arXiv subjects

P. Abraham

Publications and source records attributed to P. Abraham.

31 records · Page 2Linked to original sources

arXiv2007

The impact of main belt asteroids on infrared--submillimetre photometry and source counts

<<>> Among the components of the infrared and submillimetre sky background, the closest layer is the thermal emission of dust particles and minor bodies in the Solar System. This contribution is especially important for current and future infrared and submillimetre space instruments --like those of Spitzer, Akari and Herschel -- and must be characterised by a reliable statistical model. <<>> We describe the impact of the thermal emission of main belt asteroids on the 5...1000um photometry and source counts, for the current and future spaceborne and ground-based instruments, in general, as well as for specific dates and sky positions. <<>> We used the statistical asteroid model (SAM) to calculate the positions of main belt asteroids down to a size of 1km, and calculated their infrared and submillimetre brightness using the standard thermal model. Fluctuation powers, confusion noise values and number counts were derived from the fluxes of individual asteroids. <<>> We have constructed a large database of infrared and submillimetre fluxes for SAM asteroids with a temporal resolution of 5 days, covering the time span January 1, 2000 -- December 31, 2012. Asteroid fluctuation powers and number counts derived from this database can be obtained for a specific observation setup via our public web-interface. <<>> Current space instruments working in the mid-infrared regime (Akari and Spitzer Space Telescopes) are affected by asteroid confusion noise in some specific areas of the sky, while the photometry of space infrared and submillimetre instruments in the near future (e.g. Herschel and Planck Space Observatories) will not be affected by asteroids. Faint main belt asteroids might also be responsible for most of the zodiacal emission fluctuations near the ecliptic.

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arXiv2007

The 2004--2006 outburst and environment of V1647 Ori

(Abridged) We studied the brightness and spectral evolution of the young eruptive star V1647 Ori during its recent outburst in the period 2004 February - 2006 Sep. We performed a photometric follow-up in the bands V, R_C, I_C, J, H, K_s as well as visible and near-IR spectroscopy. The main results are as follows: The brightness of V1647 Ori stayed more than 4 mag above the pre-outburst level until 2005 October when it started a rapid fading. In the high state we found a periodic component in the optical light curves with a period of 56 days. The delay between variations of the star and variations in the brightness of clump of nearby nebulosity corresponds to an angle of 61+/-14 degrees between the axis of the nebula and the line of sight. A steady decrease of HI emission line fluxes could be observed. In 2006 May, in the quiescent phase, the HeI 1.083 line was observed in emission, contrary to its deep blueshifted absorption observed during the outburst. The J-H and H-K_s color maps of the infrared nebula reveal an envelope around the star. The color distribution of the infrared nebula suggests reddening of the scattered light inside a thick circumstellar disk. We show that the observed properties of V1647 Ori could be interpreted in the framework of the thermal instability models of Bell et al. (1995). V1647 Ori might belong to a new class of young eruptive stars, defined by relatively short timescales, recurrent outbursts, modest increase in bolometric luminosity and accretion rate, and an evolutionary state earlier than that of typical EXors.

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arXiv2006

Constraints on the nature of dust particles by infrared observations

The far-infrared (FIR) emissivity of dust is an important parameter characterizing the physical properties of the grains. With the availability of stellar databases and far-infrared data from Infrared Space Observatory (ISO) it is possible to compare the optical and infrared properties of dust, and derive the far-infrared emissivity with respect to the optical extinction. In this paper we present the results of a systematic analysis of the FIR emissivity of interstellar clouds observed with ISOPHOT (the photometer onboard ISO) at least at two infrared wavelengths, one close to ~100um and one at 200um. We constructed FIR emission maps, determined dust temperatures, created extinction maps using 2MASS survey data, and calculated far-infrared emissivity for each of these clouds. We present the largest homogeneously reduced database constructed so far for this purpose. During the data analysis special care was taken on possible systematic errors. We find that far-infrared emissivity has a clear dependence on temperature. The emissivity is enhanced by a factor of usually less than 2 in the low dust temperature regime of 12K<=T_d<=14K. This result suggests larger grain sizes in those regions. However, the emissivity increase of typically below 2 restricts the possible grain growth processes to ice-mantle formation and coagulation of silicate grains, and excludes the coagulation of carbonaceous particles on the scales of the regions we investigated. In the temperature range 14K<=T_d<=16K a systematic decrease of emissivity is observed with respect to the values of the diffuse interstellar matter. Possible scenarios for this behaviour are discussed in the paper.

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arXiv2006

Filling the Infrared Gap: ISO Observations of 1 Jy BL Lacertae Objects

The large majority of BL Lacertae objects belonging to the 1 Jy sample, the class prototype for radio-selected sources, are thought to emit most of their synchrotron power in the far IR band. Ironically, this spectral region is very sparsely sampled, with only a minority of the objects having IRAS data (most of them being upper limits or low-quality detections). We aim at filling this IR gap by presenting new, simultaneous ISOCAM and ISOPHOT observations over the 7 - 200 micron range for half the sample. A measurement of the position of the synchrotron peak frequency, nu_peak, can provide information about particle acceleration mechanisms and constrain the inverse Compton radiation that will be detected by up-coming new gamma-ray missions. We have observed 17 1 Jy BL Lacertae objects with the camera and the photometer on board the Infrared Space Observatory (ISO) satellite. The ISOPHOT data reduction was done employing a novel correction, which mitigates the effect of chopping for faint sources. Using our new ISO data, complemented by nearly-simultaneous radio and optical observations for 10 and 4 objects respectively, and other multi-frequency data, we have built the spectral energy distributions of our sources (plus a previously published one) and derived the rest-frame nu_peak. Its distribution is centered at 10^13 Hz (30 micron) and is very narrow, with 60% of the BL Lacs in the 1 - 3 10^13 Hz range. Given our set of simultaneous infrared data, these represent the best determinations available of the synchrotron peak frequencies for low-energy peaked BL Lacs. A comparison with previous such estimates, based on non-simultaneous optical and near IR data, may indicate strong nu_peak variations in a number of sources, possibly associated with large flares as observed in the high-energy peaked BL Lac MKN 501. (abridged)

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arXiv2006

Nearby debris disk systems with high fractional luminosity reconsidered

By searching the IRAS and ISO databases we compiled a list of 60 debris disks which exhibit the highest fractional luminosity values (fd>10^-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special care was taken to exclude bogus disks from the sample. We computed the fractional luminosity values using available IRAS, ISO, and Spitzer data, and analysed the galactic space velocities of the objects. The results revealed that stars with disks of high fractional luminosity often belong to young stellar kinematic groups, providing an opportunity to obtain improved age estimates for these systems. We found that practically all disks with fd>5x10^-4 are younger than 100Myr. The distribution of the disks in the fractional luminosity versus age diagram indicates that (1) the number of old systems with high fd is lower than was claimed before; (2) there exist many relatively young disks of moderate fractional luminosity; and (3) comparing the observations with a current theoretical model of debris disk evolution a general good agreement could be found.

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arXiv2006

First AU-scale observations of V1647 Ori with VLTI/MIDI

The young eruptive star V1647 Ori was observed with MIDI, the mid-infrared interferometric instrument at the Very Large Telescope Interferometer (VLTI), on March 2, 2005. We present the first spectrally resolved interferometric visibility points for this object. Our results show that (1) the mid-infrared emitting region is extended, having a size of ~ 7 AU at 10 um; (2) no signatures of a close companion can be seen; (3) the 8-13 um spectrum exhibits no obvious spectral features. Comparison with similar observations of Herbig Ae stars suggests that V1647 Ori probably possesses a disk of moderate flaring. A simple disk model with T ~ r^{-0.53}, Σ~ r^{-1.5}, M_d = 0.05 M_Sun is able to fit both the spectral energy distribution and the observed visibility values simultaneously.

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arXiv2005

SS 433 : a phenomenon imitating a Wolf-Rayet star

We present mid-infrared (2-12 micron) spectra of the microquasar SS 433 obtained with the Infrared Space Observatory (spectroscopic mode of ISOPHOT and ISOCAM). We compare them to the spectra of four Wolf-Rayet stars: WR78, WR134, WR136, and WR147 in the same wavelength range. The mid-infrared spectrum of SS 433 mainly shows HI and HeI emission lines and is very similar to the spectrum of WR147, a WN8(h)+B0.5V binary. The 2-12 micron continuum emission of SS 433 corresponds to optically thin and partially optically thick free-free emission, from which we calculate a mass loss rate of 2-3 x 10^{-4} Msun/yr if the wind is homogeneous and a third of these values if it is clumped. This is consistent with a strong stellar wind from a WN star. However, following recent studies concluding that the mass donor star of SS 433 is not a Wolf-Rayet star, we propose that this strong wind out flows from a geometrically thick envelope of material that surrounds the compact object like a stellar atmosphere, imitating the Wolf-Rayet phenomenon. This wind could also wrap the mass donor star, and at larger distances (~ 40 AU), it might form a dust envelope from which the thermal emission, detected with ISOPHOT at 25 micron and 60 micron, would originate. This wind also probably feeds the material that is ejected in the orbital plane of the binary system and that forms the equatorial outflow detected in radio at distances > 100 AU.

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arXiv2005

The rapid fading of V1647 Orionis: the sudden end of a FUor-type eruption?

V1647 Ori is a young eruptive star, which went into outburst at the end of 2003. Since then, the object is gradually fading. In October 2005, however, V1647 Ori started a very rapid fading, approximately 1 mag/month in I_C as opposed to 0.1 mag/month previously. According to the colour changes, the rapid fading is not caused by increasing extinction, which suggests that the observed brightness changes are intrinsic. Supposing that the fading rate remains unchanged, the star will return to the pre-outburst state by mid-December 2005.

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arXiv2005

The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004

We present results from the most recent (2002-2004) observing campaigns of the eclipsing system KH 15D, in addition to re-reduced data obtained at Van Vleck Observatory (VVO) between 1995 and 2000. Phasing nine years of photometric data shows substantial evolution in the width and depth of the eclipses. The most recent data indicate that the eclipses are now approximately 24 days in length, or half the orbital period. These results are interpreted and discussed in the context of the recent models for this system put forward by Winn et al. and Chiang & Murray-Clay. A periodogram of the entire data set yields a highly significant peak at 48.37 +/- 0.01 days, which is in accord with the spectroscopic period of 48.38 +/- 0.01 days determined by Johnson et al. Another significant peak, at 9.6 days, was found in the periodogram of the out-of-eclipse data at two different epochs. We interpret this as the rotation period of the visible star and argue that it may be tidally locked in pseudosynchronism with its orbital motion. If so, application of Hut's theory implies that the eccentricity of the orbit is e = 0.65 +/- 0.01. Analysis of the UVES/VLT spectra obtained by Hamilton et al. shows that the v sin(i) of the visible star in this system is 6.9 +/- 0.3 km/sec. Using this value of v sin(i) and the measured rotation period of the star, we calculate the lower limit on the radius to be R = (1.3 +/- 0.1), R_Sun, which concurs with the value obtained by Hamilton et al. from its luminosity and effective temperature. Here we assume that i = 90 degrees since it is likely that the spin and orbital angular momenta vectors are nearly aligned.

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arXiv2004

SS 433: a WR X-ray binary or a WR-type phenomenon ?

We present mid-infrared spectra of the microquasar SS 433 obtained with the Infrared Space Observatory (spectroscopic mode of ISOPHOT) and compare them to the spectra of four Wolf-Rayet stars. The mid-infrared spectrum of SS 433 shows mainly HI and HeI emission lines and is very similar to the spectrum of WR 147, a WN8(h)+B0.5V binary with a colliding wind. The 2-12 micron continuum emission corresponds to optically thin and partially optically thick free-free emission from which we calculate a mass loss rate of 1.4-2.2x10^{-4} M_sun/yr if the wind is homogeneous and a third of these values if it is clumped, which is consistent with a strong WN stellar wind. We propose that this strong wind outflows from a geometrically thick envelope of material surrounding the compact object like a stellar atmosphere, imitating the Wolf-Rayet phenomenon.

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arXiv2002

Small-scale structure of the galactic cirrus emission

We examined the Fourier power spectrum characteristics of cirrus structures in 13 sky fields with faint to bright cirrus emission observed with ISOPHOT in the 90--200µm wavelength range in order to study variations of the spectral index $α$. We found that $α$ varies from field to field with --5.3 $\le$ $α$ $\le$ --2.1. It depends on the absolute surface brightness and on the hydrogen column density. We also found different spectral indices for the same sky region at different wavelengths. Longer wavelength measurements show steeper power spectra. This can be explained by the presence of dust at various temperatures, in particular of a cold extended component. For the faintest areas of the far-infrared sky we derive a wavelength independent spectral index of $α$ = --2.3$\pm$0.6 for the cirrus power spectrum. The application of the correct spectral index is a precondition for the proper disentanglement of the cirrus foreground component of the Cosmic Far-Infrared Background and its fluctuations.

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arXiv2001

Sky Confusion Noise in the Far-Infrared: Cirrus, Galaxies and the Cosmic Far-Infrared Background

We examined the sky confusion noise in 40 sky regions by analysing 175 far-infrared (90--200 $μ$m) maps obtained with ISOPHOT, the photometer on-board the Infrared Space Observatory. For cirrus fields with $\rm < \rm B > > 5$ MJysr$^{-1}$ the formula based on IRAS data (Helou & Beichman, \cite{Helou+Beichman_90}) predicts confusion noise values within a factor of 2 to our measurements. The dependence of the sky confusion noise on the surface brightness was determined for the wavelength range 90 $\le λ\le$ 200 $μ$m. We verified that the confusion noise scales as N ~ ^{1.5}, independent of the wavelength and confirmed N ~ lambda^{2.5}$ for lambda >= 100$um. The scaling of the noise value at different separations between target and reference positions was investigated for the first time, providing a practical formula. Since our results confirm the applicability of the Helou & Beichman (1990) formula, the cirrus confusion noise predictions made for future space missions with telescopes of a similar size can be trusted. At 90 and 170um a noise term with a Poissonian spatial distribution was detected in the faintest fields ( <= 3-5 MJysr^{-1}), which we interpret as fluctuations in the Cosmic Far-Infrared Background (CFIRB). Applying ratios of the fluctuation amplitude to the absolute level of 10% and 7% at 90 and 170um, respectively, as supported by model calculations, we achieved a new simultaneous determination of the fluctuation amplitudes and the surface brightness of the CFIRB. The fluctuation amplitudes are 7(+/-)2 mJy and 15(+/-)4 mJy at 90 and 170um, respectively. We obtained a CFIRB surface brightness of B(0)=0.8(+/-)0.2 MJysr^{-1} (nuI_nu=14(+/-)3nWm^-2sr^-1) at 170um and an upper limit of 1.1 MJysr^{-1} (nuI_nu=37 nWm^-2sr^-1) at 90um.

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arXiv2000

Deep far infrared ISOPHOT survey in "Selected Area 57", I. Observations and source counts

We present here the results of a deep survey in a 0.4 sq.deg. blank field in Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared Space Observatory (ISO) at both 60 um and 90 um. The resulting sky maps have a spatial resolution of 15 x 23 sq.arcsec. per pixel which is much higher than the 90 x 90 sq.arcsec. pixels of the IRAS All Sky Survey. We describe the main instrumental effects encountered in our data, outline our data reduction and analysis scheme and present astrometry and photometry of the detected point sources. With a formal signal to noise ratio of 6.75 we have source detection limits of 90 mJy at 60 um and 50 mJy at 90 um. To these limits we find cumulated number densities of 5+-3.5 per sq.deg. at 60 um and 14.8+-5.0 per sq.deg.at 90 um. These number densities of sources are found to be lower than previously reported results from ISO but the data do not allow us to discriminate between no-evolution scenarios and various evolutionary models.

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