IRTF Observations Lab Report

Shannon Dulz

Abstract

Observations of exoplanet candidates were taken at the NASA IRTF on August 27, 2015. We obtained spectra of 3 targets: LHS371, GJ740, and EQ Peg A, as well as a standard star, AlfCrB. Also flats and darks were taken for future processing of the collected data.

Introduction

The purpose of these observations is to confirm the nature of suspected planetary candidates around m dwarf stars. This is done through the radial velocity method of exoplanet finding, which involves taking spectra of a star at several points along its planet’s orbital period. The gravitational effect of the orbiting planet pulls the host star slightly. The spectra from the star then appears red-shifted or blue-shifted at different points on the orbital period which can be measured from earth to determine minimum mass of the exoplanet. In this study, the exoplanets have already been observed via other methods and this study provides confirmation that these are truly planetary objects. We conducted observations on the night of August 27, 2015. Observations were conducted on the NASA Infrared Telescope Facility on Mauna Kea in Hawaii. A spectrograph called CSHELL was used to take spectra of several targets.

Procedures

The observations for this study including on this night follow a common sequence. This sequence is as follows:

1.     Logging into CSHELL and the gas cell laptop and ensuring the gas cell is in the light beam.

2.     Moving to look at a standard A star, in this case AlfCrB

3.     The telescope operator focuses the telescope, we also refocused later when moving to the first science target

4.     Observing the A standard star

5.     Offsetting the telescope so that the deepest gas star line falls on a particular pixel of the spectrograph

6.     Closing the slit and taking spectra while making small adjustments to maximize the flux through the slit

7.     Calculating the signal to noise ratio (SNR) and taking several cycles of spectra

8.     Turning on the gas lamp and taking flat images

9.     Repeating the procedure to look at science targets

10. Taking flats after each new target

11. Taking darks at the end of the night

At all points, along the procedure it was important to keep an eye on the weather as high humidity or fog can require the telescope dome to be closed.

Results and Discussion

We looked at 3 science targets: LHS371, GJ740, and EQ Peg A; with 20, 5, and 15 images respectively. Observations of GJ740 were cut short and flats quickly taken because high humidity necessitated closing the dome for approximately one hour. Afterwards, observations resumed as normal by moving on to target EQ Peg A. Below is a screen capture of one of the first spectra obtained on this night of LHS371.

Figure 1: Screen capture from CSHELL desktop observing LHS371

The top half of the above figure is the spectral image obtained, while the bottom graph is the Gaussian fit of the flux along a specific wavelength (vertical line in the spectra). The sum of the area under this Gaussian curve can be used to obtain an estimate of the SNR for each exposure by taking the square root. A higher SNR can be obtained by maximizing the flux through the spectrograph via small movements of the telescope. The figure below shows making these small adjustments.

Figure 2: Screen capture of moving the telescope to maximize flux

The number of cycles needed for each target was calculated to obtain a summed SNR of approximately 200.

Conclusions

Once corrected via flat field and dark image corrections these spectra should be useful for confirming the exoplanets LHS371, GJ740, and EQ Peg A. While seeing was far from perfect due to the high humidity, these spectra may still contain useful results.