Colors Of Stars Research Articles

A near-infrared study of AGB and red giant stars in the Leo I dSph galaxy

A near-infrared imaging study of the evolved stellar populations in the dwarf spheroidal galaxy Leo I is presented. Based on JHK observations obtained with the WFCAM wide-field array at the UKIRT telescope, we build a near-infrared photometric catalogue of red giant branch (RGB) and asymptotic giant branch (AGB) stars in Leo I over a 13.5 arcmin square area. The V-K colours of RGB stars, obtained by combining the new data with existing optical observations, allow us to derive a distribution of global metallicity [M/H] with average [M/H] = -1.51 (uncorrected) or [M/H] = -1.24 +/- 0.05 (int) +/- 0.15 (syst) after correction for the mean age of Leo I stars. This is consistent with the results from spectroscopy once stellar ages are taken into account. Using a near-infrared two-colour diagram, we discriminate between carbon- and oxygen-rich AGB stars and obtain a clean separation from Milky Way foreground stars. We reveal a concentration of C-type AGB stars relative to the red giant stars in the inner region of the galaxy, which implies a radial gradient in the intermediate-age (1-3 Gyr) stellar populations. The numbers and luminosities of the observed carbon- and oxygen-rich AGB stars are compared with those predicted by evolutionary models including the thermally-pulsing AGB phase, to provide new constraints to the models for low-metallicity stars. We find an excess in the predicted number of C stars fainter than the RGB tip, associated to a paucity of brighter ones. The number of O-rich AGB stars is roughly consistent with the models, yet their predicted luminosity function is extended to brighter luminosity. It appears likely that the adopted evolutionary models overestimate the C star lifetime and underestimate their K-band luminosity.

Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis

We derive precise system parameters for the pre-cataclysmic binary, NN Ser. From light curve fitting we find an orbital inclination of i = 89.6 +/- 0.2 deg. From the HeII absorption line we find K_{WD}= 62.3 +/- 1.9 km/s. The irradiation-induced emission lines from the surface of the secondary star give a range of observed radial velocities. The corrected values give a radial velocity of K_{sec}= 301 +/- 3 km/s, with an error dominated by the systematic effects of the model. This leads to a binary separation of a = 0.934 +/- 0.009 R_{sun}, radii of R_{WD} = 0.0211 +/- 0.0002 R_{sun} and R_{sec} = 0.149 +/- 0.002 R_{sun} and masses of M_{WD} = 0.535 +/- 0.012 M_{sun} and M_{sec} = 0.111 +/- 0.004 M_{sun}. The masses and radii of both components of NN Ser were measured independently of any mass-radius relation. For the white dwarf, the measured mass, radius and temperature show excellent agreement with a `thick' hydrogen layer of fractional mass M_{H}/{M}_{WD} = 10^{-4}. The measured radius of the secondary star is 10% larger than predicted by models, however, correcting for irradiation accounts for most of this inconsistency, hence the secondary star in NN Ser is one of the first precisely measured very low mass objects to show good agreement with models. ULTRACAM r', i' and z' photometry taken during the primary eclipse determines the colours of the secondary star as (r'-i')_{sec}= 1.4 +/- 0.1 and (i'-z')_{sec} = 0.8 +/- 0.1 which corresponds to a spectral type of M4 +/- 0.5. This is consistent with the derived mass, demonstrating that there is no detectable heating of the unirradiated face, despite intercepting radiative energy from the white dwarf which exceeds its own luminosity by over a factor of 20.

PPG launches CRRC, Energy Star color registration initiative

PPG launches CRRC, Energy Star color registration initiative

Star Coloring and Acyclic Coloring of Locally Planar Graphs

It is proved that every graph embedded in a fixed surface with sufficiently large edge-width is acyclically 7-colorable and that its star chromatic number is at most $2s_0^*+3$, where $s_0^*\leq20$ is the maximum star chromatic number for the class of all planar graphs.

A NEW CLASS OF LUMINOUS TRANSIENTS AND A FIRST CENSUS OF THEIR MASSIVE STELLAR PROGENITORS

The progenitors of SN 2008S and the 2008 luminous transient in NGC 300 were deeply dust-enshrouded massive stars, with extremely red mid-infrared (MIR) colors and relatively low bolometric luminosities (≈5 × 104 L☉). The transients were optically faint compared to normal core-collapse supernovae (ccSNe), with peak absolute visual magnitudes of −13 ≳ MV ≳ −15, and their spectra exhibit narrow Balmer and [Ca ii] emission lines. These events are unique among transient–progenitor pairs and hence constitute a new class. Additional members of this class may include the M85 transient, SN 1999bw, 2002bu, and others. Whether they are true supernovae or bright massive-star eruptions, we argue that their rate is of order ∼20% of the ccSN rate in star-forming galaxies. This fact is remarkable in light of the observation that a very small fraction of all massive stars in any one galaxy, at any moment, have the infrared colors of the progenitors of SN 2008S and the NGC 300 transient. We show this by extracting MIR and optical luminosity, color, and variability properties of massive stars in M33 using archival imaging. We find that the fraction of massive stars with colors consistent with the progenitors of SN 2008S and the NGC 300 transient is ≲10−4. In fact, only ≲10 similar objects exist in M33 (and perhaps ≲1)—all of which lie at the luminous red extremum of the asymptotic giant branch sequence. That these transients are simultaneously relatively common with respect to supernovae, while their progenitors are remarkably rare compared to the massive star population, implies that the dust-enshrouded phase is a short-lived phenomenon in the lives of many massive stars. This shrouded epoch can occur only in the last ≲104 yr before explosion, be it death or merely eruption. We discuss the implications of this finding for the evolution and census of "low-mass" massive stars (i.e., ∼8–12 M☉), and we connect it with theoretical discussions of electron-capture supernovae (ecSNe) near this mass range. Other potential mechanisms, including the explosive birth of massive white dwarfs and massive star outbursts, are also discussed. A systematic census with (warm) Spitzer of galaxies in the local universe (D ≲ 10 Mpc) for analogous progenitors would significantly improve our knowledge of this channel to massive stellar explosions, and potentially to others with obscured progenitors.

A silhouette envelope around GGD30IR detected bySpitzer

This paper presents a study of the envelope of the young stellar object (YSO) GGD30IR. What distinguishes this from most other YSOs is the elongated absorption feature seen in silhouette against the background emission in the Spitzer Galactic Legacy Infrared Midplane Survey Extraordinaire 8 μm Infrared Array Camera image of the region. The size and the symmetrical placement of GGD30IR in the centre of this feature suggest that it is an extended envelope, perhaps the remnant of the collapse of the GGD30 core. We have used the extinction in the envelope measured from (i) the reduction in the 8 μm background intensity and (ii) field star colour excesses, to estimate the envelope mass, obtaining values of 0.6 ± 0.2 and 0.5 ± 0.3 M ⊙ , respectively. To investigate the envelope further, we have obtained Australia Telescope Compact Array 3 mm continuum and HCO+ line observations of the region. The continuum emission at 3 mm arises from both a compact (unresolved; ≤730 au) core embedded in an extended envelope ∼18 000 au x 38 000 au in extent. We estimate the core mass to be 0.11 ± 0.02 M ⊙ . The HCO+ emission is extended in a direction perpendicular to the long axis of the envelope, suggesting it comes from an outflow. The spectral energy distribution (SED) provides a 2-24 μm spectral index, α = 1.0, which places GGD30IR in the Class I YSO category. Integrating the SED provides a luminosity of L * ≃ 25 ± 5 L ⊙ .

The UV-Excess survey of the northern Galactic plane

The UV-Excess Survey of the Northern Galactic Plane images a 10x185 degree wide band, centered on the Galactic Equator using the 2.5m Isaac Newton Telescope in four bands (U,g,r,HeI5875) down to ~21st-22nd magnitude (~20th in HeI5875). The setup and data reduction procedures are described. Simulations of the colours of main-sequence stars, giant, supergiants, DA and DB white dwarfs and AM CVn stars are made, including the effects of reddening. A first look at the data of the survey (currently 30% complete) is given.

SPITZERSAGE INFRARED PHOTOMETRY OF MASSIVE STARS IN THE LARGE MAGELLANIC CLOUD

We present a catalog of 1750 massive stars in the Large Magellanic Cloud (LMC), with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 1268 of these stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE survey database, for which we present uniform photometry from 0.3 to 24 μm in the UBVIJHKs +IRAC+MIPS24 bands. The resulting infrared color-magnitude diagrams illustrate that the supergiant B[e], red supergiant, and luminous blue variable (LBV) stars are among the brightest infrared point sources in the LMC, due to their intrinsic brightness, and at longer wavelengths, due to dust. We detect infrared excesses due to free-free emission among ~900 OB stars, which correlate with luminosity class. We confirm the presence of dust around 10 supergiant B[e] stars, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs. The similar luminosities of B[e] supergiants (log L/L ☉ ≥ 4) and the rare, dusty progenitors of the new class of optical transients (e.g., SN 2008S and NGC 300 OT), plus the fact that dust is present in both types of objects, suggests a common origin for them. We find the infrared colors for Wolf-Rayet stars to be independent of spectral type and their SEDs to be flatter than what models predict. The results of this study provide the first comprehensive roadmap for interpreting luminous, massive, resolved stellar populations in nearby galaxies at infrared wavelengths.

Uncovering strong MgII absorbing galaxies

Context. The nature of the galaxies that give rise to absorption lines , such as damped Lyman-α systems (DLAs) or strong Mgii lines, in quasar spectra is diffi cult to investigate in emission. These galaxies can be very faint and located close to the lines of sight of the much brighter background quasars. Aims. Taking advantage of the total absorption of the QSO light bluewards of the Lyman limit of two DLAs at z> 3.4, we look for the continuum emission from intervening galaxies at z≈ 2 that are identified via strong metal absorption lines. The M gii absorbers have equivalent width large enough to be potential DLA systems. Methods. Deep images are obtained with the FOcal Reducer and Spectrograph (FORS1) on the Very Large Telescope for the fields towards SDSS J110855+120953 and SDSS J140850+020522. These quasars have Mgii absorption lines at z = 1.87 (Wr(Mgii) = 2.46 A) and z = 1.98 (Wr(Mgii) = 1.89 A), respectively, and each QSO has two intervening higher redshift DLAs at z> 3. The U and R bands of FORS1 lie blue and redwards of the Lyman limit of the background DLAs, allowing us to search for emission from the foreground galaxies directly along the lines of sight to the QSOs. Results. No galaxies are found close to the sight line of the QSO to a point source limit of UAB ∼ 28.0. In both fields, the closest objects lie at an impact parameter of∼5 ′′ corresponding to∼40 kpc in projection at z = 2, and have typical colours of star forming galaxies at that redshift. However, the currently availabl e data do not allow us to confirm if the galaxies lie at the same r edshifts as the absorption systems. A more extended structure is visible in the SDSS J14085+020522 field at an impact parameter of 0. ′′ 8 or 7 kpc. If these objects are at z≈ 2 their luminosities are 0.03‐0.04 L ∗ in both fields. The star formation rates estimated from the UV flux are 0.5‐0.6 M⊙ yr −1 , while the SFRs are half these values if the U band flux is due to Lyα emission alone. Conclusions. The non-detection of galaxies near to the line of sight is most likely explained by low metallicities and luminosities of the Mgii galaxies. Alternatively, the Mgii clouds are part of extended halos or in outflows from low-meta llicity galaxies.

GALAXY CLUSTERS IN THE IRAC DARK FIELD. II. MID-INFRARED SOURCES

We present infrared (IR) luminosities, star formation rates (SFR), colors, morphologies, locations, and active galactic nuclei (AGNs) properties of 24 μm detected sources in photometrically detected high-redshift clusters in order to understand the impact of environment on star formation (SF) and AGN evolution in cluster galaxies. We use three newly identified z = 1 clusters selected from the IRAC dark field; the deepest ever mid-IR survey with accompanying, 14 band multiwavelength data including deep Hubble Space Telescope imaging and deep wide-area Spitzer MIPS 24 μm imaging. We find 90 cluster members with MIPS detections within two virial radii of the cluster centers, of which 17 appear to have spectral energy distributions dominated by AGNs and the rest dominated by SF. We find that 43% of the star-forming sample have IR luminosities LIR > 1011 L☉ (luminous IR galaxies). The majority of sources (81%) are spirals or irregulars. A large fraction (at least 25%) show obvious signs of interactions. The MIPS-detected member galaxies have varied spatial distributions as compared to the MIPS-undetected members with one of the three clusters showing SF galaxies being preferentially located on the cluster outskirts, while the other two clusters show no such trend. Both the AGN fraction and the summed SFR of cluster galaxies increase from redshift zero to one, at a rate that is a few times faster in clusters than over the same redshift range in the field. Cluster environment does have an effect on the evolution of both AGN fraction and SFR from redshift one to the present, but does not affect the IR luminosities or morphologies of the MIPS sample. SF happens in the same way regardless of environment making MIPS sources look the same in the cluster and field, however the cluster environment does encourage a more rapid evolution with time as compared to the field.

Evolution and colors of helium-core white dwarf stars with high-metallicity progenitors

Motivated by the recent detection of single and binary He-core white dwarfs in metal-rich clusters, we present a full set of evolutionary calculations and colors appropriate for the study of such white dwarfs. The paper is also aimed at investigating whether stable hydrogen burning may constitute a main source of energy for massive He-core white dwarfs resulting from high-metallicity progenitors. White dwarf sequences are derived by taking into account the evolutionary history of progenitor stars with supersolar metallicities. We also incorporate a self-consistent, time-dependent treatment of gravitational settling and chemical diffusion, as well as of the residual nuclear burning. We find that the influence of residual nuclear burning during the late stages of white dwarf evolution is strongly dependent on the occurrence of chemical diffusion at the base of the hydrogen-rich envelope. When no diffusion is considered, residual hydrogen burning strongly influences the advanced stages of white dwarf cooling, introducing evolutionary delays of several Gyr. By contrast, when diffusion is taken into account the role of residual nuclear burning is strongly mitigated, and the evolution is dictated only by the thermal content stored in the ions. In addition, for all of our sequences, we provide accurate color and magnitudes on the basis of new and improved non gray model atmospheres which explicitly include Ly$\alpha$ quasi-molecular opacity.

Star coloring planar graphs from small lists

AbstractA star coloring of a graph is a proper vertex‐coloring such that no path on four vertices is 2‐colored. We prove that the vertices of every planar graph of girth 6 (respectively 7, 8) can be star colored from lists of size 8 (respectively 7, 6). We give an example of a planar graph of girth 5 that requires 6 colors to star color. © 2009 Wiley Periodicals, Inc. J Graph Theory 63: 324–337, 2010

STELLAR LOCUS REGRESSION: ACCURATE COLOR CALIBRATION AND THE REAL-TIME DETERMINATION OF GALAXY CLUSTER PHOTOMETRIC REDSHIFTS

We present stellar locus regression (SLR), a method of directly adjusting the instrumental broadband optical colors of stars to bring them into accord with a universal stellar color–color locus, producing accurately calibrated colors for both stars and galaxies. This is achieved without first establishing individual zero points for each passband, and can be performed in real-time at the telescope. We demonstrate how SLR naturally makes one wholesale correction for differences in instrumental response, for atmospheric transparency, for atmospheric extinction, and for Galactic extinction. We perform an example SLR treatment of Sloan Digital Sky Survey data over a wide range of Galactic dust values and independently recover the direction and magnitude of the canonical Galactic reddening vector with 14–18 mmag rms uncertainties. We then isolate the effect of atmospheric extinction, showing that SLR accounts for this and returns precise colors over a wide range of air mass, with 5–14 mmag rms residuals. We demonstrate that SLR-corrected colors are sufficiently accurate to allow photometric redshift estimates for galaxy clusters (using red sequence galaxies) with an uncertainty σ(z)/(1 + z) = 0.6% per cluster for redshifts 0.09 < z < 0.25. Finally, we identify our objects in the 2MASS all-sky catalog, and produce i-band zero points typically accurate to 18 mmag using only SLR. We offer open-source access to our IDL routines, validated and verified for the implementation of this technique, at http://stellar-locus-regression.googlecode.com.

Efficient Computation of Sparse Hessians Using Coloring and Automatic Differentiation

The computation of a sparse Hessian matrix H using automatic differentiation (AD) can be made efficient using the following four-step procedure: (1) Determine the sparsity structure of H, (2) obtain a seed matrix S that defines a column partition of H using a specialized coloring on the adjacency graph of H, (3) compute the compressed Hessian matrix B ≡ HS, and (4) recover the numerical values of the entries of H from B. The coloring variant used in the second step depends on whether the recovery in the fourth step is direct or indirect: a direct method uses star coloring and an indirect method uses acyclic coloring. In an earlier work, we had designed and implemented effective heuristic algorithms for these two NP-hard coloring problems. Recently, we integrated part of the developed software with the AD tool ADOL-C, which has recently acquired a sparsity detection capability. In this paper, we provide a detailed description and analysis of the recovery algorithms and experimentally demonstrate the efficacy of the coloring techniques in the overall process of computing the Hessian of a given function using ADOL-C as an example of an AD tool. We also present new analytical results on star and acyclic coloring of chordal graphs. The experimental results show that sparsity exploitation via coloring yields enormous savings in runtime and makes the computation of Hessians of very large size feasible. The results also show that evaluating a Hessian via an indirect method is often faster than a direct evaluation. This speedup is achieved without compromising numerical accuracy.

THE ABUNDANCE GRADIENT IN THE EXTREMELY FAINT OUTER DISK OF NGC 300

In an earlier work, we showed for the first time that the resolved stellar disk of NGC 300 is very extended with no evidence for truncation, a phenomenon that has since been observed in other disk galaxies. We revisit the outer disk of NGC 300 in order to determine the metallicity of the faint stellar population. With the Gemini Multi Object Spectrograph camera at Gemini South, we reach 50% completeness at (g', i') = (26.8–27.4, 26.1–27.0) in photometric conditions and 07 seeing. At these faint depths, careful consideration must be given to the background galaxy population. The mean colors of the outer disk stars fall within the spread of colors for the background galaxies, but the stellar density dominates the background galaxies by ∼2:1. The predominantly old stellar population in the outer disk exhibits a negative abundance gradient—as expected from models of galaxy evolution—out to about 10 kpc where the abundance trend changes sign. We present two scenarios to explain the flattening, or upturn, in the metallicity gradient of NGC 300 and discuss the implication this has for the broader picture of galaxy formation.

Star coloring of sparse graphs

AbstractA proper coloring of the vertices of a graph is called a star coloring if the union of every two color classes induces a star forest. The star chromatic number χs(G) is the smallest number of colors required to obtain a star coloring of G. In this paper, we study the relationship between the star chromatic number χs(G) and the maximum average degree Mad(G) of a graph G. We prove that: If G is a graph with , then χs(G)≤4. If G is a graph with and girth at least 6, then χs(G)≤5. If G is a graph with and girth at least 6, then χs(G)≤6. These results are obtained by proving that such graphs admit a particular decomposition into a forest and some independent sets. © 2009 Wiley Periodicals, Inc. J Graph Theory 62: 201–219, 2009

MMT HYPERVELOCITY STAR SURVEY

We describe a new survey for unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their most likely origin. We investigate the possible contribution of unbound runaway stars, and show that the physical properties of binaries constrain low mass runaways to bound velocities. We measure radial velocities for HVS candidates with the colors of early A-type and late B-type stars. We report the discovery of 6 unbound HVSs with velocities and distances exceeding the conservative escape velocity estimate of Kenyon and collaborators. We additionally report 4 possibly unbound HVSs with velocities and distances exceeding the lower escape velocity estimate of Xue and collaborators. These discoveries increase the number of known HVSs by 60%-100%. Other survey objects include 19 newly identified z~2.4 quasars. One of the HVSs may be a horizontal branch star, consistent with the number of evolved HVSs predicted by Galactic center ejection models. Finding more evolved HVSs will one day allow a probe of the low-mass regime of HVSs and will constrain the mass function of stars in the Galactic center.

Structure and Star Formation in Galaxies out toz= 3: Evidence for Surface Density Dependent Evolution and Upsizing

We present an analysis of galaxies in the CDF-South. We find a tight relation to -->z = 3 between color and size at a given mass, with red galaxies being small, and blue galaxies being large. We show that the relation is driven by stellar surface density or inferred velocity dispersion: galaxies with high surface density are red and have low specific star formation rates, and galaxies with low surface density are blue and have high specific star formation rates. Surface density and inferred velocity dispersion are better correlated with specific star formation rate and color than stellar mass. Hence stellar mass by itself is not a good predictor of the star formation history of galaxies. In general, galaxies at a given surface density have higher specific star formation rates at higher redshift. Specifically, galaxies with a surface density of -->(1?3) ? 109 M? kpc?2 are red and dead at low redshift, approximately 50% are forming stars at -->z = 1, and almost all are forming stars by -->z = 2. This provides direct additional evidence for the late evolution of galaxies onto the red sequence. The sizes of galaxies at a given mass evolve like -->1/(1 + z)0.59 ? 0.10. Hence galaxies undergo significant upsizing in their history. The size evolution is fastest for the highest mass galaxies and quiescent galaxies. The persistence of the structural relations from -->z = 0 to -->z = 2.5, and the upsizing of galaxies imply that a relation analogous to the Hubble sequence exists out to -->z = 2.5, and possibly beyond. The star-forming galaxies at -->z ? 1.5 are quite different from star-forming galaxies at -->z = 0, as they have likely very high gas fractions, and star formation timescales comparable to the orbital time.

Assessment of the near infrared identification of carbon stars

Context. Thousands of C stars have been identified in Local Group galaxies using narrow band photometry. To survey C stars at larger distances and alternative to the narrow-band approach is needed. Aims. We obtain, from ESO Archive data, NIR magnitudes and colours of previously known C stars in two dwarf irregular Local Group galaxies, namely IC 1613 and NGC 3109.Methods. We compare the NIR magnitudes and colours of the C star populations, previously identified from narrow band techniques to estimate the star formation history of their intermediate-age populations.Results. We demonstrate that the photometric properties of C stars show a wider range in the NIR than in the optical. As confirmed by models we see, among the four galaxies investigated with a 0.7 dex metallicity range, little if any metallicity effect in the C star colour distribution.

NTT follow-up observations of star cluster candidates from the FSR catalogue

We are conducting a large program to classify newly discovered Milky Way star cluster candidates from the list of Froebrich et al. Here, we present deep near-infrared (NIR) follow-up observations from European Southern Observatory–New Technology Telescope (ESO/NTT) of 14 star cluster candidates. We show that the combined analysis of star density maps (SDMs) and colour–colour/magnitude diagrams derived from deep NIR imaging is a viable tool to reliably classify new stellar clusters. This allowed us to identify two young clusters with massive stars, three intermediate age open clusters and two globular cluster candidates among our targets. The remaining seven objects are unlikely to be stellar clusters. Among them is the object FSR 1767 which has previously been identified as a globular cluster using Two-Micron All-Sky Survey (2MASS) data by Bonatto et al. Our new analysis shows that FSR 1767 is not a star cluster. We also summarize the currently available follow-up analysis of the FSR candidates and conclude that this catalogue may contain a large number of new stellar clusters, probably dominated by old open clusters.