Showing papers by "Eli Dwek published in 2004"
TL;DR: In this article, Li et al. presented new interstellar dust models which have been derived by simultaneously fitting the far ultraviolet to near infrared extinction, the diffuse infrared emission, and, unlike previous models, the elemental abundances in dust for the diffuse interstellar medium.
Abstract: We present new interstellar dust models which have been derived by simultaneously fitting the far ultraviolet to near infrared extinction, the diffuse infrared emission, and, unlike previous models, the elemental abundances in dust for the diffuse interstellar medium We found that dust models consisting of a mixture of spherical graphite and silicate grains, polycyclic aromatic hydrocarbon (PAH) molecules, in addition to porous composite particles containing silicate, organic refractory, and water ice, provide an improved t to the UV-to-infrared extinction and infrared emission measurements, while consuming the amounts of elements well within the uncertainties of adopted interstellar abundances, including B star abundances These models are a signicant improvement over the recent Li & Draine (2001, ApJ, 554, 778) model which requires an excessive amount of silicon to be locked up in dust: 48 ppm (atoms per million of H atoms), considerably more than the solar abundance of 34 ppm or the B star abundance of 19 ppm
628 citations
TL;DR: In this paper, the authors used the most recent data on the EBL to construct twelve different realizations representing all possible permutations between EBL limits and the detections in different wavelength regions.
Abstract: Very high energy (~ TeV) $\gamma$-rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity for determining the EBL intensity if their intrinsic spectrum is known. Conversely, knowledge of the EBL intensity can be used to determine the intrinsic blazar spectrum. Unfortunately, neither the EBL intensity nor the intrinsic blazar spectrum is known with high enough precision to accurately derive one quantity from the other. In this paper we use the most recent data on the EBL to construct twelve different realizations representing all possible permutations between EBL limits and the detections in the different wavelength regions. We use these realizations to explore the effects of the EBL on the inferred spectra of blazars. In particular, we show that the frequently cited "IR background-TeV gamma-ray crisis" does not exist, and derive the intrinsic spectra and peak energies of the blazars Mrk 421, 501 and H1426+428 for EBL realizations that give rise to physically viable intrinsic blazar spectra. We also show that the intrinsic spectrum of Mrk~421 during a period of intense flaring activity has a peak energy that seems to shift to higher energies at higher flux states. Finally, we also explore the effect of the uncertainties in the absolute calibration of the gamma-ray energies on derived TeV opacities and the intrinsic blazar spectra.
129 citations
TL;DR: In this article, a more plausible interpretation of the observations was proposed, in which the cold dust emission is generated by conducting needles with properties that are completely determined by the combined submillimeter and X-ray observations of the remnant.
Abstract: Recently, Dunne et al. (2003) obtained 450 and 850 micron SCUBA images of CasA, and reported the detection of 2-4 M_sun of cold, 18K, dust in the remnant. Here we show that their interpretation of the observations faces serious difficulties. Their inferred dust mass is larger than the mass of refractory material in the ejecta of a 10 to 30 M_sun star. The cold dust model faces even more difficulties if the 170 micron observations of the remnant are included in the analysis, decreasing the cold dust temperature to ~ 8K, and increasing its mass to > 20 M_sun. We offer here a more plausible interpretation of their observation, in which the cold dust emission is generated by conducting needles with properties that are completely determined by the combined submillimeter and X-ray observations of the remnant. The needles consist of metallic whiskers with <1% of embedded impurities that may have condensed out of blobs of material that were expelled at high velocities from the inner metal-rich layers of the star in an asymmetric explosion. The needles are collisionally heated by the shocked gas to a temperature of 8K. Taking the destruction of needles into account, a dust mass of only 1E-4 to 1E-3M_sun is needed to account for the observed SCUBA emission. Aligned in the magnetic field, needles may give rise to observable polarized emission. The detection of submillimeter polarization will therefore offer definitive proof for a needle origin for the cold dust emission. Supernovae may still be proven to be important sources of interstellar dust, but the evidence is still inconclusive.
65 citations
TL;DR: In this paper, a more plausible interpretation of the observations was proposed, in which the cold dust emission is generated by conducting needles in the ejecta, and the needle properties were completely determined by the combined submillimeter and X-ray observations of the remnant.
Abstract: Ejecta from core-collapse supernovae contain a few solar masses of refractory elements and therefore can be the most important source of interstellar dust if these elements condense efficiently into solids. However, infrared observations of young supernova remnants, such as Cas A or Kepler, and observations of SN 1987A have detected only � 10 � 3 Mof hot dust in these objects. Recently, Dunne et al. obtained 450 and 850 � mS CUBA images of Cas A and reported the detection of 2-4 Mof cold (18 K) dust in the remnant. Here we show that their interpretation of the observations faces serious difficulties. Their inferred dust mass ignores the effect of grain destruction by sputtering and is larger than the mass of refractory material in the ejecta of a 10-30 Mstar. The cold dust model faces even more difficulties if the 170 � m observations of the remnant are included in the analysis, which decreases the cold dust temperature to � 8 K and increases its mass to k20 M� .W e offer here a more plausible interpretation of their observation, in which the cold dust emission is generated by conducting needles in the ejecta. The needle properties are completely determined by the combined submillimeter and X-ray observations of the remnant. The needles are collisionally heated by the shocked gas. They are very efficient emitters at submillimeter wavelengths and, with a resistivity of a few � � cm, can readily attain a temperature of 8 K. Taking the destruction of needles into account, a dust mass of only 10 � 4 to 10 � 3 Mis needed to account for the observed SCUBA emission. The needles consist of metallic whiskers with P1% of embedded impurities, which may have condensed out of blobs of material that were expelled at high velocities from the inner metal- rich layers of the star in an asymmetric explosion. Conductive needles may also be the source of the cold dust emission detected by Morgan et al. in Kepler. When aligned in the magnetic field, needles may give rise to observable polarized emission. The detection of submillimeter polarization will therefore offer definitive proof for a needle origin for the cold dust emission. Supernovae may yet be proven to be important sources of interstellar dust, but the evidence is still inconclusive.
61 citations
TL;DR: In this paper, the extinction curve derived from Infrared Space Observatory mid-infrared observations of the Galactic center exhibits a surprisingly flat behavior in the 3-8 μm region, contrary to the deep minimum expected from standard interstellar dust models consisting of bare silicate and graphite dust particles.
Abstract: The extinction curve derived from Infrared Space Observatory mid-infrared observations of the Galactic center (GC) exhibits a surprisingly flat behavior in the ~3-8 μm region, contrary to the deep minimum expected from standard interstellar dust models consisting of bare silicate and graphite dust particles. We show that this extinction is likely caused by the presence of metallic needles in the interstellar medium (ISM) toward the GC. If the needles contribute only to the 3-8 μm extinction, they must have a long-wavelength cutoff at ~8 μm and therefore a typical length over radius ratio of ~600, smaller than the ~3 × 103 aspect ratio determined for the needles in Cas A. Homogeneously distributed throughout the ISM, they comprise only a minor mass fraction of the ISM, with a needle-to-H mass ratio of ~5 × 10-6, which is equivalent to 0.14% of the silicate dust mass. Their total ISM abundance then can be readily explained by the combined production in supernovae and O-rich stellar outflows. The GC observations show that metallic needles, in spite of their low abundance or nonuniform distribution, can be the dominant source of opacity in the 3-8 μm wavelength region. However, expelled into the intergalactic medium, their abundance is too low to cause any dimming of cosmological sources, and their length is too short to make them a significant source of submillimeter emission.
33 citations
TL;DR: In this article, the extinction curve derived from ISO mid-infrared (IR) observations of the Galactic center (GC) exhibits a surprisingly flat behavior in the 3 to 8 micron region, contrary to the deep minimum expected from standard interstellar dust models consisting of bare silicate and graphite dust particles.
Abstract: The extinction curve derived from ISO mid-infrared (IR) observations of the Galactic center (GC) exhibits a surprisingly flat behavior in the ~ 3 to 8 micron region, contrary to the deep minimum expected from standard interstellar dust models consisting of bare silicate and graphite dust particles. We show that this extinction is likely caused by the presence of metallic needles in the interstellar medium (ISM) towards the Galactic center. If the needles contribute only to the 3 - 8 micron extinction, they must have a long wavelength cutoff at ~ 8 microns, and therefore a typical length over radius ratio of about 600, smaller than the 3000 aspect ratio determined for the needles in Cas A. Homogeneously distributed throughout the ISM, they comprise only a minor mass fraction of the ISM, with a needle-to-H mass ratio of \~5x10^{-6}, which is equivalent to 0.14% of the silicate dust mass. Their total ISM abundance can then be readily explained by the combined production in SNe and O-rich stellar outflows. It is currently unclear how universal the GC extinction law is. Local 2 - 5 micron extinction measurements seem to be consistent with the standard extinction law, suggesting a non uniform distribution of needles in the ISM. The GC observations show that metallic needles, in spite of their low abundance or non uniform distribution, can be the dominant source of opacity in the 3 - 8 micron wavelength region. However, expelled into the intergalactic medium, their abundance is too low to cause any dimming of cosmological sources, and their length is too short to make them a significant source of submillimeter emission.
33 citations
TL;DR: The authors reviewed the ingredients in constructing models for the evolution of dust, stressing the current uncertainties in the yield of dust from supernovae and AGB stars, and briefly described the effect of dust evolution on the spectral energy distribution of galaxies, and presented a rough estimate of the redshift when galaxies first become opaque.
Abstract: This paper reviews our current understanding of interstellar dust models, what constitutes a viable dust model, what observational constraints are essential for deriving such model, and the current viable dust models. Interstellar dust exhibits spatial and temporal variations, and the paper reviews the ingredients in constructing models for the evolution of dust, stressing the current uncertainties in the yield of dust from supernovae and AGB stars. Finally, the paper briefly describes the effect of dust evolution on the spectral energy distribution of galaxies, and, using a very simple criterion, present a rough estimate of the redshift when galaxies first become opaque.
2 citations