and other open clusters, and no significant deviation from the standard behavior was found.

Integrated spectral properties of 22 small angular diameter galactic open clusters

Aims. Flux-calibrated integrated spectra of a sample of 22 Galactic open clusters of small angular diameter are presented. With one exception (ESO 429-SC2), all objects have Galactic longitudes in the range 208 ◦ < l < 33 ◦ . The spectra cover the range ≈3600−6800 A, with a resolution of ≈14 A. The properties of the present cluster sample are compared with those of well-studied clusters located in two 90 ◦ sectors, centred at l = 257 ◦ and l = 347 ◦ . The dissolution rate of Galactic open clusters in these two sectors is examined. Methods. Using the equivalent widths of the Balmer lines and comparing line intensities and continuum distribution of the cluster spectra with those of template cluster spectra with known properties, we derive both foreground reddening values and ages. Thus, we provide information independent of that determined through colour–magnitude diagrams. Results. The derived E(B − V) values for the whole sample vary from 0.0 in ESO 445-SC74 to 1.90 in Pismis 24, while the ages range from ∼3 Myr (NGC 6604 and BH 151) to ∼3.5 Gyr (Ruprecht 2). For six clusters (Dolidze 34, ESO 429-SC2, ESO 445-SC74, Ruprecht 2, BH 151 and Hogg 9) the foreground E(B − V) colour excesses and ages are determined for the first time. The results obtained for the remaining clusters show, in general terms, good agreement with previous photometric results. Conclusions. The age and reddening distributions of the present sample match those of known clusters in the two selected Galactic sectors. The present results would favour a major dissolution rate of star clusters in these two sectors. Two new solar-metallicity templates are defined corresponding to the age groups of (4−5) Myr and 30 Myr among those of Piatti et al. (2002, MNRAS, 335, 233). The Piatti et al. templates of 20 Myr and (3−4) Gyr are here redefined.

The Benchmark Cluster Reddening Project. III. A Comparison of Reddening Values Derived from F and K Stars in NGC 752

A protocol for reddening analysis developed in previous papers in this series is reviewed here and applied to NGC 752. The protocol includes (1) detailed scrutiny of the histories of reddening determinations for program clusters, (2) adherence to statistical norms, (3) quality control of all reddening techniques, (4) a primary focus on techniques with limited metallicity sensitivity, (5) zero-point control of source data, (6) use of the solar neighborhood to establish a zero point for reddening values, (7) use of a standard permitting meaningful quantities to be as small as a few millimagnitudes, and (8) rejection of ad hoc data assessments. After certain published results are set aside, it is found that a reddening value derived for F stars by using a technique that is insensitive to blanketing disagrees by about 0.04 mag with a value derived for K giants by using DDO photometry. This disagreement does not appear to be an artifact of the use of questionable data, and it persists if additional techniques are considered. Partly because two comparable reddening results for K giants are afterward found to disagree with each other, the problem is attributed to a blanketing effect (with an unknown source) on the reddening values derived for the K giants. Pending a final resolution of that problem, the adopted value of E(B - V) for the cluster (44 ± 3.4 mmag) is based solely on the initial F-star analysis. A discussion of the potential usefulness of small standard errors like the one derived here for E(B - V) is given. In a parallel investigation, it is found that no definitive value of [Fe/H] can be assigned to NGC 752 at present because a statistically significant disagreement between two published high-precision metallicities is detected.

Uvby– H$\mathsf{_{\beta}}$ CCD photometry and membership segregation of the open cluster NGC 2682 (M 67)

Following deep astrometric and photometric study of the cluster NGC2682 (M67), we are able to accurately determine its fundamental parameters. Being an old and rich cluster, M67 is a relevant object for the analysis of the Galactic disk evolution. M67 is well studied but the lack of a wide and deep Stromgren photometric study makes our results worthwhile. The brightest stars of the open cluster M67 were used as uvby-Hbeta standard stars in our studies of NGC1817 and NGC2548, and the extension of the field covered, as well as the amount of observations, allowed to obtain the best set of Stromgren data ever published for this cluster. We discuss the results of our CCD uvby-Hbeta intermediate-band photometry, covering an area of about 50'x50' down to V 19. Moreover, a complete membership segregation based on astrometric and photometric criteria is obtained. The photometric analysis of a selected sample of stars yields a reddening value of E(b-y)= 0.03\pm0.03, a distance modulus of V_0-M_V= 9.7\pm0.2 and [Fe/H]= 0.01\pm0.14. Through isochrone fitting we found an age of log t= 9.6\pm0.1 (4.2\pm0.2 Gyr). A clump of approximately 60 stars around V= 16, (b-y)= 0.4 could be interpreted as a population of pre-cataclysmic variable stars (if members), or as a stream of field G-type stars placed at twice the distance of the cluster (if non-members).

A δ Scuti Distance to the Large Magellanic Cloud

We present results from a well-studied δ Scuti star discovered in the Large Magellanic Cloud (LMC). The absolute magnitude of the variable was determined from the period-luminosity (P-L) relation for Galactic δ Scuti stars and from theoretical modeling of the observed B,V,I light curves with nonlinear pulsation models. The two methods give distance moduli for the LMC of 18.46 ± 0.19 and 18.48 ± 0.15, respectively, for a consistent value of the stellar reddening of E(B - V) = 0.08 ± 0.02. We have also analyzed 24 δ Scuti candidates discovered in the OGLE II survey of the LMC, and seven variables identified in the open cluster LW 55 and in the galaxy disk by Kaluzny and coworkers . We find that the LMC δ Scuti stars define a P-L relation whose slope is very similar to that defined by the Galactic δ Scuti variables, and yield a distance modulus for the LMC of 18.50 ± 0.22 mag. We compare the results obtained from the δ Scuti variables with those derived from the LMC RR Lyrae stars and Cepheids. The corresponding distance moduli are as follows: δ Scuti stars, 18.48 ± 0.02 mag (standard deviation of the weighted average of the three above solutions); RR Lyrae stars, 18.49 ± 0.06 mag; and Cepheids, 18.53 ± 0.02 mag. We have assumed an average color excess of E(B - V) = 0.08 ± 0.02 mag for both δ Scuti stars and Cepheids. Within the observational uncertainties, the three groups of pulsating stars yield very similar distance moduli. These moduli are all consistent with the long astronomical distance scale for the Large Magellanic Cloud.

Integrated spectroscopy and individual spectra of stars of open cluster remnants and candidates

The integrated light of open cluster remnants (OCRs) is expected to be similar to that of open clusters of comparable ages. Despite erosion of the mass function in remnants as a consequence of dynamical evolution, their integrated spectra could still be similar to those of the clusters of comparable age as long as a sufficient number of bright stars remain. In the present work we employ integrated spectroscopy and individual spectra of stars to analyze 8 OCRs and candidates which were previously classified as open clusters and a new unclassified object. The OCRs and possible open cluster remnants (POCRs) are underpopulated with respect to usual open clusters, but they show a significant density contrast of bright stars when compared to the Galactic field. The spectroscopic observations were carried out with the 2.15-m telescope at CASLEO (San Juan, Argentina) in the range 3800-6800 Å, during 2003 and 2004. We derive foreground interstellar reddening values and ages. We find in most of the sample that the derived properties are compatible with what should be expected for OCRs.

VIPhotometry of Globular Clusters NGC 6293 and NGC 6541: The Formation of the Metal-poor Inner Halo Globular Clusters

We present VI photometry of the metal-poor inner halo globular clusters NGC 6293 and NGC 6541 using the WFPC2 on board the Hubble Space Telescope (HST). Our color-magnitude diagrams of the clusters show well-defined blue horizontal-branch populations, consistent with their low metallicities and old ages. NGC 6293 appears to have blue straggler stars in the cluster's central region. We discuss the interstellar reddening and the distance modulus of NGC 6293 and NGC 6541 and obtain E(B - V) = 0.40 and (m - M)0 = 14.61 for NGC 6293 and E(B - V) = 0.14 and (m - M)0 = 14.19 for NGC 6541. Our results confirm that NGC 6293 and NGC 6541 are clearly located in the Galaxy's central regions (RGC ≤ 3 kpc). We also discuss the differential reddening across NGC 6293. The interstellar reddening value of NGC 6293 appears to vary by ΔE(B - V) ≈ 0.02-0.04 mag within our small field of view. The most notable result of our study is that the inner halo clusters NGC 6293 and NGC 6541 essentially have the same ages as M92, confirming the previous result from the HST NIC3 observations of NGC 6287.

The Broadband Spectrum and Infrared Variability of the Magnetar AXP 1E 1048.1−5937

We present photometry of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 in the infrared and optical, taken at Magellan and the VLT. The object is detected in the I, J, and Ks bands under excellent conditions. We find that the source has varied greatly in its infrared brightness and present these new magnitudes. No correlation is found between the infrared flux and spin-down rate, but the infrared flux and X-ray flux may be anticorrelated. Assuming nominal reddening values, the resultant spectral energy distribution (SED) is found to be inconsistent with the only other AXP SED available (for 4U 0142+61). We consider the effect of the uncertainty in the reddening to the source on its SED. We find that although both the X-ray and infrared fluxes have varied greatly for this source, the most recent flux ratio is remarkably consistent with what is found for other AXPs. Finally, we discuss the implications of our findings in the context of the magnetar model.

Uvby – H$_{\beta}$ CCD photometry and membership segregation of the open cluster NGC 2548; gaps in the Main Sequence of open clusters

Deep CCD photometry in the uvby-Hbeta intermediate-band system is presented for the cluster NGC 2548 (M 48). A complete membership analysis based on astrometric and photometric criteria is applied. The photometric analysis of a selected sample of stars yields a reddening value of E(b-y)=0.06\pm0.03, a distance modulus of V_0-M_V=9.3\pm0.5 (725 pc) and a metallicity of [Fe/H]= -0.24\pm0.27. Through isochrone fitting we find an age of log t = 8.6\pm0.1 (400 Myr). Our optical photometry and JHK from 2MASS are combined to derive effective temperatures of cluster member stars. The effective temperature distribution along the main sequence of the cluster shows several gaps. A test to study the significance of these gaps in the main sequence of the HR diagram has been applied. The method is also applied to several other open clusters (Pleiades, Hyades, NGC 1817 and M 67) to construct a sequence of metallicities and ages. The comparison of the results of each cluster gives four gaps with high significance (one of them, centred at 4900 K, has not been previously reported).

Uvby–H$_{\beta}$ CCD photometry of NGC 1817 and NGC 1807

We have investigated the area of two NGC entries, NGC 1817 and NGC 1807, with deep CCD photometry in the uvby-Hbeta intermediate-band system. The photometric analysis of a selected sample of stars of the open cluster NGC 1817 yields a reddening value of E(b-y) = 0.19 +/- 0.05, a distance modulus of V-0-M-V = 10.9 +/- 0.6, a metallicity of [Fe/H] = -0.34 +/- 0.26 and an age of log t = 9.05 +/- 0.05. Our measurements allow us to confirm that NGC 1807 is not a physical cluster.

HST NIC3 PHOTOMETRY OF METAL-RICH GLOBULAR CLUSTERS PALOMAR 6, LILLER 1, AND 47 TUC (NGC 104)

We present HST NIC3 photometry of metal-rich globular clusters Palomar 6, Liller 1 and 47 Tuc (NGC 104). We discuss the interstellar reddening law for the HST NICMOS F110W/F160W photometric system which depends on the temperature of the source. The distance moduli and interstellar reddening values for Palomar 6 and Liller 1 are estimated by comparing the magnitudes and colors of RHB stars in the clusters with those of 47 Tuc. We obtain <TEX>$(m-M)_0=14.48$</TEX>mag and E(B-V)=1.34mag for Palomar 6 and <TEX>$(m-M)_0=15.17$</TEX>mag and E(B-V)=2.50 mag for Liller 1.

The Recent Star Formation History of the M31 Disk

The star formation history of the northern and southern M31 disk is measured using samples of BV photometry for 4' × 4' regions taken from the KPNO/CTIO Local Group Survey. The distances, mean reddening values, and age distributions of the stars in these regions were measured using the routines of Dolphin. Independent measurements of overlapping fields show that the results are stable for most samples. A slight distance gradient is seen across the major axis of the southern disk, and a mean distance of 24.47 ± 0.03 is found by combining the results. Higher mean reddening values are seen to follow the spiral structure. The stellar age distributions are consistent with episodic star formation confined mainly to the gas-rich arm regions. If these episodes were caused by propagating density waves, the waves did not cause significant star formation episodes in the gas-poor interarm regions. A combination of all of the results provides the total star formation rate for 1.4 deg2 of the M31 disk for six epochs. These results suggest that star formation in the disk declined by ~50% from ~250 to ~50 Myr ago. The lowest star formation rate occurred ~25 Myr ago, followed by a ~20% increase to the present. The mean star formation rate for this large portion of M31 over the past 60 Myr is 0.63 ± 0.07 M⊙ yr-1, suggesting a total mean rate for the disk of ~1 M⊙ yr-1.

On the calibration of the COBE/IRAS dust emission reddening maps

In this work we study the spectral properties (3600-6800 A) of the nuclear region of early-type galaxies at low (|b| < 25 ◦ ), intermediate (including surroundings of the Magellanic Clouds) and high (South Polar Cap) Galactic latitudes. We determine the E(B − V) reddening values of the galaxies by matching their continuum distribution with respect to those of reddening-free spectral galaxy templates with similar stellar populations. We also compare the spectroscopic reddening value of each galaxy with that derived from 100 µm dust emission (E(B −V)FIR) in its line of sight, and we find that there is agreement up to E(B − V) = 0.25. Beyond this limit E(B − V)FIR values are higher. Taking into account the data up to E(B − V) ≈ 0.7, we derive a calibration factor of 0.016 between the spectroscopic E(B − V) values and Schlegel et al.'s (1998) opacities. By combining this result with an AK extinction map built within ten degrees of the Galactic centre using Bulge giants as probes (Dutra et al. 2003), we extended the calibration of dust emission reddening maps to low Galactic latitudes down to |b| = 4 ◦ and E(B −V) = 1. 6( AV ≈ 5). According to this new calibration, a multiplicative factor of ≈0.75 must be applied to the COBE/IRAS dust emission reddening maps.

CCD [CLC][ITAL]uvby[/ITAL][/CLC][ITAL]C[/ITAL][CLC][ITAL]a[/ITAL][/CLC]Hβ Photometry of Clusters. III. The Most Metal-rich Open Cluster, NGC 6253

CCD photometry on the intermediate-band uvbyCaHβ system is presented for the old open cluster NGC 6253. Despite a high level of field star contamination because of its location toward the Galactic center, combination of the data from the multiple color indices with the core cluster sample derived from radial star counts leads to the identification of a set of highly probable, single cluster members. Photometric analysis of a select sample of 71 turnoff stars produces a reddening value of E(b-y) = 0.190 ± 0.002 (s.e.m.) or E(B-V) = 0.260 ± 0.003 (s.e.m.). The metallicity indices, δm1 and δhk, both identify this cluster as the most metal-rich object studied on either system to date. Simple extrapolation of the available metallicity calibrations leads to [Fe/H] values ranging from +0.7 to +0.9. Metal-rich isochrones with overshoot imply an age between 2.5 and 3.5 Gyr, with an apparent distance modulus between (m - M) = 11.6 and 12.2, depending upon the isochrones used. The improvement in the fit using α-enhanced isochrones may indicate that the cluster [Fe/H] is closer to +0.4, but the photometric indices are distorted by an elemental distribution other than a scaled solar. The Galactocentric position of the cluster, in conjunction with data for other clusters and Cepheids, is consistent with the inner disk reaching and maintaining a metallicity well above solar since the early history of the disk, unlike the solar neighborhood.

Dating star clusters in the Small Magellanic Cloud by means of integrated spectra

In this study flux-calibrated integrated spectra in the range (3600-6800) A are presented for 16 concentrated star clusters in the Small Magellanic Cloud (SMC), approximately half of which constitute unstudied objects. We have estimated ages and foreground interstellar reddening values from the comparison of the line strengths and continuum distribution of the cluster spectra with those of template cluster spectra with known parameters. Most of the sample clusters are young blue clusters (6-50 Myr), while L 28, NGC 643 and L 114 are found to he intermediate-age clusters (1-6 Gyr). One well known SMC cluster (NGC 416) was observed for comparison purposes. The sample includes clusters in the surroundings and main body of the SMC, and the derived foreground reddening values are in the range 0.00 ≤ E(B - V) ≤ 0.15. The present data also make up a cluster spectral library at SMC metallicity.

The very reddened open clusters Pismis 23 (Lyngå 10) and Stephenson 2

Pismis 23 and Stephenson 2 are two very reddened open clusters, projected close to the Galactic plane. We present VI and JH photometry for Pismis 23 and I and Gunn z for Stephenson 2. We derive a relatively old age of ~700 Myr for Pismis 23, with a reddening value of $E(B-V) = 1.73$ and a distance $d_{\odot}\approx 2.6$ kpc from the Sun. Stephenson 2 has a clump of red supergiants, indicating a young age of about 20 Myr, somewhat younger and more distant ($d_{\odot}\approx 6$ kpc) than recently reported in the literature. The age estimate for such a young object is crucial for the distance estimate. The high reddening of these two clusters relative to predictions from the diffuse interstellar medium at their distances is due to discrete intervening clouds.

Analysis of colour-magnitude diagrams of rich LMC clusters: NGC 1831

We present the analysis of a deep colour-magnitude diagram (CMD) of NGC 1831, a rich star cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W (~V) and F814W (~I) filters, reaching m_555 ~ 25. We discuss and apply a method of correcting the CMD for sampling incompleteness and field star contamination. Efficient use of the CMD data was made by means of direct comparisons of the observed to model CMDs. The model CMDs are built by an algorithm that generates artificial stars from a single stellar population, characterized by an age, a metallicity, a distance, a reddening value, a present day mass function and a fraction of unresolved binaries. Photometric uncertainties are empirically determined from the data and incorporated into the models as well. Statistical techniques are presented and applied as an objective method to assess the compatibility between the model and data CMDs. By modelling the CMD of the central region in NGC 1831 we infer a metallicity Z = 0.012, 8.75 < log(tau) < 8.80, 18.54 < (m-M)_0 < 18.68 and 0.00 < E(B-V) < 0.03. For the position dependent PDMF slope (alpha = -dlog(Phi(M))/dlog(M)), we clearly observe the effect of mass segregation in the system: for projected distances R < 30 arcsec, alpha ~ 1.7, whereas 2.2 < alpha < 2.5 in the outer regions of NGC 1831.

CCD photometric search for peculiar stars in open clusters

In total, more than 550 objects within the fields of five open clusters were investigated in order to find classical chemically peculiar stars. One bona-fide CP candidate was detected in each of the open clusters NGC 3960, NGC 6192 and NGC 6451. All three objects seem to be members of the corresponding cluster, taking the results from our photometry as well as from the literature. For NGC 6192 and NGC 6451 we found widely different interstellar reddening values and therefore distances and ages in the literature. From an analysis of published Johnson UBVRI photometry, we were able to rule out a rather old age for NGC 6451. This is also supported by the finding of one peculiar object in this cluster with spectral type of about A2. Five variable objects within the field of NGC 6134 were detected. The variability exceeds 10 times (or typically 0.03 mag) the standard deviation of the weighted individual measurements. But since our observations were obtained mostly within one hour (typical integration times of five minutes per filter) on several nights within 45 days, no light curves could be derived. For this cluster, some Am candidates were reported for which we were not able to detect a significant positive Δa-value.

The ‘remarkable’ M31 globular cluster 037–B327 revisited

The M31 globular cluster candidate 037-B327 has long been known to be an extremely red, non-stellar object. The first published spectrum of this object is used to confirm that it is a globular cluster belonging to M31, with rather typical values of v_r = -338 +/- 12 km/s and [Fe/H]= -1.07 +/- 0.20 dex. Using the spectroscopic metallicity to predict the intrinsic colours, we derive a reddening value of E(B-V)=1.30 +/- 0.04 in good agreement with the value obtained using reddening-free parameters. The extinction-corrected magnitude of 037-B327 is V_0=12.73 (absolute magnitude M_V = -11.74), which makes it the most luminous globular cluster in M31. We examine van den Bergh's (1968) argument regarding the brightest and most-reddened globular cluster in in M31; we find that the brightest clusters are more heavily-reddened than average, but this can be explained by selection effects rather than a different R_V in M31.