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Field Trip To Observatoire De Haute-Provence Report

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Published: Tue, 10 Apr 2018

  • Shaoshan Zeng

Abstract

A group of 9 students from University College London (UCL) will be visiting the Observatoire de Haute-Provence (OHP) in France between 14th February and 22nd February 2015. Students are expected to use the CCD camera on the 1.20m telescope to image two RR Lyrae variable stars: XY CVn and RR Gem. Also, students will use the Aurelie spectrograph on the 1.52m telescope to obtain high resolution spectra of bright spectroscopic binary systems within the spectral region 4070-4130Å. The main content of this report will focus on the observing facilities available at the OHP and background information of the objects to be targeted. And the last section of this report is the observing diary containing all the data obtained at the telescope.

Contents

  1. Overview of l’Observatorie de Haute-Provence (OHP)
    1. Brief History
    2. Location
    3. Previous scientific work
    4. Observing conditions
  1. Observing Instruments
    1. The 1.20m Telescope
    2. The 1.52m Telescope
    3. The Aurelie Spectrogrph
  1. Scientific Programme for 1.2m Telescope
  1. Scientific Programme for 1.52m Telescope
  1. Targets
    1. RR Lyrae variable stars
    2. Spectroscopic binaries
  1. Observing Diary

References

Appendix 1. Light curves of RR Lyrae variables.

Appendix 2. Finder Charts

1. Overview of l’Observatorie de Haute-Provence (OHP)

  1. History

The observatory is owned by the Centre National de la Recherche Scientifique (CNRS) and is funded by the Institut National des Sciences de l’Univers (INSU). Originally, OHP was built as a national facility for Frence astronomers in 1937 and later on in 1949, the facilities at the OHP were available to be used by foreign visiting astronomers. In 1943, the first astronomical observations were made with the 1.20m telescope and the first research paper were published a year later.

1.2 Location

OHP is located in St Michel, near Forcalquier in the Alpes de Haute-Provence, south-east France. The coordinates of OHP are:

  • Longitude = 0h 22m 52s E
  • Latitude = +43° 55’ 46”
  • Altitude = 650m
  1. Previous scientific work

OHP has done some remarkable work on the detection of extra-solar planets. This studies started with the discovery of 51 Peg-b in 1995 using the ELODIE spectrograph on the 1.93m telescope and followed by many more discoveries such as Glises 876, the third closest known star to the Sun were found in 1998 and HD190228b was announced in 2000 as a giant planet orbiting the host star. In 2006, ELODIE was replaced by a stabilised high resolution spectrograph called SOPHIE at the 1.93m telescope of OHP. It is operated by using a large survey for search for northern extrasolar planets through the radial velocity method. Nowadays, SOPHIE plays a significant contribution to the follow-up of transiting exoplanet candidates from photometric surveys include SWASP, CoRoT and Kepler.

1.4 Observing Conditions

The reason for OHP grounded on this site is mainly because the benefit of having higher probabilities on clear sky and favourable weather conditions throughout the year and the average atmospheric extinction at OHP is approximately twice that for ESO at La Silla. On average, about 60% of nights are considered to be suitable for astronomical observations. This can be showed by the yearly breakdown which based on a statistics made from 1965 to 2004: 170 nights are excellent conditions, 50 nights with very slight cloud and 70 nights are being partly cloudy. Despite of the weather conditions, the image quality is also the key factor to provide good observation. At OHP, the seeing disk is around 2 arcsec and can be lower down to 1 arcsec occasionally which compares to the image quality at ULO is about 4-5 arcsec. However, about 45 days per year on average (commonly in winter) cold wind flows from the northwest which is known as Mistral would cause the degradation of seeing, sometimes the quality of the seeing can severely decline to over 10 arcsec. But the advantage of having the Mistral winds is good weather usually follow as the winds usually clear up the sky.

2. Observing Instruments

There are four main telescopes operate at OHP: 1.93m, 1.52m, 1.20m and 0.80m. Typically, 1.52m and 1.20m telescopes will be used to undertake studies during the field trip.

2.1 The 1.20m Telescope

This is the first telescope installed at OHP and operates since 1943. It only has a Newton focus which is corresponded to a focus ratio of f/6. As an improvement, the telescope is now equipped with a CCD camera for direct imaging and photometry. It is usually operated for studies of variability of X-ray sources, imaging of galaxies and H II regions as well as the faint solar system objects.

2.2 The 1.52m Telescope

This is the telescope that have been in use since 1967 at OHP and is used accompanied with the high resolution Aurelie spectrograph which is positioned at the Coude focus. Thus, most of the spectroscopic studies are carried out by using this telescope. The Coude focus is the only focus of this telescope and the focal ratio is f/27.6 which is almost identical to the 1.52m telescope at the ESO at La Silla. Even though it needed to be pointed manually, all other functions are automated. The telescope is equipped with a CCD camera, used from acquiring the target and automatic guiding. The camera has a field of view of 3’ x 4’.

2.3 The Aurelie Spectrograph

The Aurelie spectrograph is a high resolution spectrograph that has been developed and installed on the 1.52m telescope in 1989 at the OHP. The goal of using such high quality instrument is to obtain spectra at very high resolutions over the spectral range of 3900 to 10000Å and because of the average seeing conditions at the OHP, the Aurelie spectrograph is also designed to obtain the largest optical efficiency and small amount of scattered light with an entrance aperture at about 3 arcsec wide. The detector attached to the spectrograph is called Thomson TH7832 which is a linear array CCD-like detector. The array is made of 2048 pixels of which 2036 are usable. The advantage of using this detector is: it is very clean meaning it exhibits no interference fringes or persistence effects and this would give the benefit to detect very weak absorption lines.

3. Scientific Programme for 1.2m Telescope

The main purpose of this programme is to obtain images of two RR Lyrae variable stars RR Gem and XY CVn by using Cousins B- and V-band filters followed by calibrating these images with respect to stars of known magnitude near to the target star. This will be down over the course of 6 nights with 1.2m telescope. As a result, light curves of these stars will be obtained over several cycles of variation in order to calculate the pulsation period of each of the two stars. The light curve of XY CVn will be compared to that of RR GEM as the light curve of XY CVn is more symmetrical. From experience gained last few years (between 2003 and 2014) of UCL field trips who also completed the task with the same telescope, CCD and filters conclude that the exposure times should be around 1-3 minutes in each of the filter for RR Gem and because XY CVn is fainter than RR Gem, it requires longer exposure time of around 4 minutes in each of the B and V-bands. If the star has magnitude at around 11, a signal to noise ratio of at least 100 should be obtained with a 60 second or even shorter exposure in the V-band. The same signal to noise ratio is also kept for the B-band.

Other than evaluating the result obtaining in the programme, observations of these two targets will also be compared to those obtained during the UCL field trip of 2000 to 2014. This should give more accurate calculated period and overtone modulation of the light curves to the stars.

RR Lyrae stars are pulsating variables with about half the mass of the Sun but probably much older and hotter than the Sun. They belong to low mass Population II and they are abundant in globular clusters. RR Lyrae variables are special because they growing larger and smaller in size with their brightness changing significantly. In general, they have periods of 0.2 to 1 day and spectral types of A2 to F6 which have an average effective temperature of 7000K and a luminosity typical around 80 Lsun. Some of them have similar light curves to those of Cepheid variables and obey a period luminosity relation which is approximately:

Log10 P = -0.85M + constant

These properties make RR Lyrae variables become excellent standard candles as if the period of time it takes for an RR Lyrae to go through its cycle of brightening and dimming is known, then the absolute luminosity of that star can be estimated. The absolute luminosity shows how bright a star would be if it was a certain distance away from us. From this, the distance to the star can be determined if the measure brightness of the star appear to us compared to its absolute luminosity.

4. Scientific Programme for 1.52m Telescope

For this programme, the Aurelie spectrograph on the 1.52m telescope will be used to obtain high resolution spectra of a selection of targets which are known to be spectroscopic binary systems that in the spectral region of 4070-4130Å. By analysing these spectra, weak absorption lines should be resolved to measure the changes in the radial velocity of a star by applying the cross correlation technique. Since the field trip group in 2006 and 2010-2014 also completed the similar task with Aurelie, the analysed results from this field trip will be compared to those obtained in previous years and hopefully this will result in an improvement of phase coverage for all of the systems, especially for targets with long period. By combining all the data sets, a number of physical parameters such as the mass ratio, the mass function, the period and the orbital eccentricity can be estimated for each binary system. As Hδ (λ4101Å) is included in the targeted spectral region, the appearance of this line in obtained spectra also need to be investigated as it changes as a function of spectral type.

In order to resolve the weak lines and measure their wavelengths accurately, a spectrographic resolving power of at least R = 40000 is required. Such high resolution that in use should also provide accurate information of radial and rotational velocities for each target. A wavelength coverage of at least 60Å is needed to include enough weak lines simultaneously and to make sure the continuum level outside the Hδ line could be estimated. Despite of these, useful weak lines can only be obtain with a signal to noise ratio of at least 250 due to the fact that the equivalent widths of the weak lines are expected to be with several mÅ. And more importantly, the total integration time of all of the start should be controlled to be 120 minutes or less because the resolution of orbital phase of 1.52m telescope would become poor if longer exposure time is used.

Spectroscopic binaries involve two stars orbiting around their common centre of mass. These two stars are so close together that can only be seen as one object, and over period of time, there is Doppler shift change in the observing spectrum. In other words, if the star does exist in a binary system, they are of similar luminosity, each spectral line will twice over the course of one orbit, split into two, reach a maximum separation and then move back together again due to the Doppler shift caused by their radial velocity. Among all the targets, the star α Uma is of particular interest as it has a long period of 44 years while other target stars have relatively short periods and has an orbit that is not clearly determined. It thought to have passed periastron in 2000, so it would be interesting to compare its measured radial velocity with that from spectral obtained during 2006 to 2014.

5. Target stars

5.1 RR Lyrae Variable Stars

The stars that will be observing for the investigation of the light curves of RR Lyrae variable stars on the 1.2m telescope are shown below in Table 1 along with their coordinate, magnitudes and period.

Table 1. Investigate RR Lyrae variable stars

Target

RA(J2000.0) h m s

Dec(J2000.0) ° ’ ”

Vmas – Vmin

Period (day)

RR Gem

07 21 33.5

+30 52 59

10.62—11.99

0.40

XY CVn

13 48 01.9

+29 11 47

13.5—14.5

0.36

RR Gem is a type “a” RR Lyrae variable, it is well located for observation between UT 18:00 to 24:00 in late January to February at OHP. This light curve of this type of star will show a steep increase in brightness at first, the brightness will then gently fade away until a minimum is reached.

The finding charts for RR Gem can be found in Appendix 1. (A1) and the calibration stars are shown in Table 2 below.

Table 2. Calibration stars for RR Gem

Calibration star

RA(J2000.0) h m s

Dec(J2000.0) ° ’ ”

B

V

Number of the finding chart

HD 57247

07 21 40.42

+30 52 23.9

9.52

9.37

94

TYC 2452-1557-1

07 21 33.35

+30 54 42.9

10.62

10.33

103

XY CVn is a type “c” RR Lyrae variable, it is well located for observation in the morning in January and February at OHP. Different to RR Gem, the light curve of this type of star is more symmetrical.

The finding charts for both target stars can be found in Appendix 1.

5.2 Spectroscopic binaries targets

The stars for which I am responsible for background research for the programme occurring on the 1.52m telescope are shown in Table 3 below.

Table 3. Spectroscopic binaries targets

Star

RA(J2000.0) h m s

Dec(J2000.0) ° ’ ”

V Mag

Total Integration (minutes)

Spectral Type

SB8 Number

Period (days)

HR 4072

10 24 07.9

+65 33 59

4.97

500

A0p Si:Sr:Hg:

608

11.58

ι CrB

16 01 26.6

+29 51 04

4.99

300

A0p Hg

878

35.47

Both of the targets are circumpolar which will never set throughout the night, this makes them easily observable in night time. For HR 4072, it would be best observed at around 0:00-1:00 UT and ι CrB should be best observed at 6:00-7:00 UT as these time will be the target just across the meridian i.e at their highest point in the sky. However, 6:00-7:00UT would correspond to 7:00-8:00 local time at OHP, would pass the sun rise time and the sky is bright already. Thus, target ι CrB can be observed in the early morning before sunrise when it still high up in the sky before crossing the meridian.

The finding charts for both target stars can be found in Appendix 2.

6. Observing Journal

References

http://www.obs-hp.fr -the OHP website

http://www.simbad.com -online star catalogue

http://www.aavso.org/ -online database for variable stars, obtain light curves and finding charts

2015 UCL Field Trip Observation Plans (Stephen Boyle, September 2014)

Appendix 1

A1. Finding Chart of RR Gem (AAVSO)

A2. Finding chart of XY CVn (AAVSO)


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