* Astronomy

Title: The dusty Nebula surrounding HR Car: a Spitzer view
Authors: G. Umana, C. S. Buemi, C. Trigilio, J. L. Hora, G. G. Fazio, P. Leto

We present mid-IR observations of the Galactic Luminous Blue Variable (LBV) HR Car and its associated nebula carried out with the Spitzer Space Telescope using both IRAC and IRS, as part of a GTO program aimed to study stellar ejecta from evolved stars. Our observations reveal a rich mid-IR spectrum of the inner nebula showing both solid state and atomic gas signatures. Strong low-excitation atomic fine structure lines such as  26.0 \mu m {Fe}{2} and  34.8 \mu m {Si}{2}, indicate, for the first time, the presence of a PDR in this object class. While the physics and chemistry of the low-excitation gas appears to be dominated by photodissociation, a possible contribution due to shocks can be inferred from the evidence of gas phase Fe abundance enhancement. The presence of amorphous silicates, inferred from the observed characteristic broad feature at 10 \mu m located in the inner nebula, suggests that dust has formed during the LBV outburst. This is in contrast with the detection of crystalline dust in other probably more evolved Galactic LBVs, which is similar to the crystalline dust observed in red supergiants. This has been considered to be evidence of dust production during evolutionary phases prior to the outburst.

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HR Carinae is a supergiant, luminous blue variable star located in the constellation Carina. It is surrounded by a vast nebula of ejected nuclear-processed material because this star has a multiple shell expanding atmosphere. This star is among the most luminous stars in our galaxy. It has very broad emission wings of the Balmer lines, reminiscent from the broad lines observed in the spectra of Of and Wolf-Rayet stars. A kinematic distance of 5.4 kpc and a bolometric magnitude of -9.4 put HR Car among the most luminous stars of the galaxy.

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