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Small Magellanic Cloud-Type Interstellar Dust in the Milky Way
It is well known that the sight line toward HD 204827 in the clusterTrumpler 37 shows a UV extinction curve that does not follow the averageGalactic extinction relation. However, when a dust component, foregroundto the cluster, is removed, the residual extinction curve is identicalto that found in the SMC within the uncertainties. The curve is verysteep and has little or no 2175 Å bump. The position of HD 204827in the sky is projected onto the edge of the Cepheus IRAS bubble. Inaddition, HD 204827 has an IRAS bow shock, indicating that it may beembedded in dust swept up by the supernova that created the IRAS bubble.Shocks due to the supernova may have led to substantial processing ofthis dust. The HD 204827 cloud is dense and rich in carbon molecules.The 3.4 μm feature indicating a C-H grain mantle is present in thedust toward HD 204827. The environment of the HD 204827 cloud dust maybe similar to the dust associated with HD 62542, which lies on the edgeof a stellar wind bubble and is also dense and rich in molecules. Thissight line may be a Rosetta Stone if its environment can be related tothose in the SMC having similar dust.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Dust properties in the direction of Trumpler 37.
The fourteen brightest OB stars of the Trumpler 37 Cluster have beenobserved in the V,R,I,J,H,K photometric bands. These have been used toderive accurate measurements of the quantity R_V_=A_V_/E(B-V). Differentfrom previous studies it is found that in Trumpler 37 isclose to the mean interstellar value of 3.1. Most of the reddeningtowards Trumpler 37 is found to be of foreground origin. This isconsistent with the lack of variations, observed by us, of R_V_ with theprojected distance from the center of the cluster. In two highlyreddened stars of the cluster, whose reddening is certainly mostlylocal, the observed R_V_ is lower than the mean interstellar value of3.1. This is to be compared with the behaviour of the Trapezium stars inOrion, whose R_V_ is quite above such value whereas the age of thecluster is similar to the one of Trumpler 37.
| Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg
The literature on all OB associations was reviewed, and their IRAS pointsource content was studied, between galactic longitude 55 and 150 deg.Only one third of the 24 associations listed by Ruprecht et al. (1981)have been the subject of individual studies designed to identify thebrightest stars. Distances to all of these were recomputed using themethod of cluster fitting of the B main sequence stars, which makes itpoossible to reexamine the absolute magnitude calibration of the Ostars, as well as for the red supergiant candidate stars. Also examinedwas the composite HR diagram for these associations. Associations withthe best defined main sequences, which also tend to contain very youngclusters, referred to here as OB clusters, have extremely few evolved Band A or red supergiants. Associations with poorly defined mainsequences and few OB clusters have many more evolved stars. They alsoshow an effect in the upper HR diagram referred to as a ledge byFitzpatrick and Garmany (1990) in similar data for the Large MagellanicCloud. It is suggested that the differences in the associations are notjust observational selection effects but represent real differences inage and formation history.
| The interstellar extinction law towards the open cluster TR-37
Near-infrared photometry of OB stars in the young open cluster TR-37 hasbeen obtained. These data have been combined with previous visual andultraviolet observations of the same sample and a detailed extinctioncurve has been derived, ranging from the near-infrared to thefar-ultraviolet. The extinction towards this sample of stars seems to beanomalous, consistent with a shift in the grain-size distribution withrespect to an average galactic extinction curve.
| Anomalous dust in the open cluster Trumpler 37
New IUE data were obtained towards stars in the open cluster Trumpler37. Extinction curves derived from these data show that the anomalousdust detected towards HD 204827 and HD 210072 is not unique to thoselines of sight but occurs generally in that region of the sky. The dustis apparently associated with the Trumpler 37 cluster, which forms thecenter of Cepheus OB2. This is a complex region of star formation wherethe evolution of dust grains through shock waves could be consideredlikely. A supernova in a slightly older star-forming region of Cep OB2located next to Trumpler 37 is a possible source of such a shock wave.
| Determination of size, color indices, and spectral classes of stars in an area centered on R.A.=21h24m, Dec=+58.5deg.
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