Mean Extinction Curve Research Articles

Toward a better understanding of the mid-infrared emission in the Large Magellanic Cloud

Context. The scarcity of spectroscopic data with a high signal-to-noise ratio in the interstellar medium between 20 and 100 μm has led to the development of several dust models with distinct dust properties that are poorly constrained in this broad wavelength range. Some of them require the presence of graphites, whereas others consider small amorphous or small aromatic carbon grains, with various dust sizes. Aims. We aim to constrain the dust emission in the mid- to far-infrared domain in the Large Magellanic Cloud (LMC) for the first time with the use of the Spitzer IRS and MIPS spectral energy distribution (SED) data, combined with Herschel data. We also consider ultraviolet extinction predictions derived from modeling. Methods. We selected ten regions that were observed as part of the SAGE-Spec program (PI: F. Kemper) to probe dust properties in various environments (diffuse, molecular, and ionized regions). All data were smoothed to the 40″ angular resolution before we extracted the dust emission spectra and photometric data. The SEDs were modeled with dust models available in the DustEM package, using the standard Mathis radiation field, as well as three additional radiation fields, with stellar clusters ages ranging from 4 Myr to 600 Myr. Results. Previous analyses of molecular clouds in the LMC have reasonably well reproduced the SEDs of the different phases of the clouds constructed from near- to far-infrared photometric data using the DustEM models. However, only by using spectroscopic data and by changing the dust abundances and size distributions in comparison with our Galaxy we were able to derive new constraints on the small- grain component. Standard dust models (with free dust abundances) that were used to reproduce the Galactic diffuse medium are clearly not able to reproduce the dust emission in the mid-infrared wavelength domain. This analysis shows the need of adjusting the parameters describing the dust size distribution, which shows a clearly distinct behavior depending on the type of environment. In addition, whereas the small-grain emission always seems to be negligible at long wavelengths in our Galaxy, the contribution of this small-dust component might be stronger than expected in the submillimeter to millimeter range in the LMC-averaged SED. Conclusions. The properties of the small-dust component of the LMC are clearly different from those of our Galaxy. Its abundance, which is significantly enhanced, might be the result of the shattering of large grains through strong shocks or turbulence. In addition, this grain component in the LMC systematically shows smaller grain sizes in the ionized regions than in the diffuse medium. Predictions of extinction curves show significantly distinct behaviors depending on the dust models, but they are also different from one region to the next. A comparison of model predictions with the LMC mean extinction curve shows that no model agrees satisfactorily when the Mathis radiation field is used, but a harder radiation field tends to improve the agreement.

Atmospheric extinction properties above Mauna Kea from the Nearby SuperNova Factory spectro-photometric data set

We present a new atmospheric extinction curve for Mauna Kea spanning 3200–9700 Å. It is the most comprehensive to date, being based on some 4285 standard star spectra obtained on 478 nights spread over a period of 7 years obtained by the Nearby SuperNova Factory using the SuperNova Integral Field Spectrograph. This mean curve and its dispersion can be used as an aid in calibrating spectroscopic or imaging data from Mauna Kea, and in estimating the calibration uncertainty associated with the use of a mean extinction curve. Our method for decomposing the extinction curve into physical components, and the ability to determine the chromatic portion of the extinction even on cloudy nights, is described and verified over the wide range of conditions sampled by our large dataset. We demonstrate good agreement with atmospheric science data obtain at nearby Mauna Loa Observatory, and with previously published measurements of the extinction above Mauna Kea.

Is GRB 050904 at z = 6.3 absorbed by dust?

Claim of dust extinction for this GRB has been debated in the past. We suggest that the discrepant results occur primarily because most of previous studies have not simultaneously investigated the X-ray to near-IR spectral energy distribution of this GRB. The difficulty with this burst is that the X-ray afterglow is dominated by strong flares at early times and is poorly monitored at late times. In addition, the Z band photometry, which is the most sensitive to dust extinction, has been found to be affected by strong systematics. In this paper we carefully re-analyze the Swift/XRT afterglow observations of this GRB, using extensive past studies of X-ray flare properties when computing the X-ray afterglow flux level and exploiting the recent reanalysis of the optical (UV rest frame) data of the same GRB. We extract the X-ray to optical/near-IR afterglow SED for the three epochs where the best spectral coverage is available: 0.47, 1.25, and 3.4 days after the trigger. A spectral power-law model has been fitted to the extracted SEDs. We discuss that no spectral breaks or chromatic temporal breaks are expected in the epochs of interest. To fit any UV rest-frame dust absorption, we tested the Small Magellanic Cloud (SMC) extinction curve, the mean extinction curve (MEC) found for a sample of QSO at $z>4$ and its corresponding attenuation curve, as well as a starburst attenuation curve, and the extinction curve consistent with a supernova dust origin (SN-type). The SMC extinction curve and the SN-type one provide good fit to the data at all epochs, with an average amount of dust absorption at $\lambda_{rest} = 3000 \AA$ of $A_{3000} = 0.25\pm 0.07$ mag. These results indicate that the primeval galaxy at $z = 6.3$ hosting this GRB has already enriched its ISM with dust.

The extinction law at high redshift and its implications

We analyze the optical-near infrared spectra of 33 quasars with redshifts 3.9<z<6.4 to investigate the properties of dust extinction at these cosmic epochs. The SMC extinction curve has been shown to reproduce the dust reddening of most quasars at z<2.2; we investigate whether this curve also provides a good description of dust extinction at higher redshifts. We fit the observed spectra with synthetic absorbed quasar templates obtained by varying the intrinsic slope (alpha), the absolute extinction (A3000), and by using a grid of empirical and theoretical extinction curves. We find that seven quasars in our sample are affected by substantial extinction (A3000 >0.8), and characterized by very steep intrinsic slopes (alpha<-2.3). All of the individual quasars require extinction curve deviating from that of the SMC, with a tendency to flatten at lambda<2000 A (in the rest frame of the source). We obtain a mean extinction curve at z>4, both by performing a simultaneous fit of all quasars and by averaging the extinction curves inferred for individual quasars. In the case of broad absorption line quasars the mean extinction curve deviates from the SMC at a confidence level >95%. The difference between extinction curves in quasars at z>4 and in quasars at lower redshift is indicative of either a different dust production mechanism at high redshift, or a different mechanism for processing dust into the ISM. We suggest that the same transitions may also apply to normal, star-forming galaxies at z>4. In particular, the observed change in the average spectral slope of galaxies at z>4 may be partially ascribed to a variation in the extinction curve, rather than a lower dust content at high redshift. In this scenario, the extinction curve inferred at z>4 would imply a cosmic star-formation corrected for dust attenuation a factor of ~2 higher than estimated in the past.

Residual extinction effects in spectra of newly formed stars

Extinction laws observed in spectra of Be stars are shown to be evidently different from the mean extinction curve. They do not contain in many cases the prominent extinction feature - the 2200A bump. The results are derived both from multicolour (ranging from the far-IR until the ultraviolet ANS) photometry and from TD-1 UV spectra. It is strongly suggested that the observed extinction phenomena are originated in circumstellar, disk-shaped shells as the shape of resultant extinction curve suggests both the presence of big, core-mantle grains and continuous infrared emission of circumstellar origin.

A SPHERICAL, NON-LTE, BLANKETED MODEL STELLAR ATMOSPHERE FOR PHI CASSIOPEIAE (F0 IA)

By constructing a model atmosphere, I have calculated the theoretical energy distribution of the emergent radiation that best matches the observed energy distribution of the star Cassiopeiae, and so derived its effective temperature and surface gravity. For an accurate comparison between theory and observations, it is desirable to determine the interstellar extinction situ and then correct the observations for it. Fortunately, (J) Cas is very likely a member of the open cluster NGC 457. Five early-B-type stars near the main-sequence turnoff in NGC 457 were observed at low dispersion with the WE satellite. In order to use the pair to derive the extinction curve for NGC 457, 1 have developed a new method of choosing a lightly reddened comparison star. It consists of matching the equivalent widths of spectral features that are particularly strong and sensitive to temperature and luminosity. The UV extinction in NGC 457 deviates from the average extinction for the Milky Way and is uniform across the cluster. For the visible region, monochromatic fluxes of 4> Cas were dereddened with a mean extinction curve. Thus, the intrinsic energy distribution of § Cas was determined from 1500 A to 5800 A. A detailed new spherical model atmosphere was constructed. The equation of transfer was solved with the constraints of hydrostatic and pure radiative equilibrium. The statistical equilibrium of 106 transitions was treated. Several tests have demonstrated this model to be reliable. Comparison of the observed UV and visible energy distributions and the HO, Ca II K, and Mg II h and k line profiles with the theoretical results indicates that the best model for Cas.

Further evidence for disk accretion in FU Orionis objects

Optical and IR spectroscopy data on the T Tauri star FU Ori obtained with instrumentation at Kitt Peak and the Mt. Hopkins Multiple Mirror telescope were examined for double peaks predicted by a disk accretion model. The data, collected in January 1986, were used to calculate a mean extinction curve and optical spectra. Comparison of optical profiles were also recorded for V1057 Cyg and V1515 Cyg. The FU Ori data exhibited the double peak predicted by the disk model. A wavelength-dependent rotation rate that was observed is attributed to the absence of any companion star. Mass is instead being accreted onto a disk from remnant material from the star formation process. 19 references.

The extinction law in the open cluster NGC 457 and the intrinsic energy distribution of Phi Cassiopeiae (F0 Ia)

Five early B-type stars near the main-sequence turnoff in NGC 457 have been observed at low dispersion with the short-wavelength prime and the long-wavelength redundant cameras of the IUE satellite. The equivalent widths of spectral features that are particularly strong and sensitive to temperature and luminosity were computed in the cluster stars and in 20 lightly reddened stars of types O9-B3 and luminosity classes III-V. The comparison of the equivalent widths provides a reliable method for finding matching pairs. Having identified the best comparison star for each program star, binned fluxes were used to determine the mean extinction curve. In order to cover the visible region, monochromatic fluxes of Phi Cas were derived from observations with the intensified Reticon scanner mounted on the No. 2 0.9 m telescope of KPNO, and they were dereddened with the mean extinction curve of Savage and Mathis. Thus, the intrinsic energy distribution of Phi Cas were determined from 1500 to 5800 A for use in a detailed model-atmosphere analysis.

Near Infrared Observations of Galactic WN Stars

The observations reported here have been obtained with the ESO Reticon system attached to the Boller and Chivens spectrograph at the 3.6 m telescope. The reciprocal dispersion was 228 A mm−1 with a useful spectral range extending from 5800 A to 11000 A. The slit was 200 µm wide which corresponds to 1.4 arcsec on the sky and to about 33.5 µm(∼ 7.6 A) in the plane of the detector. Sky substraction was performed by means of observations at 93 arcsec. East and West of the object. The correction for atmospheric extinction was based on a mean extinction curve of La Silla (Tug, 1977). As a consequence the strongest atmospheric bands are still present on the final spectra. The data were also corrected for the instrument sensitivity response by using calibrated stars selected from Breger's catalog (1976). As these reference data are from filter photometry, they are not well suited to the resolution of our spectra, at least in some regions. This problem can be significant, as clearly illustrated by Johnson (1980).

Infrared photometry, extinctions andR values for some Northern Milky Way stars

Infrared wideband photometry for 21 early-type stars in the Northern Milky Way is used to determine extinction values by the colour-difference method. The mean extinction curve is similar to the van de Hulst theoretical curve No. 15, and there is no significant difference in the results for stars lying towards and away from nebulosity. None of the stars exhibit infrared excesses at 3.5 μ. A mean value ofR=3.15±0.15 is deduced for the ratio of total to selective extinction.R is found to be normal towards the open cluster IC 1805 for which high values have been suggested; a distance of 2.0±0.2 kpc is deduced for this cluster.

A Discussion on astronomy in the ultraviolet - Preliminary results of ultraviolet interstellar extinction from TD1 satellite observations

The interstellar extinction law in the far ultraviolet has been derived from observations of about 100 stars for three galactic regions in the direction of Cygnus, galactic centre and anticentre. For each region the weighted mean extinction curve has an accuracy of about 6 % and shows no variation for the different directions. The profile of the broad extinction band centred near 2200 A is derived and shown to be symmetrical. It is concluded that the extinction characteristics will need more than one type of particle for their explanation.

Reddening Curves for Planetary Nebulae

view Abstract Citations (16) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Reddening Curves for Planetary Nebulae Pipher, Judith L. ; Terzian, Yervant Abstract Absolute reddening curves are derived for selected planetary nebulae from a comparison of radio- continuum observations with observations of hydrogen recombination lines. The curves are compared with a suitably scaled mean extinction curve. The scaling factor is in agreement with the factor derived by the method of Aller and Minkowski. Significant deviations of Ha and other lines from the reddening curves are discussed Publication: The Astrophysical Journal Pub Date: February 1969 DOI: 10.1086/149883 Bibcode: 1969ApJ...155..475P full text sources ADS | data products SIMBAD (9)