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An Analysis of the Shapes of Interstellar Extinction Curves. VI. The Near-IR Extinction Law
We combine new observations from the Hubble Space Telescope's AdvancedCamera of Survey with existing data to investigate the wavelengthdependence of near-IR (NIR) extinction. Previous studies suggest a powerlaw form for NIR extinction, with a "universal" value of the exponent,although some recent observations indicate that significant sightline-to-sight line variability may exist. We show that a power-law modelfor the NIR extinction provides an excellent fit to most extinctioncurves, but that the value of the power, β, varies significantlyfrom sight line to sight line. Therefore, it seems that a "universal NIRextinction law" is not possible. Instead, we find that as βdecreases, R(V) ≡ A(V)/E(B – V) tends to increase,suggesting that NIR extinction curves which have been considered"peculiar" may, in fact, be typical for different R(V) values. We showthat the power-law parameters can depend on the wavelength interval usedto derive them, with the β increasing as longer wavelengths areincluded. This result implies that extrapolating power-law fits todetermine R(V) is unreliable. To avoid this problem, we adopt adifferent functional form for NIR extinction. This new form mimics apower law whose exponent increases with wavelength, has only two freeparameters, can fit all of our curves over a longer wavelength baselineand to higher precision, and produces R(V) values which are consistentwith independent estimates and commonly used methods for estimatingR(V). Furthermore, unlike the power-law model, it gives R(V)s that areindependent of the wavelength interval used to derive them. It alsosuggests that the relation R(V) = -1.36 \frac{E(K-V)}{E(B-V)} - 0.79 canestimate R(V) to ±0.12. Finally, we use model extinction curvesto show that our extinction curves are in accord with theoreticalexpectations, and demonstrate how large samples of observationalquantities can provide useful constraints on the grain properties.
| An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-through-UV Curve Morphology
We study the IR-through-UV interstellar extinction curves towards 328Galactic B and late-O stars. We use a new technique which employsstellar atmosphere models in lieu of unreddened "standard" stars. Thistechnique is capable of virtually eliminating spectral mismatch errorsin the curves. It also allows a quantitative assessment of the errorsand enables a rigorous testing of the significance of relationshipsbetween various curve parameters, regardless of whether theiruncertainties are correlated. Analysis of the curves gives the followingresults: (1) In accord with our previous findings, the central positionof the 2175 A extinction bump is mildly variable, its width is highlyvariable, and the two variations are unrelated. (2) Strong correlationsare found among some extinction properties within the UV region, andwithin the IR region. (3) With the exception of a few curves withextreme (i.e., large) values of R(V), the UV and IR portions of Galacticextinction curves are not correlated with each other. (4) The largesightline-to-sightline variation seen in our sample implies that anyaverage Galactic extinction curve will always reflect the biases of itsparent sample. (5) The use of an average curve to deredden a spectralenergy distribution (SED) will result in significant errors, and arealistic error budget for the dereddened SED must include the observedvariance of Galactic curves. While the observed largesightline-to-sightline variations, and the lack of correlation among thevarious features of the curves, make it difficult to meaningfullycharacterize average extinction properties, they demonstrate thatextinction curves respond sensitively to local conditions. Thus, eachcurve contains potentially unique information about the grains along itssightline.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| Correlation between the very broad structure and the continuum in FUV extinction curves
There exists a correlation between the amount of the far UV extinctionand the relative strength of the very broad structure in the 500 to 600nm wavelength region of the interstellar extinction curves. For a givencolor excess E(B-V) values of the UV extinction large than averagecorrespond to larger strengths of the very broad structure (VBS) andvice versa. The VBS seems to be a common dust-related feature.
| The interstellar 217 NM band - A third catalogue of equivalent widths
A catalog of equivalent widths of the 217 nm interstellar absorptionband as well as other parameters characterizing the extinction curve inthe ultraviolet has been compiled for 790 O and B stars. A relativelytight correlation between the equivalent width of the 217 nm band andE(B-V) indicates that the absorber of this band is connected with thepopulation of larger interstellar grains responsible for the visualextinction. The parameter characterizing the amount of extinction in thefar UV is only weakly correlated with E(B-V), a result in accord withthe assumption that a second population of very small grains causes therapid increase of the far-UV extinction.
| A catalog of ultraviolet interstellar extinction excesses for 1415 stars
Ultraviolet interstellar extinction excesses are presented for 1415stars with spectral types B7 and earlier. The excesses with respect to Vare derived from Astronomical Netherlands Satellite (ANS) 5-channel UVphotometry at central wavelengths of approximately 1550, 1800, 2500, and3300 A. A measure of the excess extinction in the 2200-A extinction bumpis also given. The data are valuable for investigating the systematicsof peculiar interstellar extinction and for studying the character of UVinterstellar extinction in the general direction of stars for which theextinction-curve shape is unknown.
| H-beta photometry of southern early-type stars
H-beta photoelectric photometry is presented for 209 southern hemisphereearly-type stars from the HD catalog with galactic latitudes /b/ greaterthan 6 deg. Four-color photometry exists for all these stars and MKtypes for most of them. Absolute magnitudes have been estimated for allbut the emission-line stars and distances from the sun and the galacticplane determined.
| Four colour photometry of southern early-type stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978MNRAS.182..629K&db_key=AST
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| קבוצת-כוכבים: | שדרית |
| התרוממות ימנית: | 08h10m53.15s |
| סירוב: | -51°11'26.1" |
| גודל גלוי: | 7.962 |
| מרחק: | 505.051 פארסק |
| תנועה נכונה: | -2.7 |
| תנועה נכונה: | 6.5 |
| B-T magnitude: | 8.259 |
| V-T magnitude: | 7.987 |
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