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V 1751 Cyg

Students: Gergana Muteva, Daniela Doneva

Team leather: Ivan Ivanov

Special gratitudes for:

Katq Tsvetkova – our scientific adviser for the whole project;

Peter Pesev – about his help in making up the practical exercise;

Veselka Radeva and National Astronomical Observatory of Rojen – for the images that they gave us;

    1. The star V 1751 Cyg  

1.1. History of the star

The fast irregular brightness variations of V 1751 Cyg were discovered in the course of the program for investigations of flare stars and other non-stabile stars in a region, which contains several indicators of recent star formation – T-and OB associations, a large number of Halpha emission stars and nebulae, reflection nebulae and dark nebulae.

In 1980 K. Tsvetkova reported about fast irregular changes in the brightness of a star in the region around Gamma Cyg (Tsvetkova, 1980, IBVS No. 1890).

In 1971 some observations of H alpha stars were made and some new H alpha stars were found in the region around Gamma Cyg of 8 sqr degrees and the centre with the coordinates a 1950 =20 h 22 m.5, d 1950 =39 ° 40' (Kazaryan and Parsamyan, Astrofisika, 7, 671, 1971). It turned out that a sqr degree contains about 8 emission stars. The star with No. 30 from the list of Kazaryan and Parsamyan was the star in which Tsvetkova noticed fast irregular brightness changes, which have not been recorded till time in the General Catalogue of Variable Stars (GCVS, 1969), GCVS 1, 2 and 3 Suppl (1971 – 1976), 62, 63, 64 Name list of variable stars (IBVS, Nos.1248, 1414, 1581). The coordinates that are given are a 1950 =20 h 26 m.3, d 1950 =39 ° 45', m ph =16.2 and intensity of H alpha line in three degree scale 3. This new variable star received the designation V 1751 Cyg given by Kholopov et al. (1981, IBVS No.2024).

1.2. Position of the star

The star is situated in the dark nebulae Barnard 347, which is projected on the NGC 1318c emission nebulae. The right coordinates of the stars are a 2002 =20 h 28 m 1 s.44, d 2002 = 39 ° 53`21``.2. In SIMBAD astronomical information system the following coordinates of the star are given: a 2002 =20 h 28 m 7 s.0, d 2002 =39 ° 55`48``. They do not correspond to the real location of the star, which is given above. The roughly measured coordinates given from Kazaryan and Parsamyan, who first paid attention on the star, probably cause this.

On this map is roughly put a cross mark where the position of V 1751 Cyg is.

1.3. Type of brightness variations

The type of the variability, given on the base of the observations of Tsvetkova (1980) is INS. The definition of this variable star class can be find in the GCVS, vol. I (Moscow, Nauka, 1985).

“IN – Orion variables. Irregular eruptive variables connected with bright or dark diffuse nebulae or observed in the region of these nebulae. Some of them may show cyclic light variation due to axial rotation. In the spectrum – luminosity diagram they are found in the area of main sequence and subgiant. Probably young objects, which in the course of further evolution will become light-constant stars of zero age main sequence. The range of brightness variations may reach several magnitudes. In the case of rapid light changes having been observed (up to 1 m in 1 d – 10 d ), the letter is added to the symbol of the type (INS).”

1.4. Amplitude of variations and photometry results

The U filter monitoring observations of Tsvetkova (1980) are with total effective time 85h50m. They have been done in the period between September 1979 and November 1980 and made by the method of multiple exposures, which gives us possibility to observe a quick variability of the order of one 10 minute exposure. Using the data of these observations the brightness of the star V 1751 Cyg in U filter is changing in the limits:

m max =15 m.5 ; m max =16 m.8

The UBV photographic photometry made be Tsvetkova (Tsvetkova, PhD Thesis, 1985) gives us the following results:

V: 15.86

B-V: 0.20

U-B: -0.36

1.5 Spectra of the star

In the poster “V1751 Cyg and V1826Cyg in the star forming region Cygnus T2” presented during the ESO workshop “Low Mass star formation and pre-main sequence objects” in 1989 (European Southern Observatory, Scientific Preprint No. 673, 1989, poster abstracts) Tsvetkova, Petrov, Tsvetkov and Duemmler reported about spectral observations of V 1751 Cyg.

Image tube spectra were obtained at the “2.6 m telescope of the Crimean Observatory equipped with a SPEM spectrograph with dispersion of 10nm/mm and resolution of about 0.8 nm in August 1988. According to Tsvetkova (2002, private communication) in the spectra the emission line of H gamma, H beta, Fe II492.4, Fe II501.8, Fe II516.9 (blue range 430 – 550 nm), He I587.6, Na I D, [O] 630.0, [N II] 654.8, H alpha, [N II]658.4, He I 667.8 (red range 560 – 680 nm) are seen.

1.6 CCD observations of the star

We obtained CCD images of the star using the 50/70 cm Schmidt telescope of the National Astronomical Observatory Rozhen of the Bulgarian Academy of Sciences in May 2002. The telescope is equipped with the ST8 CCD camera (Pixel Array – 1530 x 1020 pixels, 13.8 x 9.2 mm, Total Pixels – 1,500,000, Pixel Size - 9 x 9 microns, Field of View – 24 x 16 arcminutes, Pixel Size – 9 x.9 arcseconds).

The image in R filter with 180s exposure time is presented here.he star V 1751 Cyg is pointed by an arrow.

 

1.7. Type of variability of the star

In the work of Tsvetkova, Petrov, Tsvetkov and Duemmler (1989) a suggestion is made that the star V 1751 Cyg is most probably a T Tau type variable, with dense circumstellar envelope.

2. T Tauri type of stars  

2.1. History of the star

The variable star T Tauri was discovered on an October night in 1852 by John Russell Hind. Hind, a noted asteroid hunter, is credited with having discovered 11 minor planets. On this particular night, however, while scanning the sky with his telescope through the Pleiades and in the direction of Hyades, Hind spotted a tenth magnitude star that was missing from the chart that he was using. The missing star, as it turns out, was the variable now known as T Tauri. This star is the prototype for a class of very young stars. 

2.2. Main characteristics of the type

The T Tauri stars are young stars, still in process of gravitational contraction, which have cleared away most of the material out of which hey were formed, but which are still active (in other words they are still ejecting material from their vicinities).

In a Hertzsprung-Russell Diagram they are plotted (~2 m ) above the main sequence, where the protostars should be located. T Tauri stars appear almost always within dark clouds. They gather in dark clouds, because there the star formation is most active.

It’s a Hertzsprung-Russell Diagram where the position of T Tauri stars is shown.

This is the reason to think that these stars are ones of the youngest stars, observed in the Galaxy. This belief is strengthen by the circumstance that the T Tau stars are usually found in groups – T-associations

T Tauri stars are mostly:

  1. between 10 5 and 10 8 years in age;
  2. of low mass (0.5 to 0.3 solar masses);
  3. surrounded by hot dense envelopes; and
  4. losing mass via stellar winds with typical v= ~100km/s

“In fact an infant star going trough its T-Tau stage can loose as much as 0.4 solar masses before it settles down on the main sequence” (Kaufmann p.258)

As a generalisation of the facts above we can say:” Stars with masses roughly 0.2 to three times the Sun’s and with ages 100 000 to

1 000 000 years typifies the T Tauri regime”( Cohen 1981)

Our own Sun presumably passed through the T Tauri stage some 4,5 billion years ago. Therefore, these stars may be able to offer us a peak into the evolution of our own Sun, solar system, as well as other planetary systems.

2.3. Brightness changes

Brightness changes detected in these stars are not due to evolutionary effects, per se, but may be due to such processes as instabilities in the disk, violent activity in the atmosphere of the star, and may also be due in part to moving clouds of dust and gas from which they were conceived.

Unlike many other types of variable stars, classification of T Tauri variables can not be based only upon light curves. The behaviour of these young stars is just too erratic as found by Alfred Joy in 1945:”The variations in light of the T Tauri stars are so irregular and unpredictable that classification by means of their light curves is practically impossible…”

The figure above shows the variations in the brightness of the T-Tauri star, which is believed to be the prototype of this class. We can see that the changes are totally erratic. 

2.4. Classification system

Since T Tauri-like stars can not be classified based on their light curves, astronomers have scrambled to find a common link between these unusual stars. In 1945, Alfred Joy was the first to systematically study and categorise these stars.

In Joy’s 1945 paper, he states the following:

“…The distinctive characteristics are:

  1. irregular light-variations of about 3mag.;
  2. spectral type F5-G5 with emission lines resembling the solar chromosphere ;
  3. low luminosity and
  4. associations with dark or bright nebulosity …There are situated in or near the Milky Way dark clouds…These stars differ from other known variables , especially in their low luminosity and high in the intensity of bright H and K in their spectra…”

2.5. Spectra of the T Tauri stars.

Hundreds of T Tauri stars are now known, learned and classified mainly through spectroscopic searches. In fact it is found that the spectra of these stars can range from F through M with B-V ranging from ~0.7 (ru Lup) to ~1.33 (V410 Tau). The luminosity class is IV – V. In the spectra of the star usually can be found intense lines of Fe I 4063, Fe I 4132, also the forbidden lines of [O I] and [S II]. There is also various emission lines of H alpha, He I, Ca II 3933, Ca II 3968, Fe II, Ti II. Strong absorption line of Li I 6707. The width of the lines in the spectra of T Tauri stars and the P Cygni profile in the Halpha line prove the mass lost. 

This figure shows the spectra of the star T Tauri. 

3. Comparison between T Tauri stars and other eruptive variable stars.  

A type of star that has similar characteristics to T Tauri type of stars is FU Ori. The characteristics that connect this star and T Tauri are the strong absorption line of Li I in the spectra and also the availability of expanding surrounding. The stars are of that types are supposed to be pre-mail sequence stars. These stars are surrounded by hot and dense envelope and accretion disc. Although the FU Ori phenomena is rare perhaps a lot of T Tauri stars go through this episode of their lifetime. It is known, for instance, that the FU Ori star V1057 Cyg was a confirmed T Tauri star prior to outburst. Herbig suggested that perhaps the FUoris recur in the average T Tauri star after roughly 10 4 years. Kenyon (1999) stated that based on statistics for known FUors, the young stars probably undergo 10-20 FUor eruptions before reaching the main sequence.

The T Tauri stars differ from the other eruptive variable star, such as nova and supernova, because they are pre-main sequence star while the other eruptive stars which are in the end of there life.

4. How can we define the type of V1751 Cyg?

We believe that the type of V1751 Cyg is most probably T Tauri type. It can be proved by some facts:

  1. There are fast irregular changes in the brightness of the star V1751 Cyg. They are typical for T Tauri type of stars too.
  2. In the spectrum of V1751 Cyg and T Tauri type of stars there are emission lines of H gamma, H beta, Fe II, He I, [O], H alpha, [N II]. The spectrum of V1751 Cyg was observed with low resolution. So the similarity between the spectrum of V 1751 Cyg and T Tauri stars should be further investigated.
  3. The amplitude of the brightness changes in U filter is D m=1 m.3. This is typical for T Tauri stars.
  4. The star V1751 Cyg has a dense envelope as the spectrum suggests. This conclusion is based on the observed proportion between the intensity of the emission lines of He I587.6 and He I667.8.
  5. The star is situated in the dark nebulae of Barnard 347, which is projected on the NGC 1318c emission nebulae. Usually the existence of such nebula indicates a region of active star formation.
  6. When we plot V1751 Cyg on the Hertzsprung-Russell Diagram V/B-V (V on the Y-axis, (B-V) – X-axis.) we can see that the star is situated above the main sequence in the region of T Tauri stars.

The mass of the star can not be defined from the existing observations. The mass would be a strong argument for or against the classification of the star type.

5. Conclusions

As it is seem from the presented collection of observational data for the star V 1751 Cyg, further intensive observations are needed. In this direction we consider the presentation of the right coordinates of the star given here as very important step.

6. Practical exercise: Star photometry of CCD imges.

6.1. Background

All the information we obtain about the sky objects is based on the exploration of weak electromagnetic flows that reach us through the outer space. The star photometry of astronomical images is a weighty spared method about measuring of the registered signals. We can make important conclusions about the nature of objects (stars, star clusters or even a whole galaxy), the surrounding where the beams have been traveling and about the physics processes that occurs in them. The images we work with could be obtained on photografic plates or with CCD receivers. The CCD matrix are prodused in 1970 in Bell Labs as an attempt to be found a compact substitute of ferite memories. They have a wide currency during the last two decades and they are the main receiver of of light in the ground and space obserwatories. That’s why we will mainly examine the reduction of CCD images. But we should say that we have not to underestimate the role of photographic emulsions, that where wide spread detectors during the bigger part of 20 th century. Due to their help a huge observation material is piled up. It is frequently a very important part of solving various problems.

The first astronomical image on CCD matrix is obtained in 1975 by the team of JPL with 61inch telescope of Arizona University Mt. Lemmon. It was an image of the planet Uranus at the wavelength 8900A (890nm). The advantages of CCD matrixes are:

  1. High amount of received quantums.
  2. Lineal reaction of the receiver in the hole dynamic wavelengths.
  3. Wide field of the images.
  4. The images are received directly in numerical type.

6.2. Threatment of CCD images.

The treatment of the numerical images of CCD consists of the following steps:

Task1: Removing the darc field from each frame. For this puprose we should get such images that no signal can reach the receiver. They should have the same exposure as the images that are reduced. The signal that is collect in the pixels entirely due to defects of the matrix. Because of the linear reaction of the receiver we can also use proportional parts of the dark field with longer exposure. The dark field is minimized in the modern matrixes and BIAS (dark field with zero exposure) is used. This is a chock that is put artificially in the receiver.

Task2: Divide the made up in this way frame on flat field. It should be done to equal the reaction of each pixel of the matrix. For this purpose we irradiate the surface of the receiver with homogeneous in intensity and spectral composition light and we detect how each single element reacts.

6.3. Photometry of such made up images.

After that we can proceed to the photometry of such made up images.

There are different programs for astronomy photometry measurements.But they all use in some way the method of the aperture photometry. It consists in the following.In the focal plane of the telescope because of some reasons the image of the star is actually a two-dimensional gausian curve.

Practically to determine the instrumental magnitude of the star we have to measure the volume of the figure that the curve determines.

For this purpose we use the following methods:

Task1: We determine the maximum of the two-dimensional curve

Task2: Gradually we measure the volume which the curve circumscribes with with increasing radiuses centered upon the maximum value of the curve

Task3: We build an graph of the volume according to the radius of the aperture. When the volume ceases growing we consider that we have reached the level of the bachground and we have determined the instrumental magnitude of the object.

Task4: The instrumental magnitude of the star depends on the observation system that is used (telescope, filter , receiver ) and for being able to compare the results with another observations we have to turn them into standard photometry system (UBV system for example).In which system we are going to carry out the observations depends mainly on the object., which we are observing and the physical prosseces in it..For turning the results into standard system we use the so called standards. These are stars which star magnitudes are previously known with high accuracy.. If we have a standard star in the frame we can determine the standard star magnitude by means of comparison between the signal from the object and the signal from the standard. If we don’t have standards in the field we turn to some of the well-known standard fields and determine the equations of the transformation. These equations allow us to determine the magnitude of our object in every moment of the observation. For this purpose its good to take a shot of the standard in two different zenith distances. Especially for observations of variable stars it is useful to place a standard near the observed object in the field of the receiver. This is the way to avoid the influence of the accidental changes in the condition of the atmosphere., because they will effect equally to the object and to the standard.

6.4. Examples.

These are some pictures of V1751Cyg. The first is the original image. The second is threatened according to Task 1 in part 7.2. and the third one in threatened according Task 1 and Task2 in Part 7.2. In our future work we are going to make the tasks in part 7.3.

 

 

 

7. References.

7.1. Astronomy, N. Nikolkov, M. Kalinkov, “St Klimen Ohridski”, Sofiq 1998g.

7.2. Tsvetkova, 1980, IBVS No. 1890.

7.3. Kazaryan and Parsamyan, Astrofisika, 7, 671, 1971.

7.4. Kholopov, 1981, IBVS No.2024.

7.5. GCVS, (Moscow, Nauka, 1985).

7.6. http://peripatus.gen.nz/astronomy/ttausta.html

7.7. http://www.aavso.org/vstar/vsotm/0201.stm

7.8. http://www.aavso.org/vstar/vsotm/0202.stm