Alexa Hart

Supergiant temperatures and linear radii from near-infrared interferometry

We present angular diameters for 42 Luminosity Class (LC) I stars and 32 LC II stars that have been interferometrically determined with the Palomar Testbed Interferometer. Derived values of radius and effective temperature are established for these objects, and an empirical calibration of these parameters for supergiants will be presented as a function of spectral type and colours. For the effective temperature versus (V - K) 0 colour, we find an empirical calibration with a median deviation of AT = 70K in the range of 0.7 < (V - K) 0 < 5.1 for LC I stars; for LC II, the median deviation is AT = 120K from 0.4 < (V - K) 0 < 4.3. Effective temperature as a function of spectral type is also calibrated from these data, but shows significantly more scatter than the T EFF versus (V - K) 0 relationship. No deviation of T EFF versus spectral type is seen for these high-luminosity objects relative to LC II giants. Directly determined diameters range up to 400R ⊙ , though are limited by poor distance determinations, which dominate the error estimates. These temperature and radii measures reflect a direct calibration of these parameters for supergiants from empirical means.

Identification of Three New Protoplanetary Nebulae Exhibiting the Unidentified Feature at 21 μm

Among its great findings, the Infrared Astronomical Satellite mission showed the existence of an unidentified mid-IR feature around 21 μm. Since its discovery, this feature has been detected in all C-rich protoplanetary nebulae (PPNe) of intermediate spectral type (F-G) and—weakly—in a few PNe and asymptotic giant branch (AGB) stars, but the nature of its carriers remains unknown. In this paper, we show the detection of this feature in the spectra of three new stars transiting from the AGB to the PN stage obtained with the Spitzer Space Telescope. Following a recent suggestion, we try to model the spectral energy distributions of our targets with amorphous carbon and FeO, which might be responsible for the unidentified feature. The fit thus obtained is not completely satisfactory, since the shape of the feature is not well matched. In an attempt to relate the unidentified feature to other dust features, we retrieved mid-IR spectra of all 21 μm sources currently known from Infrared Space Observatory and Spitzer online archives and noticed a correlation between the flux emitted in the 21 μm feature and that emitted at 7 and 11 μm (polycyclic aromatic hydrocarbon bands and hydrogenated amorphous carbon broad emission). Such a correlation may point to a common nature of the carriers.

Interferometric Studies of the Extreme Binary ϵ Aurigae: Pre-Eclipse Observations

We report new and archival K-band interferometric uniform disk diameters obtained with the Palomar Testbed Interferometer for the eclipsing binary star Aurigae, in advance of the start of its eclipse in 2009. The observations were intended to test whether low-amplitude variations in the system are connected with the F supergiant star (primary), or with the intersystem material connecting the star with the enormous dark disk (secondary) inferred to cause the eclipses. Cepheid-like radial pulsations of the F star are not detected, nor do we find evidence for proposed 6% per decade shrinkage of the F star. The measured 2.27 ± 0.11 mas K-band diameter is consistent with a 300 solar radius F supergiant star at the Hipparcos distance of 625 pc. These results provide an improved context for observations during the 2009-2011 eclipse.

Spitzer observations of a circumstellar nebula around the candidate luminous blue variable MWC 930

Context. The source MWC 930 is a star just ∼2 ◦ above the Galactic plane whose nature is not clear and has not been studied in detail so far. While a post-asymptotic giant branch classification was proposed in the past, studies of its optical spectrum and photometry pointed toward strong variability, therefore, the object was reclassified as a luminous blue variable (LBV) candidate. Aims. The LBVs typically undergo phases of strong mass loss in the form of eruptions that can create shells of ejecta around the star. Our goal is to search for the presence of such a circumstellar nebula in MWC 930 and investigate its properties. Methods. To do so, we make use of space-based infrared data from our Spitzer campaign performed with the InfraRed Array Camera (IRAC) and the InfraRed Spectrograph (IRS), as well as data from optical and infrared (IR) surveys. Results. In our Spitzer images, we clearly detect an extended shell around MWC 930 at wavelengths longer than 5 μm. The midinfrared spectrum is dominated by the central star and mostly shows forbidden lines of [FeII] with an underlying continuum that decreases with wavelengths up to ∼15 μm and then inverts its slope, displaying a second peak around 60 μm, which is evidence of cold dust grains formed in a past eruption. By modeling the spectral energy distribution, we identify two central components, beside the star and the outer shell. These extra sources of radiation are interpreted as material close to the central star, which may be due to a recent ejection. Features of C-bearing molecules or grains are not detected.

Identification of three new protoplanetary nebulae exhibiting the unidentified feature at 21 μm

Since its discovery, the feature at 21 μm has been detected in all C-rich proto-PNe of intermediate spectral type (A–G) and – weakly – in a few PNe and AGB stars, but the nature of its carriers remains unknown. In this paper, we show the detection of this feature in the spectra of three new stars obtained with the Spitzer Space Telescope. In the attempt to relate the unidentified feature to other dust features, we retrieved mid-IR spectra of all the 21 μm sources currently known from ISO and Spitzer archives and noticed a correlation between the flux emitted in the 21 μm feature and that emitted at 7 and 11 μm (PAH bands and HAC broad emission). Such a correlation may point to a common nature of the carriers.