arXiv subjects

I. S. Glass

Publications and source records attributed to I. S. Glass.

At least 19 recordsLinked to original sources

arXiv2014

Towards stellar effective temperatures and diameters at one per cent accuracy for future surveys

The apparent size of stars is a crucial benchmark for fundamental stellar properties such as effective temperatures, radii and surface gravities. While interferometric measurements of stellar angular diameters are the most direct method to gauge these, they are still limited to relatively nearby and bright stars, which are saturated in most of the modern photometric surveys. This dichotomy prevents us from safely extending well calibrated relations to the faint stars targeted in large spectroscopic and photometric surveys. Here, we alleviate this obstacle by presenting SAAO near-infrared JHK observations of 55 stars: 16 of them have interferometric angular diameters, and the rest are in common with the 2MASS (unsaturated) dataset, allowing us to tie the effective temperatures and angular diameters derived via the Infrared Flux Method to the interferometric scale. We extend the test to recent interferometric measurements of unsaturated 2MASS stars, including giants, and the metal-poor benchmark target HD122563. With a critical evaluation of the systematics involved, we conclude that a 1% accuracy in fundamental stellar parameters is usually within reach. Caution, however, must be used when indirectly testing a temperature scale via colour relations, as well as when assessing the reliability of interferometric measurements, especially at sub-milliarcsec level. As a result, rather different effective temperature scales can be compatible with a given subset of interferometric data. We highlight some caveats to be aware of in such a quest, and suggest a simple method to check against systematics in fundamental measurements. A new diagnostic combination seismic radii with astrometric distances is also presented.

astro-ph.SR↗

arXiv2009

Mid-IR period-magnitude relations for AGB stars

Asymptotic Giant Branch variables are found to obey period-luminosity relations in the mid-IR similar to those seen at K_S (2.14 microns), even at 24 microns where emission from circumstellar dust is expected to be dominant. Their loci in the M, logP diagrams are essentially the same for the LMC and for NGC6522 in spite of different ages and metallicities. There is no systematic trend of slope with wavelength. The offsets of the apparent magnitude vs. logP relations imply a difference between the two fields of 3.8 in distance modulus. The colours of the variables confirm that a principal period with log P > 1.75 is a necessary condition for detectable mass-loss. At the longest observed wavelength, 24 microns, many semi-regular variables have dust shells comparable in luminosity to those around Miras. There is a clear bifurcation in LMC colour-magnitude diagrams involving 24 micron magnitudes.

astro-ph.SR↗

arXiv2008

Interstellar Extinction and Long-Period Variables in the Galactic Center

We use the Spitzer IRAC catalogue of the Galactic Center (GC) point sources (Ramirez et al. 2008) and combine it with new isochrones (Marigo et al. 2008) to derive extinctions based on photometry of red giants and asymptotic giant branch (AGB) stars. This new extinction map extends to much higher values of Av than previoulsy available. Our new extinction map of the GC region covers 2.0 x 1.4 degree (280 x 200 pc at a distance of 8 kpc). We apply it to deredden the LPVs found by Glass et al. (2001) near the GC. We make period-magnitude diagrams and compare them to those from other regions of different metallicity. The Glass-LPVs follow well-defined period-luminosity relations (PL) in the IRAC filter bands at 3.6, 4.5, 5.8, and 8.0 micron. The period-luminosity relations are similar to those in the Large Magellanic Cloud, suggesting that the PL relation in the IRAC bands is universal. We use ISOGAL data to derive mass-loss rates and find for the Glass-LPV sample some correlation between mass-loss and pulsation period, as expected theoretically.The GC has an excess of high luminosity and long period LPVs compared to the Bulge, which supports previous suggestions that it contains a younger stellar population.

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arXiv2007

Semi-Regular Variables in the Solar Neighbourhood

Period-luminosity sequences have been shown to exist among the Semi-Regular Variables (SRVs) in the Magellanic Clouds (Wood et al, 1999), the Bulge of the Milky Way galaxy (Glass & Schultheis, 2003) and elsewhere. Using modern period and revised Hipparcos parallax data, this paper demonstrates that they also appear among the M-giant SRVs of the Solar Neighbourhood. Their distribution in the K, log P diagram resembles that of Bulge stars more closely than those in the Magellanic Clouds. The prevalence of mass-loss among local M-type SRVs and its dependence on period and spectral sub-type are also discussed. K -- [12], a measure of circumstellar dust emission, increases clearly with V amplitude, M giant sub-type and log P.

astro-ph↗

arXiv2006

SRVs in the Solar Neighbourhood

Period-luminosity sequences have been shown to exist among the Semi-Regular Variables (SRVs) of the Magellanic Clouds (Wood et al, 1999), the Bulge of the Milky Way galaxy (Glass & Schultheis, 2003) and elsewhere. It would clearly be useful to have absolute (trigonometric) calibrations of these relations. This paper investigates whether the sequences can be seen among the M-type giant SRVs of the solar neighbourhood. Mass loss phenomena among these stars and their dependence on period and spectral sub-type are also discussed.

astro-ph↗

arXiv2006

Recent star formation in the inner Galactic Bulge seen by ISOGAL. I - Classification of bright mid-IR sources in a test field

Context: The stellar populations in the central region of the Galaxy are poorly known because of the high visual extinction and very great source density in this direction. Aims: To use recent infrared surveys for studying the dusty stellar objects in this region. Methods: We analyse the content of a 20x20 arcmin^2 field centred at (l,b)=(-0.27,-0.06) observed at 7 and 15 microns as part of the ISOGAL survey. These ISO observations are more than an order of magnitude better in sensitivity and spatial resolution than the IRAS observations. The sources are cross-associated with other catalogues to identify various types of objects. We then derive criteria to distinguish young objects from post-main sequence stars. Results: We find that a sample of about 50 young stellar objects and ultra-compact HII regions emerges, out of a population of evolved AGB stars. We demonstrate that the sources colours and spatial extents, as they appear in the ISOGAL catalogue, possibly complemented with MSX photometry at 21 microns, can be used to determine whether the ISOGAL sources brighter than 300 mJy at 15 microns (or [15] < 4.5 mag) are young objects or late-type evolved stars.

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arXiv2004

Late-type giant variables in NGC6522, LMC and SMC: How do they differ?

Complete samples of 2MASS stars in 3 fields of differing metallicities (and possibly differing age distributions) have been extracted and cross-correlated with MACHO and ISO data to determine their variability and mass-loss properties. In each M_K histogram a fall-off in numbers is seen above the tip of the RGB. The luminosity of the tip increases with metallicity as does the average J-K at given M_K. The proportion of stars with high H-K increases at lower metallicities. This results from the increasing proportion of C stars. All 3 fields contain similar types of variables, mainly short-period, Mira-like and double-period (having long and short periods). The proportion of variable stars decreases at lower metallicities and the minimum period associated with a given amplitude gets longer. Various trends are seen in the K, log P diagrams of each field. The Magellanic Cloud fields largely resemble each other but the Bulge field is noticeably different. The stars that follow the 'A' K,log P relation in the Bulge hardly extend beyond the M giant tip and the other groups also appear truncated in M_K relative to their Magellanic Cloud counterparts. In the Clouds there are many stars with periods around 200-300d that follow the 'C' or Mira relation but have relatively small amplitudes. The mid-IR sample detected by ISOCAM during the Magellanic Cloud Mini-Survey appears to be reliable and complete for sources with M_K more luminous than -7 mag, i.e., for those close to the top of the AGB. The various colour-colour and colour-magnitude diagrams reflect the increasing dominance of C stars at low metallicity. Mira magnitude vs log P relations exist up to at least 7 microns. Mass-loss from longer-period and double-period SRVs occurs at similar rates in each field, despite differences in metallicity.

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arXiv2004

SiO Maser Survey of IRAS Sources in the Inner Galactic Disk

We have surveyed 401 color selected IRAS sources in the Galactic disk in the SiO J=1--0 v= 1 and 2 maser lines at 43 GHz, resulting in 254 (239 new) detections. The observed sources lie mostly in a strip of the inner Galactic disk with boundaries -10<l<40 deg and |b|<3 deg. This survey provides radial velocities of inner-disk stars for which optical measurements cannot be made due to interstellar extinction. The SiO $l$--$v$ diagram in the area $-10<l<40 deg exhibits fewer objects coincident with the molecular ring feature than the OH 1612 MHz source $l$--$v$ map does, indicating a slight difference of stellar type between SiO and OH emitting stars. After identifying all of the SiO detected sources in the 2MASS near-infrared catalog, we computed their luminosity distances based on the infrared fluxes. We then mapped these objects onto the first quadrant of the Galactic plane. Combining the distances with the SiO radial-velocities, we obtained a pattern speed for SiO maser sources, Omega_P=21 (+- 13) km s^{-1} kpc^{-1}, between the distances 1 and 5.5 kpc, without the use of any dynamical models. The increase of the pattern speed toward the Galactic center (up to 60 km s^{-1} kpc^{-1} between the distances, 5.5 and 7 kpc) suggests the presence of two pattern speeds in the Galaxy.

astro-ph↗

arXiv2004

SiO Maser Survey of the Large-Amplitude Variables in the Galactic Center

We have surveyed ~400 known large-amplitude variables within 15' of the galactic center in the SiO J=1--0 v=1 and 2 maser lines at 43 GHz, resulting in 179 detections. SiO lines were also detected from 16 other resulting in 180 detections. SiO lines were also detected from 16 other sources, which are located within 20" (the telescope half beamwidth) of the program objects. The detection rate of 48 percent is comparable to that obtained in Bulge IRAS source surveys. Among the SiO detections, five stars have radial velocities greater than 200 km/s. The SiO detection rate increases steeply with the period of light variation, particularly for stars with P>500 d, where it exceeds 80%. We found that, at a given period, the SiO detection rate is approximately three times that for OH. These facts suggest that the large-amplitude variables in the Nuclear Disk region are AGB stars similar in their overall properties to the inner and outer Bulge IRAS/SiO sources. From the set of radial velocity data, the mass distribution within 30 pc of the galactic center is derived by a new method which is based on the collisionless Boltzmann equation integrated along the line of sight. The mass within 30 pc is about 6.4 [\pm 0.7] \times 10^7 M_{\odot} and the mass of the central black hole is 2.7 [\pm 1.3] \times 10^6 M_{\odot}. Consideration of the line-of-sight velocity of each star and its potential energy leads to the conclusion that the five high-velocity stars come from galactocentric distances as high as 300 pc. The high-velocity subsample of stars with negative radial velocities exhibits a tendency to have brighter K magnitudes than the subsample of stars with positive velocities. The origin of these high-velocity stars is discussed.

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arXiv2004

Long-term IR Photometry of Seyferts

Long-term (up to 10000d) monitoring has been undertaken for 41 Seyferts in the near-IR (JHKL). All but 2 showed variability, with K ampl in the range <0.1 to > 1.1 mags. The timescale for detectable change is from about one week to a few years. A simple cross-correlation study shows evidence for delays of up to several hundred days between the variations seen at the shortest wavelengths and the longest in many galaxies. In particular, the data for F9 now extend to twice the interval covered earlier and the delay between its UV and IR outputs persists. An analysis of the fluxes shows that, for any given galaxy, the colours of the variable component are usually independent of the level of activity. The state of activity can be parameterized. Taken over the whole sample, the colours of the variable components fall within moderately narrowly defined ranges. In particular, the H-K colour is appropriate to a black body of temperature 1600K. The H-K excess for a heavily reddened nucleus can be determined and used to find E_{B-V}, which can be compared to the values found from the visible region broad line fluxes. Using flux-flux diagrams, the flux within the aperture from the underlying galaxy can often be determined without the need for model surface brightness profiles. In many galaxies it is apparent that here must be an additional constant contribution from warm dust.

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arXiv2003

Period-magnitude relations for M giants in Baade's Window NGC6522

A large and complete sample of stars with K < 9.75 in the NGC6522 Baade's Window is examined using light curves from MACHO and IJK from DENIS. All 4 of the sequences ABCD in the K vs logP diagram of the LMC are seen in the Bulge. The Bulge sequences however show some differences from the Magellanic Clouds. The sequences may be useful as distance indicators. A new diagram of the frequency of late-type variables is presented. The catalogued SR variables of the solar nbd are found to be a subset of the total of SRs, biased towards large amplitude.

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arXiv2002

Infrared stellar populations in the central parts of the Milky Way galaxy

Near- and mid-IR survey data from DENIS and ISOGAL are used to investigate the structure and formation history of the inner 10 degree (1.4 kpc) of the Milky Way galaxy. Synthetic bolometric corrections and extinction coefficients in the near- and mid-IR are derived for stars of different spectral types, to allow the transformation of theoretical isochrones into observable colour-magnitude diagrams. The observed IR colour-magnitude diagrams are used to derive the extinction, metallicity and age for individual stars. The inner galaxy is dominated by an old population (> 7 Gyr). In addition, an intermediate-age population (200 Myr to 7 Gyr) is detected, which is consistent with the presence of a few hundred Asymptotic Giant Branch stars with heavy mass loss. Furthermore, young stars (< 200 Myr) are found across the inner Bulge. The metallicities of these stellar population components are discussed. These results can be interpreted in terms of an early epoch of intense star formation and chemical enrichment which shaped the bulk of the Bulge and nucleus, and a more continuous star formation history which gradually shaped the disk from the accretion of sub-solar metallicity gas from the halo. A possible increase in star formation about 200 Myr ago might have been triggered by a minor merger. Ever since the formation of the first stars, mechanisms have been at play that mix the populations from the nucleus, Bulge and disk. Luminosity functions across the inner galactic plane indicate the presence of an inclined (bar) structure at > 1 kpc from the galactic centre, near the inner Lindblad resonance. The innermost part of the Bulge, within about 1 kpc from the galactic centre, seems azimuthally symmetric.

astro-ph↗

arXiv2002

M giants in Macho, DENIS and Isogal

A complete sample of 174 M giants classified by Blanco (1986) and later than subtype M0 in the NGC6522 Baade's Window clear field has been investigated to establish some general properties of cool Bulge stars. Photometric information has been obtained from the MACHO database to search for variability and, where possible, to determine periods. Near- and mid-IR magnitudes have been extracted from DENIS and ISOGAL. 46 semi-regular variables and 2 irregulars were found a mongst the 174. Many M5 and all stars M6 or later show variation, whereas earlier subtypes do not. The DENIS I-J and J-K_S colours and the luminosities of the M stars increase with M sub-class. K tends to increase with log P among M-type SR variables. Almost all the variables were detected at 7 microns during the ISOGAL programme. Excess radiation at 15 microns, indicative of heavy mass loss, is associated with very high luminosity and late spectral type. The limit of sensitivity of the ISOGAL survey was such that the non-variables were not detected. Four probable M stars not listed by Blanco (1986), two of which are semi-regular variables, were detected by ISOGAL. In the case of doubly-periodic SR variables, the longer periods have K mags which place them close to the D line of Wood (2000) in a K,log P diagram. The unusual MACHO light curve of one particular star, Blanco 26, shows the commencement of a long-period variation with an anomalously short and sharp event and appears to rule out a pulsational model for this phenomenon.

astro-ph↗

arXiv2002

Detections of SiO Masers from the Large-Amplitude Variables in the Galactic Nuclear Disk

We have surveyed known large-amplitude variables within 15' of the Galactic center in the SiO J=1-0 v=1 and 2 maser lines at 43 GHz, resulting in 79 detections and 58 non-detections. The detection rate of 58 percent is comparable to that obtained in Bulge IRAS source surveys. SiO lines were also detected from four other sources near the program objects. The SiO detection rate increases steeply with the period, particularly for stars with P>500 d, where it exceeds 80%. We found at a given period that the SiO detection rate is approximately double that for OH. These facts suggest that the large-amplitude variables in the Nuclear Disk region are AGB stars similar in their overall properties to the inner and outer bulge IRAS/SiO sources.

astro-ph↗

arXiv2001

AGB variables in Baade's windows

In this work, a sample of luminous M-type giants in the Baade's Windows towards the inner Galactic Bulge is investigated in the near-infrared. The ISOGAL survey at 7 and 15 micron has given information concerning the mass-loss rates of these stars and their variability characteristics have been extracted from the MACHO database. Most are known to be semi-regular variables (SRVs). Here we discuss how their IJHK-region colours depend on period and the presence or absence of mass-loss, using results mainly taken from the DENIS and 2MASS surveys. In order to compare their colours with solar neighbourhood stars, photometric colours on the DENIS, 2MASS and ESO photometric systems have been synthesized for objects in the spectrophotometric atlas of Lancon and Wood (2000). In addition, they have been used to predict the differences in colour indicies when stars with strong molecular bands are observed using different photometric systems. The SRVs are found to inhabit the upper end of the J-K, K colour-magnitude diagram, lying just below the Miras. High mass-loss rates are associated with high luminosity. The near-infrared colours of the semi-regular variables increase in a general way with period and are reddest for the stars with significant mass-loss. The average colours of Mira variables, whose periods start at around 200 days in the Bulge, are bluer than those of the semi-regulars at this period, particularly in J-H, thanks to the association of deep water-vapour bands with large amplitude.

astro-ph↗

arXiv2001

The Nature of the Nuclear H2O Masers of NGC 1068: Reverberation and Evidence for a Rotating Disk Geometry

We report new (1995) Very Large Array observations and (1984 - 1999) Effelsberg 100m monitoring observations of the 22 GHz H2O maser spectrum of the Seyfert 2 galaxy NGC 1068. The sensitive VLA observations provide a registration of the 22 GHz continuum emission and the location of the maser spots with an accuracy of ~ 5 mas. Within the monitoring data, we find evidence that the nuclear masers vary coherently on time-scales of months to years, much more rapidly than the dynamical time-scale. We argue that the nuclear masers are responding in reverberation to a central power source, presumably the central engine. Between October and November 1997, we detected a simultaneous flare of the blue-shifted and red-shifted satellite maser lines. Reverberation in a rotating disk naturally explains the simultaneous flaring. There is also evidence that near-infrared emission from dust grains associated with the maser disk also responds to the central engine. We present a model in which an X-ray flare results in both the loss of maser signal in 1990 and the peak of the near-infrared light curve in 1994. In support of a rotating disk geometry for the nuclear masers, we find no evidence for centripetal accelerations of the redshifted nuclear masers; the limits are +/- 0.006 km/s/year, implying that the masers are located within 2 degrees of the kinematic line-of-nodes. We also searched for high velocity maser emission like that observed in NGC 4258. In both VLA and Effelsberg spectra, we detect no high velocity lines between +/- 350 km/s to +/- 850 km/s relative to systemic, arguing that masers only lie outside a radius of ~ 0.6 pc (1.9 light years) from the central engine (assuming a distance of 14.4 Mpc).

astro-ph↗

arXiv2000

The 0.1-200 keV spectrum of the blazar PKS 2005-489 during an active state

The bright BL Lac object PKS 2005-489 was observed by BeppoSAX on November 1-2, 1998, following an active X-ray state detected by RossiXTE. The source, detected between 0.1 and 200 keV, was in a very high state with a continuum well fitted by a steepening spectrum due to synchrotron emission only. Our X-ray spectrum is the flattest ever observed for this source. The different X-ray spectral slopes and fluxes, as measured by various satellites, are consistent with relatively little changes of the peak frequency of the synchrotron emission, always located below 10^{17} Hz. We discuss these results in the framework of synchrotron self-Compton models. We found that for the BeppoSAX observation, the synchrotron peak frequency is between 10^{15} and 2.5x10^{16} Hz, depending on the model assumptions.

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arXiv1999

The blazar PKS 0528+134: new results from BeppoSAX observations

BeppoSAX observed 8 times the gamma-ray bright blazar PKS 0528+134 in Feb. and Mar. 1997, during a multiwavelength campaign involving EGRET, onboard CGRO, and ground based telescopes. The source was in its faintest X-ray state observed so far, with an unabsorbed [2-10] keV flux of 2.5E-12 erg/cm2/s. The spectrum can be fitted with a power law, with energy index alpha_X=0.49+-0.07 between 0.1 and 10 keV. The high energy instrument PDS detected a flux in the 15-100 keV band which appears disconnected from the lower X-ray energies, most likely due to a contaminating source. We consider our findings in the context of the overall spectral energy distribution and discuss their implications for synchrotron and inverse Compton models.

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