P REGUNTAS DE R ESPUESTA O RAL EN P LENO

Duchamp The source-finding package duchamp (Whiting, 2012) was used to search

for maser sources within the 28′

full width at half-maximum of each of the fields. The code uses a sum-threshold algorithm over each plane in a data cube and returns the positions of flux peaks above the set threshold. After tuning the value of the threshold to result in 10−15 peaks, a spectrum for each of the targets was extracted using casa and any promising candidates were cross-referenced with AGB catalogues (e.g. Boyer et al., 2011; Riebel et al., 2012, 2015) and the SIMBAD database.

2.3

Mid-infrared spectroscopic techniques

Within this work, mid-IR spectroscopy has been used to investigate one of the largest dust producers in the LMC, the red supergiant: IRAS 05280−6910 (Chapter 7). Observing in the mid-IR comes with challenges different from those in the radio. Ground based- instruments, as opposed to space-based instruments, are easier to construct and maintain. This is why ground-based mid-IR instruments currently have more capabilities than space-based instruments. This work used spectroscopic and photometric data from the Spitzer Space Telescope to model the spectral energy distributions of evolved stars. While the instrument has a high sensitivity and is useful for isolated targets, it is less useful in crowded fields like the Galactic Centre. Instruments like the VISIR spectrograph at the Very Large Telescope (VLT) and Michelle at Gemini North can achieve higher spatial and spectral resolutions than space-based instruments as a result of their much larger-diameter instruments. However these instruments have severely limited sensitivities as a result of the high levels of thermal radiation from the sky as well as the instruments themselves.

The Earth’s atmosphere is host to a significant amount of dust and a number of

molecular features in both emission and absorption. Species like H2O, O3, CO2, N2O

and CH4 dominate the mid-IR spectrum creating a forest of telluric lines above 2.3 µm

2.3 Mid-infrared spectroscopic techniques 55

Figure 2.7: The Very Large Telescope (Credit: ESO/Jos´e Francisco Salgado)

λmax = b/T (2.1)

where λmax is the peak wavelength of a blackbody, T is the temperature (in K) and b is

Wien’s displacement constant (b = 2900 µm·K). This means that a temperature of ∼

27◦_{C, a temperature commonly found on Earth, reaches its maximum intensity around}

10 µm, where the flux of silicate emission from cool evolved stars peaks.

2.3.1

Very Large Telescope

The Very Large Telescope (VLT) is a telescope facility in Northern Chile run by the European Southern Observatory (ESO). The facility is located at the Paranal observatory which sits high in the Atacama Desert at 8,645 feet. The telescope is composed of four 8.2 m unit telescopes (UTs) with a suite of optical and infrared instruments (Fig. 2.7).

2.3 Mid-infrared spectroscopic techniques 56

VISIR This work has made use of data from the VLT spectrometer and imager for the mid-infrared (VISIR) on the Melipal telescope (UT3). VISIR is capable of spectroscopy and imaging in the M -band (5 µm), the N -band (8−11 µm), and Q-band (17−20 µm). The long-slit spectroscopic mode of VISIR can also have a range of resolving powers from 150 to 30,000.

Chopping and Nodding To compensate for the atmosphere as well as changes in sensitivity of different parts of a telescope, imperfections within the primary mirror, as well as any other excess low frequency noise, mid-IR observations often use a method of chopping and nodding. Chopping is the rapid tilting of the telescopes secondary mirror off of the target by several arcseconds and is typically done several times a second. Chopping gives on- and off-source observations that can be used to remove unwanted signal that changes over time and gives a background subtracted image. Alternating the placement of the slit on the target between two positions or ‘nodding’, in addition to chopping, allows for the suppression of the residual background image. This creates a second set of on- and off- source observations (Fig. 2.8), that can be subtracted from each other to produce the final image.

VISIR pipeline The VISIR data that are used in Chapter 7 have been reduced using the reflex software (version 2.8.5) VISIR pipeline (version 4.3.1). The pipeline takes the raw spectroscopic data and executes a full spectroscopic reduction chain which includes stacking exposures, removal of optical distortions, wavelength calibration and spectrum extraction. One step not included in this pipeline is flux calibration using a telluric standard star, which must be done manually.

After stacking the chopped and nodded frames, the data are calibrated. VISIR uses curved slits to cancel the distortion of the pre-slit optics. This however adds distortions along the direction of the dispersed light. The magnitude of the optical distortion is known analytically and the VISIR pipeline corrects for each pixel by interpolating the distortion corrected pixels using the source pixels. After correcting for the optical

2.3 Mid-infrared spectroscopic techniques 57

Figure 2.8:A diagram showing the results of chopping an nodding adapted from an image made by the Observatorio del Roque de los Muchachos (http://www.gtc.iac.es/instruments/canaricam)

2.3 Mid-infrared spectroscopic techniques 58

distortion, the data are wavelength-calibrated.

Using a model based on the atmospheric emission (similar to that in Fig. 2.1) the pipeline fits the spectral signature that maximizes the cross-correlation. The model assumes a black body of 253 K and is multiplied by the emissivity of the atmosphere, Gaussian smoothed, and then multiplied by the detector quantum efficiency. This model is then fitted to the brightest spectral signature. The calibrated spectrum is then extracted using an optimal extraction method. This method takes the level of noise along the dispersion direction into account and weights the final spectrum by this value. The calculation of the standard deviation ignores pixels above 3σ, excluding them from the final extracted spectrum.

Next, the data need to be flux calibrated. This can be done by scaling the extracted spectral flux, in analogue-to-digital units (ADUs) per second, using a standard star.

Using the ratio of ADUs s−1 _{from the standard star observations to a model of the known}

59

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LMC Maser results8