156

YOUNG STARS IN GAS-DUST DISKS. II. Vega (ααααα Lyr) and βββββ Pic

E. V. Ruban and A. A. Arkharov

Observational data for Vega, α Lyr in the 325-1080 nm range from various years are studied using

material stored at the Pulkovo Spectrophotometric Data Base (PSDB). It is shown that the

spectrophotometric temperature of the star was lower than the effective temperature according to data

from all the observational seasons. Possible reasons for this are examined: light scattering by small

particles in the ultraviolet and reflection by large particles at longer wavelengths. A difference of 0m.01-

0m.02 in the quasimonochromatic magnitudes of the star is obtained for observations in different seasons.

This difference may related to a large-scale inhomogeneity in the distribution of dust and gas in the disk.

Photometric data for the star β Pic in the Hipparcos catalog reveal periodic variations in the Vt magnitude

with a period of 4.46 d and an amplitude of 0m.0085. Three assumptions are made to explain this result:

pulsations in the star’s photosphere, a planet at a distance of ~0.1 a.u., and a region with an elevated

density of particles in the circumstellar disk that periodically eclipses the star as it undergoes Kepler

rotation. These assumptions require further confirmation.

Keywords: Vega; β Pic; spectrophotometry; photometry; microvariability

1. Introduction

The second part of this paper is a study of spectrophotometric and photometric data from observations of Vega

(α Lyr, HD 172167, BS 7001) and β Pic (HD 39060, BS 2020), which are surrounded by extended gas-dust clouds.

As noted in the first part of this paper, there is a planet in the disk of β Pic. The giant planet β Pictoris b with a

mass of 7-10 times that of Jupiter was discovered in 2010. It lies at a distance of 8-15 a.u. from the star and orbits

Astrophysics, Vol. 55, No. 2, June , 2012

0571-7256/12/5502-0156 ©2012 Springer Science+Business Media, Inc.

Original article submitted September 28, 2011; accepted for publication April 4, 2012. Translated fromAstrofizika, Vol. 55, No. 2, pp. 175-188 (May 2012).

Main (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, Russia; e-mail: ubane@mail.ru arkadi@arharov.ru

157

it with a period of ~17 years [1,2]. No planet has yet been found near Vega, but the large, solid particles made up

of primordial gas and dust in the disk surrounding the star suggest that its gas-dust disk is similar to the disk

surrounding β Pic, which is a protoplanetary system.

As noted in the first part of this paper, these studies are based on Vt- and Bt-band photometric data contained

in the Hipparcos catalog [3] and on spectrophotometric data in the 325-1080 nm range stored in the Pulkovo

Spectrophotometric Data Base (PSDB), which has served as the basis for compiling the Pulkovo Spectrophotometric

Catalog (PSC) [4,5].

2. Vega

2.1. Results of the spectrophotometric observations. The study of observational data on Vega ( α Lyr,

BS = HR 7001 [6], HD 172167) in this paper is based only on spectrophotometric data from the PSDB, since no Vt

and Bt photometric data for Vega are given in the Hipparcos catalog [3]. The H-band data given there also have

systematic errors, which we detected but could not eliminate owing to their complicated character, as opposed to the

systematic errors in the Vt and Bt bands for Fomalhaut [7], which we were able to eliminate. In addition, the H-band

is too broad for studying subtle effects of the sort examined here.

It has been repeatedly noted that the results of spectrophotometric observations are stored in the PSDB in the

form of individual seasonal catalogs. Each catalog combines the observational data obtained during a single season

for all the stars observed in that season.

The duration of a season was determined by the fixed observation site, telescope, spectral range, and detection

system and could range from a few months to several years. Data on the quasimonochromatic flux E(λ) of a star, after

eliminating the effect of the earth’s atmosphere and accounting for the spectral sensitivity of the apparatus, are given

in absolute energy units of erg/cm2/s/cm.

In seasonal catalog k, the following data are given at each wavelength λ within the spectral band being studied

with a step size of 2.5 nm (see Table 1): the quasimonochromatic magnitude of the star averaged over all observations

(i is the series number of an observation, Nk is the number of observations), i.e., ( ) ( ) k

N

ik Nmmk

∑ λ=λ1

, where

( ) ( )λ−=λ ii E.m log52 ; as well as the mean square (standard) error of the average magnitude, ( ) ( ) kNk NSSk

λ=λ ,

where ( )λkNS is the standard error for a single observation.

The mean weighted magnitudes of each star obtained from the data in the seasonal catalogs and the mean

square error of these magnitudes are listed the PSC [4,5].

It is evident that the error in the observations will be determined both by random factors and by the variability

in the emission when microvariability is present.

In this paper we study the microvariability factor. For the chosen stars, in order to monitor the presence of

possible systematic errors, we have used data obtained in the same seasons. Large systematic errors (>0m.02) were

eliminated in the course of analyzing the data and creating the PSC [4,5]. Small errors ( 020m.≤ ) were not considered

158

k Years λλ , nm Telescope Nk

Group

1 1971-1973 325-737,5 AZT-7 124

4 1985-1986 " " 16 I

9 1990-1991 " " 22

16 1987e 510-1080 Zeiss-600 28

17 1988 " " 47 II

19 1989 " " 22

TABLE 1. Observational Seasons

λ, nm

mλ

��

��

���� ����

T = 10000KT = 9500KT = 9000K

�

��

��

��

���� ���� ����

k=16k=17k=19

��

b

mλ

����

T = 10000KT = 9500KT = 9000K

����

���

����

����

k=1k=4k=9

a

��� �� �� ��� ��� ��� ���

Fig. 1. Average continuum energy distributions,

)(λλm , of Vega in the ultraviolet (a) and infrared (b)

segments of the spectrum obtained in seasons k; theerror bars indicate the error in the averagemagnitudes; the different plot points show the

theoretical values of λm for different temperatures T

(normalized to λ = 555 nm).

159

then. Now, as we are studying microvariability, it is important to protect ourselves against these errors.

Table 1 lists the series numbers k of the selected seasons [7] in which Vega was observed. Also indicated there

are the observation years (“e” denotes the second half of a year), the spectral ranges λλ, the telescopes, the number

Nk of observations of the star during a given season, and the groups combining the catalogs.

The seasonal average magnitudes mk(λ) are plotted in Fig. 1 as functions of the real continuum wavelength

in the ultraviolet (a) and infrared (b) parts of the spectrum. The plot points are joined smoothly by curves of various

types for the different seasons k. The error bars indicate the errors in the average magnitudes (the other notation is

discussed below).

2.2. Effect of the variability factor. The influence of the variability factor is quite evident from the difference

in the average magnitudes obtained in different seasons (see Fig. 1b). The spread in the magnitudes within the seasons

shown here was random according to the Fischer criterion (Eq. (6) in Ref. 7).

Fig. 2. Average continuum energy distributions,

)(λλm , in the infrared obtained in seasons k for the

stars (a) BS 1791 and (b) BS 7557. The parenthesesindicate the number of observations and the errorbars indicate the error in the average magnitudes.

λ, nm

��

�

���� ����

��

��

��

���� ���� ����

k = 16(5)k = 17(5)k = 19(2)

�

bBS 7557

mλ

mλ

���

���

��

���

��

BS 1791a

k = 16(26)k = 17(60)k = 19(30)

160

It can be seen that the data from the 17-th season differ substantially from the others: the fluxes of monochromatic

light from Vega in the 17-th season are systematically higher, by ~2%, than the fluxes observed in the other seasons.

For comparison with other stars, Fig. 2 shows the corresponding infrared data for the stars BS 1791 (a) and

BS 7557 (b) observed in the same seasons. The parentheses indicate the number of observations of the star and the

other notation is the same. Clearly, here there are no systematic differences among the data from different seasons.

The significance of the differences in the average magnitudes for Vega in seasons 16 and 17, as well as 19

and 17, was verified using Student’s criterion [8]. According to this test, if the difference in th average magnitudes

exceeds T (Eq. (2) of Ref. 7), then the difference is significant. The following quantiles of the Student distribution

were used for calculating T with a confidence of 95%: t0.95

(27) = 2.05, t0.95

(46) = 2.02, t0.95

(21) = 2.08. The values of T16,17

and T19,17

calculated using Eq. (2) of Ref. 7 at each point of the real continuum and averaged over wavelength

are 0140m,1716 .T ≈ and 0190m

,1719 .T ≈ . The differences in the average magnitudes obtained for the same seasons and

averaged over wavelength are ( ) ( ) 03201716 m.mm ≈− and ( ) ( ) 02801719 m.mm ≈− . Therefore, m(k) - m(17) > Tk,17

for

k = 16 and k = 19; that is, the difference in the averaged magnitudes is significant with 95% confidence.

2.3. Microvariability. The variability factor, which influences the average magnitudes, also affects the

dispersion in the results when they are combined over several seasons. An approximate estimate of this influence was

made using single-factor dispersion analysis [8], which decomposes the total dispersion into separate components

characterizing the random factor and the variability factor.

In part I [7] the random component of the dispersion, ( )λ20S , was determined for the two groups of seasons

listed in Table 1. The number of degrees of freedom for the random dispersion in these groups are fI

= 275

(group I) and fII

= 143 (group II).

TABLE 2. Variations in Stellar Magnitude, λ± ,MS , for Group I

λ, nm meanm ,λ λ± ,MS λ, nm meanm ,λ λ± ,MS

325.0 1.282 0.014 460.0 0.616 0.012

327.5 1.277 0.017 580.0 1.328 0.009

417.5 0.334 0.009 582.5 1.339 0.007

425.0 0.379 0.014 600.0 1.423 0.010

445.0 0.517 0.013 627.5 1.567 0.012

447.5 0.540 0.015 630.0 1.585 0.017

450.0 0.552 0.012 635.0 1.601 0.008

452.5 0.563 0.010 675.0 1.798 0.009

455.0 0.587 0.014 680.0 1.829 0.009

457.5 0.600 0.012 687.5 1.879 0.020

161

TABLE 3. Variations in Stellar Magnitude, λ± ,MS , for Group II

λ, nm meanm ,λ λ± ,MS

1 2 3

510.0 0.922 0.012

512.5 0.932 0.012

515.0 0.942 0.009

517.5 0.951 0.008

520.0 0.964 0.010

522.5 0.979 0.014

525.0 0.997 0.020

527.5 1.012 0.016

530.0 1.021 0.012

532.5 1.034 0.010

535.0 1.051 0.012

537.5 1.065 0.016

540.0 1.075 0.007

542.5 1.094 0.019

545.0 1.108 0.018

547.5 1.123 0.016

550.0 1.132 0.013

552.5 1.146 0.011

555.0 1.162 0.010

557.5 1.184 0.012

560.0 1.197 0.006

562.5 1.212 0.006

565.0 1.229 0.008

567.5 1.243 0.011

570.0 1.259 0.009

572.5 1.274 0.006

575.0 1.288 0.009

577.5 1.299 0.007

580.0 1.310 0.007

585.0 1.337 0.008

587.5 1.350 0.009

590.0 1.361 0.011

592.5 1.372 0.013

595.0 1.384 0.012

597.5 1.398 0.007

λ, nm meanm ,λ λ± ,MS

1 2 3

600.0 1.413 0.010

602.5 1.417 0.014

605.0 1.432 0.014

607.5 1.445 0.014

610.0 1.462 0.017

612.5 1.474 0.014

615.0 1.489 0.013

617.5 1.500 0.012

620.0 1.512 0.012

622.5 1.520 0.014

625.0 1.538 0.010

627.5 1.558 0.016

630.0 1.572 0.013

632.5 1.577 0.013

635.0 1.587 0.012

637.5 1.595 0.008

640.0 1.606 0.013

642.5 1.618 0.011

645.0 1.634 0.016

675.0 1.800 0.012

677.5 1.812 0.008

682.5 1.834 0.013

685.0 1.856 0.013

687.5 1.863 0.014

697.5 1.919 0.008

702.5 1.941 0.017

705.0 1.953 0.011

707.5 1.962 0.010

710.0 1.976 0.017

712.5 1.997 0.010

715.0 2.006 0.010

737.5 2.127 0.006

740.0 2.137 0.008

742.5 2.145 0.015

745.0 2.157 0.016

λ, nm meanm ,λ λ± ,MS

1 2 3

747.5 2.166 0.016

777.5 2.286 0.012

780.0 2.293 0.015

782.5 2.304 0.015

785.0 2.316 0.013

787.5 2.325 0.013

790.0 2.344 0.013

792.5 2.355 0.015

795.0 2.362 0.017

797.5 2.371 0.011

800.0 2.381 0.012

802.5 2.392 0.016

805.0 2.405 0.015

807.5 2.410 0.009

825.0 2.488 0.011

827.5 2.492 0.014

830.0 2.503 0.016

832.5 2.511 0.016

835.0 2.518 0.018

837.5 2.530 0.014

840.0 2.533 0.010

842.5 2.535 0.009

845.0 2.543 0.011

847.5 2.550 0.011

850.0 2.556 0.007

852.5 2.564 0.010

855.0 2.571 0.010

857.5 2.578 0.013

860.0 2.581 0.012

862.5 2.585 0.011

865.0 2.590 0.014

877.5 2.609 0.013

880.0 2.603 0.012

882.5 2.629 0.007

885.0 2.655 0.008

162

In order to estimate the dispersion owing to variability, it is necessary to determine the dispersion in the

average magnitudes. The dispersion ( )λ2AS in the average magnitudes for Vega was estimated using Eq. (5) of Ref.

7 with the numbers of catalogs and observations, k and Nk, given in Table 1. The calculated dispersion had f

I = 2

(group I) and fII

= 2 (group II) degrees of freedom. The significance of the ratio ( ) ( )λλ 20

2 SSA of the dispersions was

tested in each group using the Fisher test (Eq. (6) of Ref. 7): ( ) ( ) ( )095020

2 , ffFSS A.A >λλ . The Fisher quantiles for

a 95% confidence for both groups are roughly equal, with ( ) ( ) 0314322752 950950 ..F.F .. ≈≈ . The ratio ( ) ( )λλ 20

2 SSA

of the dispersions (on the average, over all wavelengths) for group I is ~7 and for group II, ~16; that is, the

inequality ( ) ( ) 0320

2 .SSA >λλ was satisfied for both groups. This means that the variability factor had a significant

effect on the dispersion in the average magnitudes with a probability >95%.

Therefore, the variability in the light was found, with high probability, to have an influence on the average

magnitudes of the star and on the dispersion in these magnitudes according to both the Student and Fisher tests.

For those wavelengths at which the inequality (6) of Ref. 7 was satisfied, we estimated the dispersion in the

variability factor, ( )λ2MS , using the approximation (7) of Ref. 7. The square root of these dispersions give the

variations in the star’s emission, λ± ,MS . These dispersions are given in magnitudes (m) in Table 2 for group I and

in Table 3 for group II. The mean square magnitudes meanm ,λ of Vega in these groups are also listed in these tables.

The results in Tables 2 and 3 are illustrated in Fig. 3. The observed mean weighted magnitudes meanm ,λ for

both groups of catalogs are plotted in Fig. 3a. The variations λ± ,MS in the observed magnitudes for the same groups

are plotted in Fig. 3b. The other notation is discussed below.

We now analyze these results.

TABLE 3. (Conclusion)

λ, nm meanm ,λ λ± ,MS

1 2 3

932.5 2.748 0.022

970.0 2.880 0.023

972.5 2.885 0.026

975.0 2.895 0.031

977.5 2.897 0.023

980.0 2.896 0.018

982.5 2.902 0.016

985.0 2.906 0.017

987.5 2.913 0.015

990.0 2.922 0.014

992.5 2.928 0.018

995.0 2.936 0.016

λ, nm meanm ,λ λ± ,MS

1 2 3

1022.5 3.018 0.023

1025.0 3.025 0.025

1027.5 3.032 0.025

1030.0 3.041 0.022

1032.5 3.049 0.020

1035.0 3.057 0.026

1037.5 3.065 0.022

1040.0 3.074 0.019

1042.5 3.083 0.019

1045.0 3.085 0.016

1047.5 3.101 0.021

1050.0 3.112 0.019

λ, nm meanm ,λ λ± ,MS

1 2 3

1052.5 3.121 0.021

1055.0 3.130 0.022

1057.5 3.135 0.018

1060.0 3.142 0.014

1062.5 3.157 0.022

1065.0 3.166 0.019

1067.5 3.175 0.017

1070.0 3.183 0.014

1072.5 3.194 0.013

1075.0 3.206 0.018

1077.5 3.216 0.015

1080.0 3.221 0.020

163

2.4. Spectrophotometric temperature. Besides the observational results, theoretical monochromatic magnitudes

mλ for temperatures of 9000, 9500, and 10000 K according to the data of Kurucz [9] are plotted in Figures 1 (a and

b) and 3a. (Here and in the following, the data are normalized to the averaged observed magnitude at a wavelength

of λ = 555 nm.)

A comparison of the observed and theoretical curves in Fig. 3a shows that the energy distribution in the

continuum spectrum of Vega corresponds to the theoretical curve for a temperature of ~9000 K. According to the

latest data [10], the rapidly rotating Vega, with its pole oriented toward the observer, has a polar temperature of 10000

K, and an average of ~9650 K. Spectrophotometric observations, in which the luminous flux from the entire star is

measured at each wavelength, should yield a temperature close to the average. But here the spectrophotometric

Fig. 3. (a) Mean-weighted continuum energy

distributions of Vega, )( , λλ meanm , for groups I and II

and theoretical values of mλ for temperatures T(normalized to λ = 555 nm); (b) variations in the

emission from Vega, λ ,MS , for groups I and II and

theoretical variations in the magnitudes, λΔ m , when

the temperature is varied by ΔT.

λ, nm

���

���

��� ���

���

���

����

����

����

Δmλ

�

T=10000K T=9500K T=9000K

(I)mmean ,λ

(II)mmean ,λ

(I)S M, λ

(II)S M, λ

500KT −=Δ500KT +=Δ

����

mλ

���

���

���

���

���

��

��

164

temperature of Vega turned out to be lower than the effective temperature. Furthermore, it differs in different parts

of the spectrum (cf. Figs. 1a and 1b). Thus, non-temperature factors have had an influence on the spectral energy

distribution.

In order to check whether the change in the monochromatic magnitudes at different wavelengths is caused

by changes in the temperature of the photosphere, theoretical variations in the magnitudes, λΔ+ m , were calculated

for temperature changes of ±500 K and are plotted in Fig. 3b. The plotted dependences of the observed

variations ( )λ± λ ,MS in the magnitudes are symmetric with respect to the horizontal axis, in the directions of both

increased and decreased radiative flux. The resulting symmetry is related to the method used for determining the

variations, i.e., taking the square root of the dispersions. However, the theoretical dependences are, strictly speaking,

not symmetric. We can still assume that the asymmetry is small for small temperature variations.

A comparison of the curves in Fig. 3b shows that the observed dependence ( )λλ ,MS does not agree with the

theoretical dependence ( )λΔ λm as the temperature is varied: in the theoretical curve, as opposed to the observed data,

the variation in the magnitude increases toward shorter wavelengths. This implies that the variations in the observed

magnitudes are not related to variations in the photospheric temperature.

2.5. Effect of a surrounding gas-dust disk. A drop in the spectrophotometric temperature indicates that layers

outside the photosphere are acting on the photospheric emission. These layers can be layers from a gas-dust disk

surrounding the star. Layers of the disk can also absorb or scatter light from the star [7]. Absorption by the gas, as

in the case of Fomalhaut, can show up as a reduction in the continuum level in the extreme ultraviolet portions of

the spectrum (see Fig. 3a) and may be related to bunching of faint absorption lines of ionized atoms. Here, given

the small variations in the emission in this region (see Fig. 3b), we may assume that the density of the gas along

the line of sight varies between different observational seasons. Given the high temperature of Vega, we may assume

that there are quite a lot of ionized atoms in the disk. However, because of the orientation of the disk in the plane

of the figure, there are not enough of them along the line of sight, so that absorption by the gas has negligible effect

on the observational results.

Fine particles with radii 10.≤ mm contribute the most to the scattering of light in the visible and near

ultraviolet regions of the spectrum with λ < 650 nm [11]. This kind of scattering can obviously explain the reduction

in the observed light flux from the star relative to the theoretical level in the ultraviolet (see Fig. 1a).

At long wavelengths λ > 650 nm, the main contribution is from large particles with radii 1≥ mm [11]. The

increased flux from the star observed at long wavelengths (see Fig. 1b) is most likely related to the superposition

of light reflected from the dust disk on the light from the star. The fraction of reflected light is obviously determined

by the albedo of the particles and the amount of these particles (the density of the medium). A difference in the

contributions from reflected light in different seasons may be caused by a large-scale inhomogeneity in the distribution

of dust over the disk and to the participation of the dust in Kepler rotation.

165

3. βββββ Pic

3.1. Results of the spectrophotometric observations. The star b Pic appears in the PSDB only in one season,

k = 1. The number of observations was 4 (Nk = 4). The real continuum wavelength-averaged standard error for a single

observation of the star, S1, is equal to 0m.0239. The random error, S

0(λ), in the observational results in the season for

group I was determined in the first part of Ref. 7. On the average, it is 0m.0345. Clearly, S1

< S0. Thus, no factors other

than random are involved in the spread in the data.

The average energy distribution in the real continuum of the star obtained in season 1 is plotted in Fig. 4.

The error bars indicate the errors in the average magnitudes. Also shown here are the theoretical monochromatic

magnitudes at different wavelengths for temperatures T = 8000 and 9000 K according to the data of Kurucz [9]. The

effective temperature of the star is 8073 K [12]. It is clear in the figure that the star’s spectral energy distribution

is essentially the same as the theoretical curve for the corresponding temperature.

3.2. Results of the photometric observations. In the Hipparcos catalog [3], the series of observational data

for the star β Pic in the Bt and Vt bands includes 164 terms. Fourier analysis was used to search for microperiodicity

in the series. Figure 5a is a power spectrum 105PW for the Vt series. The maximum peak (indicated by a dashed vertical

line) corresponds to a period p = 4.462 d and exceeds the noise level by a factor of 6.54. The error in the period

determined from the width of the peak is ±0.03 d. A phase curve for a period of 4.62 d is shown in Fig. 5b. The abscissa

is the phase ph, and the ordinate is the difference Vt – Vtmean

(Vtmean

is the average magnitude in the series of

observations). A sinusoidal regression fit with an amplitude of 0m.0085 is drawn through the points. The correlation

ratio determining the strength of the coupling of the points to the regression curve is 0.281; that is, the coupling

is significant with a confidence exceeding 95% [8]. (The power spectrum in the Bt band contained several peaks,

Fig. 4. The continuum spectral energy distribution ofβ Pic obtained in the first (k = 1) season, mλ(λ), andtheoretical magnitudes mλ for temperatures T; the errorbars indicate the errors in the average magnitudes.

λ, nm

mλ

���

��� ���

λm (k=1)

T=8000KT=9000K

���

���

���

��� ��

���

166

among which a main peak could not be distinguished, so it is not shown here.)

4. Discussion

4.1. An unknown planet? The period of the microvariability obtained for β Pic is p = 4.46±0.03 d with an

amplitude of 0m.0085. If this variability were associated with a planet orbiting the star, then, according to Kepler’s

third law, the planet would be at a distance of ~0.098 a.u. from the star. Its orbital velocity would be ~240 km/s.

However, in a study of radial velocities [13] no inner planets of the star were observed at distances ranging from 0.03

to 1.2 a.u. Recall that the only planet discovered so far [1,2], β Pic b, is at a distance of 8-15 a.u.

Fig. 5. (a) A power spectrum 105PW(Vt) for β Pic. ν isthe frequency in d-1. (b) The phase dependence of Vt– Vt

mean. p is the period and ph, the phase.

ph, p = 4.462d

���

����

�����

Vt

- V

t mea

n

��� ��� ��� ���

ν , d-1

����

����

�����

�

����

���

���

105

PW(V

t)

���� ��� ���� ����

���

���

���

�

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167

4.2. Pulsations? It might be supposed that the observed microvariability is related to pulsations of β Pic. The

existence of pulsations of the star was suggested in the above cited paper [13]. The period of the pulsations obtained

there, ~30 min, led the authors to compare them with the pulsations of type δ Scu stars. On the other hand, if pulsations

do exist, the ones obtained in the present work have a longer period, 4.46 d, than in δ Scu stars. (The period in δ

Scu stars is 0.01-0.2 d [14].) In addition, the positions of the (Vt-Vtmean

) points in the phase plot for β Pic Fig. 5b

recall the asymmetric light curve of δ Cep stars: a slow decrease in brightness and a rapid rise (the sinusoid is only

an approximation to the phase dependence). However, the pulsations of β Pic have a very low amplitude (0m.0085)

compared to those of δ Cep stars.

4.3. Large-scale inhomogeneity in the circumstellar disk? There may be a third reason for the variability

of β Pic: periodic eclipsing of the star by regions of a circumstellar disk with an elevated concentration of particles.

The existence of large-scale inhomogeneity in the surrounding disks is suggested by an analysis of observational data

from Fomalhaut and Vega. Confirmation of an inhomogeneity in the disk of β Pic will require further study.

5. Conclusion

A study of data from photometric observations of Vega at wavelengths of 325-1080 nm obtained in different

years has shown that its spectral energy distribution is influenced by a gas-dust disk surrounding the star. The effect

of the disk shows up through scattering of light in the ultraviolet by small particles, as well as in an enhanced

luminous flux at longer wavelengths owing to reflection of the star’s light by large particles. As a result, the

spectrophotometric temperature of the star turned out to be lower than the star’s effective temperature.

A microvariability of roughly 0m.01-0m.02 in the quasimonochromatic magnitudes of the star has been

detected. This variability was not associated with pulsations of the star, but was apparently determined by a large-

scale inhomogeneity in the distribution of dust and gas across the disk, where absorbing and scattering matter is

concentrated in clouds.

Because of the small number of spectrophotometric observations, the effect of a gas-dust disk surrounding the

star could not be detected in the observational data for β Pic. On the other hand, photometric observations of this

star reveal periodic variations in the Vt magnitude with a period of 4.46 d and an amplitude of 0m.0085. Three

assumptions were made to interpret this result: the existence of pulsations in the star’s photosphere, of a planet at

a distance of ~0.1 a.u., and of a region with an elevated particle density in the circumstellar disk, which periodically

eclipses the star as it participates in Kepler rotation. All three of these assumptions require confirmation.

To conclude, we note the main result of this work: a gas-dust disk surrounding the star has an effect on the

spectral energy distributions of Fomalhaut and Vega. It reduces the spectrophotometric temperature of the stars by

acting in different ways on different regions (UV and IR) of the spectrum. This interaction is nonthermal. The

difference in the effect during different seasons indicates that the disks have a large scale inhomogeneity.

168

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