A uvbyCaHbeta CCD Analysis of the Open Cluster Standard, NGC 752

Precision uvbyCaHbeta photometry of the nearby old open cluster, NGC 752, is presented. The mosaic of CCD fields covers an area ~42' on a side with internal precision at the 0.005 to 0.010 mag level for the majority of stars down to V~15. The CCD photometry is tied to the standard system using an extensive set of published photoelectric observations adopted as secondary standards within the cluster. Multicolor indices are used to eliminate as nonmembers a large fraction of the low probability proper-motion members near the faint end of the main sequence, while identifying 24 potential dwarf members between V=15.0 and 16.5, eight of which have been noted before from Vilnius photometry. From 68 highly probable F dwarf members, we derive a reddening estimate of E(b-y)= 0.025 +/- 0.003 (E(B-V) = 0.034 +/- 0.004), where the error includes the internal photometric uncertainty and the systematic error arising from the choice of the standard (b-y), Hbeta relation. With reddening fixed, [Fe/H] is derived from the F dwarf members using both m_1 and hk, leading to [Fe/H] = -0.071 +/-0.014 (sem) and -0.017 +/- 0.008 (sem), respectively. Taking the internal precision and possible systematics in the standard relations into account, [Fe/H] for NGC 752 becomes -0.03 +/-0.02. With the reddening and metallicity defined, we use the Victoria-Regina isochrones on the Stromgren system and find an excellent match for (m-M) = 8.30 +/- 0.05 and an age of 1.45 +/- 0.05 Gyr at the appropriate metallicity.Comment: 37 pages, 11 figures, 4 tables. Accepted to Astronomical Journa

A uvbyCaHβ CCD Analysis of the Open Cluster Standard NGC 752*

Precision uvbyCaHβ photometry of the nearby old open cluster, NGC 752, is presented. The mosaic of CCD fields covers an area ~42' on a side with internal precision at the 0.005–0.010 mag level for the majority of stars down to V ~ 15. The CCD photometry is tied to the standard system using an extensive set of published photoelectric observations adopted as secondary standards within the cluster. Multicolor indices are used to eliminate as nonmembers a large fraction of the low probability proper-motion members near the faint end of the main sequence, while identifying 24 potential dwarf members between V = 15.0 and 16.5, eight of which have been noted before from Vilnius photometry. From 68 highly probable F dwarf members, we derive a reddening estimate of E(b − y) = 0.025 ± 0.003 (E(B − V) = 0.034 ± 0.004), where the error includes the internal photometric uncertainty and the systematic error arising from the choice of the standard (b − y, Hβ) relation. With reddening fixed, [Fe/H] is derived from the F dwarf members using both m1 and hk, leading to [Fe/H] = −0.071 ± 0.014 (sem) and −0.017 ± 0.008 (sem), respectively. Taking the internal precision and possible systematics in the standard relations into account, [Fe/H] for NGC 752 becomes −0.03 ± 0.02. With the reddening and metallicity defined, we use the Victoria-Regina isochrones on the Strömgren system and find an excellent match for (m − M) = 8.30 ± 0.05 and an age of 1.45 ± 0.05 Gyr at the appropriate metallicity

vbyCaHbeta CCD Photometry of Clusters. VI. The Metal-Deficient Open Cluster NGC 2420

CCD photometry on the intermediate-band vbyCaHbeta system is presented for the metal-deficient open cluster, NGC 2420. Restricting the data to probable single members of the cluster using the CMD and the photometric indices alone generates a sample of 106 stars at the cluster turnoff. The average E(b-y) = 0.03 +/- 0.003 (s.e.m.) or E(B-V) = 0.050 +/- 0.004 (s.e.m.), where the errors refer to internal errors alone. With this reddening, [Fe/H] is derived from both m1 and hk, using b-y and Hbeta as the temperature index. The agreement among the four approaches is reasonable, leading to a final weighted average of [Fe/H] = -0.37 +/- 0.05 (s.e.m.) for the cluster, on a scale where the Hyades has [Fe/H] = +0.12. When combined with the abundances from DDO photometry and from recalibrated low-resolution spectroscopy, the mean metallicity becomes [Fe/H] = -0.32 +/- 0.03. It is also demonstrated that the average cluster abundances based upon either DDO data or low-resolution spectroscopy are consistently reliable to 0.05 dex or better, contrary to published attempts to establish an open cluster metallicity scale using simplistic offset corrections among different surveys.Comment: scheduled for Jan. 2006 AJ; 33 pages, latex, includes 7 figures and 2 table

uvbyCaHbeta CCD Photometry of Clusters. V. The Metal-Deficient Open Cluster NGC 2243

CCD photometry on the intermediate-band uvbyCaHbeta system is presented for the metal-deficient open cluster, NGC 2243. Restricting the data to probable single members of the cluster using the CMD and the photometric indices alone generates a sample of 100 stars at the cluster turnoff. The average E(b-y) = 0.039 (0.003 s.e.m.) or E(B-V) = 0.055 (0.004 s.e.m.), where the errors refer to internal errors alone. With this reddening, [Fe/H] is derived from both m_1 and hk, using b-y and Hbeta as the temperature index. The agreement among the four approaches is excellent, leading to a final weighted average of [Fe/H] = -0.57 (0.03 s.e.m.) for the cluster, on a scale where the Hyades has [Fe/H] = +0.12. Using a combination of photometric and spectroscopic data, 27 probable cluster members are identified and used to delineate the red giant branch and a well-defined clump at V = 13.70, while eliminating the so-called second clump at V = 14.1. Interpolation between isochrones of appropriate [Fe/H] leads to an apparent modulus of (m-M) = 13.15 +/- 0.1 and an age of 3.8 +/- 0.2 Gyr. A differential CMD comparison with Berkeley 29, a cluster with a galactocentric distance almost twice that of NGC 2243, constrains Berkeley 29 to be at least as young and as metal-rich as NGC 2243.Comment: 21 pg postscript, latex to preprint format, 15 eps figures, 2 tables. Accepted for publication in the Astronomical Journal Feb. 200

The discontinuous nature of chromospheric activity evolution

Chromospheric activity has been thought to decay smoothly with time and, hence, to be a viable age indicator. Measurements in solar type stars in open clusters seem to point to a different conclusion: chromospheric activity undergoes a fast transition from Hyades level to that of the Sun after about 1 Gyr of main--sequence lifetime and any decaying trend before or after this transition must be much less significant than the short term variations.Comment: 6 pages, 1 figure, to be published in Astrophysics and Space Scienc

A possible age-metallicity relation in the Galactic thick disk?

A sample of 229 nearby thick disk stars has been used to investigate the existence of an age-metallicity relation (AMR) in the Galactic thick disk. The results indicate that that there is indeed an age-metallicity relation present in the thick disk. By dividing the stellar sample into sub-groups, separated by 0.1 dex in metallicity, we show that the median age decreases by about 5-7 Gyr when going from [Fe/H]=-0.8 to [Fe/H]=-0.1. Combining our results with our newly published alpha-element trends for a local sample of thick disk stars, that show signatures from supernovae type Ia (SNIa), we can here draw the conclusion that the time-scale for the peak of the SNIa rate is of the order 3-4 Gyr in the thick disk. The tentative evidence for a thick disk AMR that we present here also has implications for the thick disk formation scenario; star-formation must have been an ongoing process for several billion years. This is further discussed here and appear to strengthen the hypothesis that the thick disk originates from a merger event with a companion galaxy that puffed up a pre-existing thin disk.Comment: Accepted for publication in A&A, 8 page

Energy measurement and fragment identification using digital signals from partially depleted Si detectors

A study of identification properties of a Si-Si DE-E telescope exploiting an underdepleted residual-energy detector has been performed. Five different bias voltages have been used, one corresponding to full depletion, the others associated with a depleted layer ranging from 90% to 60% of the detector thickness. Fragment identification has been performed using either the DE-E technique or Pulse Shape Analysis (PSA). Both detectors are reverse mounted: particles enter from the low field side, to enhance the PSA performance. The achieved charge and mass resolution has been quantitatively expressed using a Figure of Merit (FoM). Charge collection efficiency has been evaluated and the possibility of energy calibration corrections has been considered. We find that the DE-E performance is not affected by incomplete depletion even when only 60% of the wafer is depleted. Isotopic separation capability improves at lower bias voltages with respect to full depletion, though charge identification thresholds are higher than at full depletion. Good isotopic identification via PSA has been obtained from a partially depleted detector whose doping uniformity is not good enough for isotopic identification at full depletion.Comment: 13 pages, 10 figures 5 tables; submitted to European Physical Journal

Enhanced X-ray variability from V1647 Ori, the young star in outburst illuminating McNeil's Nebula

We report a ~38 ks X-ray observation of McNeil's Nebula obtained with XMM on 2004 April 4. V1647 Ori, the young star in outburst illuminating McNeil's Nebula, is detected with XMM and appears variable in X-rays. We investigate the hardness ratio variability and time variations of the event energy distribution with quantile analysis, and show that the large increase of the count rate from V1647 Ori observed during the second half of the observation is not associated with any large plasma temperature variations as for typical X-ray flares from young low-mass stars. X-ray spectral fitting shows that the bulk (~75%) of the intrinsic X-ray emission in the 0.5-8 keV energy band comes from a soft plasma component (0.9 keV) reminiscent of the X-ray spectrum of the classical T Tauri star TW Hya, for which X-ray emission is believed to be generated by an accretion shock onto the photosphere of a low-mass star. The hard plasma component (4.2 keV) contributes ~25% of the total X-ray emission, and can be understood only in the framework of plasma heating sustained by magnetic reconnection events. We find a hydrogen column density of NH=4.1E22 cm-2, which points out a significant excess of hydrogen column density compared to the value derived from optical/IR observations, consistent with the picture of the rise of a wind/jet unveiled from ground optical spectroscopy. The X-ray flux observed with XMM ranges from roughly the flux observed by Chandra on 2004 March 22 (~10 times greater than the pre-outburst X-ray flux) to a value two times greater than that caught by Chandra on 2004 March 7 (~200 times greater than the pre-outburst X-ray flux). We have investigated the possibility that V1647 Ori displays a periodic variation in X-ray brightness as suggested by the combined Chandra+XMM data set (abridged).Comment: 11 pages and 8 Figures. Accepted for publication by Astronomy & Astrophysic

A Uniform CO Survey of the Molecular Clouds in Orion and Monoceros

We report the results of a new large scale survey of the Orion-Monoceros complex of molecular clouds made in the J = 1->0 line of CO with the Harvard-Smithsonian 1.2m millimetre-wave telescope. The survey consists of 52,288 uniformly spaced spectra that cover an area of 432 square degrees on the sky and is the most sensitive large-scale survey of the region to date. Distances to the constituent molecular clouds of the complex, estimated from an analysis of foreground and background stars, have provided information on the three dimensional structure of the entire complex.Comment: Accepted for publication in Astronomy and Astrophysics. 19 pages with 17 colour figures - 39 if you count the sub-figures separately. The figures here have been bit-mapped with some loss of quality and beauty. The paper version in A&A will be in greyscale with the on-line version in colour. In the meantime the colour version can be obtained by following links at http://www.star.bris.ac.uk/mrwm . The 9MB PostScript is recommended if you have appropriate bandwidth or otherwise the 2.3MB PDF is usabl

CCD uvbyHbeta Photometry in Clusters: I. The Open Cluster Standard, IC 4651

CCD photometry of the intermediate-age open cluster, IC 4651, on the uvbyHbeta system is presented and analyzed. By using a combination of the information from the color-magnitude diagram (CMD) and the color-color diagrams, a sample of 98 highly probable main sequence cluster members with high photometric accuracy is isolated. From this sample, adopting the intrinsic color relation of Olsen (1988), E(b-y) = 0.062 +/- 0.003 and [Fe/H] = +0.077 +/- 0.012, where the errors quoted are the standard errors of the mean and refer to the internal errors alone. Use of the Nissen (1988) intrinsic color relation produces E(b-y) = 0.071 and [Fe/H] = +0.115. Adopting the lower reddening, a direct main-sequence fit to the Hyades with (m-M) = 3.33 leads to (m-M) = 10.15, while isochrones with convective overshoot and zeroed to the Hyades produce an age of 1.7 +/- 0.1 Gyr, with an excellent match to the morphology of the turnoff. The higher reddening produces (m-M) = 10.3 and an age lower by 0.1 Gyr. Comparison with the CMD of NGC 3680 shows that the two clusters have virtually identical morphology which, in combination with their similar compositions, produces identical ages. Coincidentally, the shifts in the CMD necessary to superpose the two clusters require that the apparent moduli of IC 4651 and NGC 3680 be the same, while E(b-y)(4651) = E(b-y)(3680) + 0.04.Comment: 28 pages and 3 tables, in latex, 11 postscript figures. Accepted for Astronomical Journa