Summary

1. Improving the Knowledge about the Spiral Structure of our Galaxy using NIR Multi-Object Spectroscopy. YSO O? Elysandra Figuerêdo1 (IAG/USP), Augusto Damineli (IAG/USP), Robert Blum (CTIO) & Peter Conti (JILA) 2 1 YSO 3 O<5 O5-6 30 % (~0,67 mag) 1 tipo-O 12 % (~0,30 mag) 6 tipo-O NGC3576 DRádio= 2,8 kpc IRS1 Some Results: Methodology: 60 48 50 Spiral Structure: 18 microns W43 G333.1-0.4 W42 W31 DRadio> 4.1 kpc DPS = 3.4 ± 0.3 kpc W31 False color image of G333.1-0.4 made by combining the three near-infrared images and adopting the colors blue, green, and red for J, H and K, respectively. In this way, the bluest stars are likely foreground objects, and the reddest stars are probably K-band excess objects, indicating the presence of hot dust for objects recently formed in the cluster). North is up and east is to the left. The coordinates of the center of the image are R.A.=16h21m03s and decl.=-50d36h19s (J2000), and the size of the image is 1'.9 x 1'.7. Distribution of GHIIR (filled circles) and HIIR (open circles) in the Galactic disk (Conti & Crowther 2003). Arrows indicate the change in position of four clusters for which we have revised distances. Dashed arrows indicate the ones that had ambiguous distances from radio surveys. The spiral pattern tracings are from Russeil (2003). Our revised distances have implications for the inferred amount of current star formation rate in the Galaxy. Object #4 W42 DRádio= 3.7 kpc DPS = 2.2 kpc In order to derive spectroscopic paralaxes of OB stars, we work in the near-infrared to overcome the obscuration at the optical wavelengths and use the recently developed spectral classification schemes (Hanson et al. 1996). CMD showing the ZAMS shifted to D = 2.8 kpc and reddened by AK = 0.48 because of the interstellar component (dashed line). When adding the average local reddening (AK = 1.29 - solid line), the ZAMS line is displaced to the right and down. We use the CMD in order to choose candidates to spectroscopy. YSO 1 W43 The absence of photospheric lines indicate that this objects are still enshrouded in theirs birth cocoon. This is corroborated by the excess emission derived from photometry and the CO head band at 2.2935 m that is usually attributed to warm (>1000K) and very dense ( = 1010 cm-3) circumstellar material near the star. DRádio= 7.0 kpc DPS = 4.3 - 5.7 kpc 2 The uncertainty in the distance estimate is completely dominated by the luminosity class assumed and the scatter in the intrinsic MV of O stars (±0.67 mag), leading to an uncertainty of 30 % for a single O type star. Additional stars will diminish the random error. 3 5 4 Fig. 6 Fig. 7 O6 O8 D = 2,9 kpc Summary The survey of Giant HII regions (GHII) in the near-infrared is very promising, since it can address important Astrophysical questions such as the study of the nature of massive star formation and tracing the spiral arms of our galaxy. The basic procedure in this work is to obtain Near-Infrared images of embedded stellar clusters, selected among the most luminous Galactic radio and IRAS sources. As such clusters are expected to be very good tracers of the spiral arms the next step is to chose in the NIR images the best candidates for the distance determinations that are based upon spectroscopic parallaxes of newly born OB stars. The problem in the current methods comes from the ambiguity of the distance from galactic rotation models. Our preliminary results break the radio ambiguity distances toward smaller distance and in cases where distances were derived by radio techniques, our distances are systematically smaller. Our emerging picture is that of less active star formation in the Milky Way than thought before. The uncertainty in our estimated distances is completely dominated by the scatter in the intrinsic Mv of O stars (±0.67 mag), leading to an uncertainty of 30% for a single O type star. Additional stars usually diminish the random error by a factor of root square of the number of objects that we have spectrum, and can reach < 10% if B-type stars are observed, since they have ten times longer Main Sequence lifetime. However, to obtain more spectra, a lot of telescope time would be necessary, making this project very time consuming. In this way, in order to advance in the knowledge about the spiral structure of our Galaxy will be very important the use of multi-object spectroscopy in the K-band. In addiction... Financial support by FAPESP and PRONEX/GAL 1email: lys@astro.iag.usp.br