COLLIDING WINDS COLLIDING WINDS IN CLASSICAL SYMBIOTIC STARSIN CLASSICAL SYMBIOTIC STARS

E. Kilpio, D. Bisikalo

Institute of Astronomy, RAS, Moscow, Russia

El Escorial, 2007

Symbiotic

stars

are

characterized

by

peculiar

spectra where

the

characteristic

features

of

a

cool

giant

are

present

together

with

emission

lines

corresponding

to

a high

excitation

level. The

red

and

infrared

spectra

of

symbiotic

stars

are

typical

of

cool

giants

while

in

the

UV range

they

are

characterized

by

a very

hot

continuum.

Symbiotic star is a wide binary consisting of a cool giant and a white dwarf surrounded by a nebulosity. It

is

a

detached

binary

with

mass

exchange

via

stellar

wind.

Symbiotic stars present nova-like outbursts. In classical symbiotic systems outbursts typically

last

a few

months

and

have

amplitudes

of

2-4m. The mechanism of outbursts in classical symbiotic systems and processes taking place during an outburst development are still poorly known.

UV range can give us a lot of information about symbiotic systems.

WDNebulosity

Accretion diskWind of a giant Hot areas formed as the result of winds collision

A. Skopal

et

al., Astron. Astrophys., 453, 279

(2006)

Most part of existing UV space missions observed symbiotic stars.

?

A typical classical symbiotic star is a detached

system with mass exchange driven by stellar wind.

EQUATIONSEQUATIONS

0u v wt x y zρ ρ ρ ρ∂ ∂ ∂ ∂+ + + =

∂ ∂ ∂ ∂

( )2

2u Pu uv uw v

t x y z xρρ ρ ρ ρ ρ

∂ +∂ ∂ ∂ ∂Φ+ + + = − + Ω

∂ ∂ ∂ ∂ ∂

( )2

2v Pv uv vw u

t x y z yρρ ρ ρ ρ ρ

∂ +∂ ∂ ∂ ∂Φ+ + + = − − Ω

∂ ∂ ∂ ∂ ∂

( )2w Pw uw vwt x y z z

ρρ ρ ρ ρ∂ +∂ ∂ ∂ ∂Φ

+ + + = −∂ ∂ ∂ ∂ ∂

E uh vh wh u v wt x y z x y zρ ρ ρ ρ ρ ρ ρ∂ ∂ ∂ ∂ ∂Φ ∂Φ ∂Φ

+ + + = − − −∂ ∂ ∂ ∂ ∂ ∂ ∂

( )1P γ ρε= −

The Roe-Osher

scheme has been used for 2D

and

SFS (simplified flux splitting) scheme for 3D.

GRIDS2D:601×601cells

689×589 cells (condensed near the accretor)3D:

253×253×85 cells

( ) ( ) ( )

22 21 1 2 2

1 2 1 2 1 22 2 2 2 2 2 2 2 21 2

12( )

GM GM GM Mx a yM Mx y z x y z x a y z

⎛ ⎞⎛ ⎞⎜ ⎟Φ = − +Γ − − Ω − +⎜ ⎟⎜ ⎟+⎝ ⎠+ + + + − + + ⎝ ⎠

M1

–

mass of the donor, r

–

distance from the center of the donor,

Г

–

parameter.

The modified potential

:

rrF 2

1

rGM

Γ=

MODELING OF A GIANTMODELING OF A GIANT’’S WINDS WIND

Z AND PARAMETERSZ AND PARAMETERS

T.Fernandez-Castro, A.Cassatella, A.Gimenez

et

al., Astrophys. J., 324, 1016 (1988)

Mass of the giant

M1 2MRadius of the giant

R1 77 R

Mass of the accretor

M2 0.6MRadius of the accretor

R2 0.07 R

Orbital period

Porb 758.8 daysSeparation A 482.53 RMass loss rate 2×10-7

M

Donor’s wind velocity 25 km/s

MECHANISM OF TRANSITION FROM MECHANISM OF TRANSITION FROM QUIESCENT TO ACTIVE STATE IN QUIESCENT TO ACTIVE STATE IN CLASSICAL SYMBIOTIC STARSCLASSICAL SYMBIOTIC STARS

NATURE OF OUTBURSTS IN SYMBIOTIC STARSNATURE OF OUTBURSTS IN SYMBIOTIC STARS

The most probable mechanism describing the observational manifestations of symbiotics

is thermonuclear burning on the

accretor's

surface (Kenyon

1986, Iben&Tutukov

1996).

The process of thermonuclear burning of hydrogen on the white dwarf surface strongly depends on the accretion rate

(Paczynski&Rudak,

1980).

The

steady

hydrogen

burning

is possible

only in a narrow range of accretion rate values

(Paczynski&Zytkow,

1978). This range depends

on the white dwarf mass.

S.J.Kenyon, The

Symbiotic

Stars, Cambridge

Univ. Press, (1986)I.Iben, A.V.Tutukov, Astrophys. J. Suppl. Ser. 105, 145 (1996)B.Paczynski

and

B.Rudak, Astron. and

Astrophys., 82, 349 (1980)B.Paczynski

and

A.Zytkow, Astrophys. J. 222, 604 (1978)

LEAVING THE STEADY BURNING RANGELEAVING THE STEADY BURNING RANGECaseCase

1.1.

Accretion rate becomes lower than the steady

burning range:

Corresponds to the observational manifestations of symbiotic novae

CaseCase

2.2.

Accretion rate exceeds the steady burning range

Timescale and brightness changes correspond to classical symbiotic stars

Accreted material becomes

exhaustedBurning

termination

Matter accumulation on the surface

Hydrogen shell

flash

Accreted matter accumulates above the

burning

shell

Expansion

to giant’s

dimensions

What is the reason of the accretion rate growthWhat is the reason of the accretion rate growth??

In order to explain the accretion rate change several mechanisms were proposed:

Significant change of the donor’s mass loss(Nussbaumer&Vogel, 1987) – not confirmed by observations

Accretion disk instabilities (Bath, 1977) – no correspondence on scale and time

REASON OF THE ACCRETION RATE REASON OF THE ACCRETION RATE GROWTHGROWTH

Using the results of gasdynamic modeling we proposed the mechanism that can provide the accretion rate growth (Bisikalo et al., 2002)

WW>1>1

–

wind accretion WW<1<1

–

disk accretion

The accretion regime is defined by the parameter

W=W=vvwindwind

/v/vorborb

DEPENDENCE OF THE FLOW STRUCTURE ON THE DEPENDENCE OF THE FLOW STRUCTURE ON THE WIND VELOCITYWIND VELOCITY

In classical symbiotics

the value of giant’s wind velocity is close to the orbital velocity so both regimes are possible.

MECHANISM PROVIDING THE ACCRETION MECHANISM PROVIDING THE ACCRETION RATE GROWTHRATE GROWTH

Minor Minor variations of variations of giantgiant’’s wind s wind

velocityvelocity

Accretion Accretion regime regime changechange

Jump of the Jump of the accretion accretion

raterate

Even

minor

variations

in

the

donor's

wind

velocity

can lead

to

the

accretion

regime

change

-

from

disk

accretion

to

wind

accretion. During

the

transition

period, namely during

the

disk

destruction

the

accretion

rate

increases

abruptly

and

exceeds

the

upper

limit

of

the

steady

burning range.

The

velocity

of

the

mass-losing

star's

wind

in

Z

And

is v=25

km/s. (Fernandez-Castro et al., 1988)

In order to check the applicability of the proposed mechanism, numerical simulations for velocities of 25±5

km/s have been carried out.

T.Fernandez-Castro, A.Cassatella, A.Gimenez

et

al., Astrophys. J., 324, 1016 (1988)

DONOR WIND VELOCITYDONOR WIND VELOCITY

V=V=20 20 KM/SKM/S

Rdisk

≈50-60R

►Accretion efficiency 22.5 -25%

►Accretion rate

►For the case of Z And

(MWD

=0.6M ), steady burning range is:

Accretion rate in the solution with the disk is within the Accretion rate in the solution with the disk is within the steady burning rangesteady burning range..

годMMaccr /105.4 8−×≈

годMMгодM accr /107.4/101.2 88 −− ×≤≤×

DONOR WIND VELOCITYDONOR WIND VELOCITY

V=V=30 30 KM/SKM/S

Accretion efficiency

11 13%accrM = −

WIND VELOCITY CHANGE 20WIND VELOCITY CHANGE 20→→30 30 KM/SKM/S

It takes about 133d

for the matter moving with increased velocity to reach the the

bow shock located in front of the

accretor.

After that the wind moves further, crushes disk and makes the matter it contained to fall onto the surface of the accretor.

This results in the accretion rate jump.

ACCRETION RATE CHANGEACCRETION RATE CHANGE

This is enough (amplitude and time that the accretion rate spends above the steady burning range) to lead to the outburst.

The

maximum value

of

the

accretion

rate

is

~ 2-2.2 times

as

compared

to

the state

with

disk

accretion.

Acc

retio

n ra

te

Time

OUTBURST DEVELOPMENT IN THE OUTBURST DEVELOPMENT IN THE FRAMEWORK OF THE COLLIDING WINDS MODELFRAMEWORK OF THE COLLIDING WINDS MODEL

There are a lot of symbiotics

where the wind from the hot component has been observed at active stages of the system (e.g. Z And). In such a case winds from both components collide and form the complex structure with shocks.

We took the stationary solution corresponding to the pre- outburst state and continued calculations with changed

boundary conditions on the accretor’s

surface.

After 70d

the waves reach their final position between the system’s components and it takes 20-30 days for the formation of shocks structure to be completed.

SHOCKS CONTRIBUTION TO THE BRIGHTNESS SHOCKS CONTRIBUTION TO THE BRIGHTNESS OF THE SYSTEMOF THE SYSTEM

Shocks formation can provide the brightness growth up to 1m

so it should be seen on the lightcurve.

Hot wind appears

(15.09.2000)

Shocks form

(~25d)

Good correspondence between the model and observations!Good correspondence between the model and observations!

U

B

V

~60d

Sokoloski

et al 2006

Z AND LIGHT CURVE FOR THE OUTBURST AT 2000Z AND LIGHT CURVE FOR THE OUTBURST AT 2000

SHOCK IONIZATION ESTIMATIONSHOCK IONIZATION ESTIMATION

Date

(state of the system) μ

15.09.1999

(quiescence) 1.1222.11.2000

(outburst, rise to the second maximum) 1.00

06.12.2000 (outburst,

near maximum) 0.76

At the maximum of the outburst 24%

of the emission of nebulosity can be the result of the shocks formed.

VnnL

e α=μ

+

The ratio of the number of ionizing photons to recombinations

CONCLUSIONSCONCLUSIONS1) In quiescent state the accretion disk exists in the system.

2) The activity of symbiotics

depends strongly on stellar wind parameters. Namely, insignificant variations of the stellar wind parameters can lead to drastic changes in the the flow

structure, to the accretion

regime

change

over

and to the outburst.

So it is of interest to have precise estimates of wind parameters.

3) The collision of winds from both components results in the formation of the structure with shocks.

4) UV observations could give us the important information on the processes taking place in the system (particularly stellar winds from the components of the system) and thus to contribute to the study of the activity of symbiotic stars.

Thank you for your Thank you for your attentionattention!!

At present a very important role in astrophysics belongs to the numerical modeling of astrophysical processes. The reasons are:

►The rapid growth of computing facilities gives new opportunities for theoretical investigation.

►The necessity of interpretation of volume of observational data that grows continuously.

CONCLUSIONBasing on the detailed 3D gasdynamic

study of the

classical symbiotic system Z And we have studied the behaviour

of the system before the outburst and

during the outburst development.

UV observations could give us important information on the processes taking place in the system (particularly stellar winds from the components of the system) and thus to contribute to the study of the activity of symbiotic stars.

The

evolution

of

the

UV spectrum

throughout

the outburst

plays

a vital

role

in

distinguishing

between

the

outburst

models

currently

in

contention

for describing

outburst

behavior. Emission

line

strengths

of

the

FUSE-band

species, especially

when combined

with

lines

of

the

same

species

from

the

other

spectral

regions, provide

diagnostics

of

the colliding

windshock

region. Note

especially

that

FUSE provides

important

information

for

the

analysis of

the

x-ray

data

as

FUSE observations

of

HI

absorption

are

at

sufficiently

high

resolution

to

allow the

separation

of

the

interstellar

component

from

the

systemic

component

of

the

absorption.

Unlike

massive

binaries, the

winds

of

a symbiotic

system

have

very different

characteristics

--

the

hot

wind

is

very

thin

and

fast,

whereas

the

cool

wind

is

slow

and

weakly

ionized

-

and

hence

they can

be

isolated

spectroscopically.

The

white

dwarf

secondary

of

symbiotic

systems

provides

a far-UV continuum

source

against

which

absorption

from

a broad

range

of

ionization

levels

can

be

seen, ranging

from

molecular

hydrogen surrounding

the

giant

star

to

highly

ionized

material

in

the

wind

collision

region. Because

of

the

high

ionization

lines

arising

in

the hot

and

shocked

gas, as

well

as

the

molecular

and

atomic

absorption

arising

in

the

red

giants

extended

atmosphere, phase- resolved

spectra

provide

the

unique

tomographic

information

we

require

to

improve

our

knowledge

of

these

systems

and

how

winds are

generated

and

interact.

The

O vi

1032,1038 emission

lines

can

convert

into

broad

and highly

polarized

emission

lines

at

lambda

6825 and

lambda

7082 in

a Raman

scattering

process

by

neutral

hydrogen. From

a comparison of

direct

and

Raman

scattered

radiation

we

extract

new

information

on

the

scattering

geometry

in

symbiotic

systems. The

nebular

O vi emission

lines

are

in

all

objects

redshifted

by

about

+40 km

s(-1) .

This

can

be

explained

as

a radiative

line

transfer

effect

in

a slowly expanding

emission

region. A comparable

redshift

is

measured

in

the

Raman

scattered

O vi

lines. In

AG Peg

the

O vi

emissions

show beside

a narrow

nebular

line

a broad

component

from

a fast

stellar

wind

outflow. Many

interstellar

absorption

lines

of

molecular hydrogen

are

detected, particularly

near

the

O vi

lambda

1038

component. With

model

calculations

we

investigate

their

impact

on the

O vi

lines. From

the

dereddened

line

fluxes

of

the

direct

and

Raman

scattered

O vi

lines

we

derive

the

scattering

efficiency, which

is

defined

as

photon

flux

ratio

N_Raman/N_O VI. The

efficiencies

derived

for

RR Tel, V1016 Cyg

and

Z And

indicate

that about

30% of

the

released

O vi

lambda

1032 photons

interact

with

1. 1. v=v=20 20 кмкм//сс::►

Исследование

возможности

формирования

диска

►

Определение

величины

темпа

аккреции►

Проверка

соответствия

темпа

аккреции

диапазону

устойчивого

горения

2.2.

v=v=30 30 кмкм//сс::►

Определение

параметров

картины

течения

в

системе

без

диска

3.3.

v=v=2020→→30 30 кмкм//сс::►

Исследование

процесса

разрушения

диска

►

Рост

темпа

аккреции►

Достаточен

ли

рост

темпа

аккреции?

►

Достаточно

ли

долго

темп

аккреции

находится

за пределами

диапазона

стационарного

горения?

ВЫПОЛНЕННЫЕВЫПОЛНЕННЫЕ

РАСЧЕТЫРАСЧЕТЫ

V=80 км/cV=40 км/cV=20 km/s

The binary nature of these stars has been proved by A. Boyarchuk

in 1967 but later in 80th UV observations with

IUE confirmed the presence of the additional UV (more exactly far UV) radiation source.

Tomov

N. et al Astron.

Astrophys.,

v.401, p.669-676,

(2003)

More

than

100 years

of

observations

have

shown

that

Z And presents

outbursts.The

last

of

them

took

place

in

2000 and

has

been

actively

observed

at

different

wavelengths.

( ) ( )

22 21 2 2

1 2 1 22 2 2 2 2 21 2

12( )

GM GM Mx a yM Mx y z x a y z

⎛ ⎞⎛ ⎞⎜ ⎟Φ = − − − Ω − +⎜ ⎟⎜ ⎟+⎝ ⎠+ + − + + ⎝ ⎠

Roche potential

CONCLUSION

According to the results of gasdynamic

modeling of the flow structure in the classical symbiotic system Z And has been modeled.

The results of modeling show that transition to the outburst as well as behavior of the system during the outburst dpends

strongly on parameters of winds from components of a symbiotic system.

In order to carry out more detailed studies we needmore information on stellar wind parameters

The estimates of stellar winds parameters equires

UV observations

Показано, что

поэтапный

характер

роста

кривой

блеска

в период

развития

вспышки

может

быть

объяснен

в

рамках

модели

сталкивающихся

ветров

от

компонентов

системы. Предложен

сценарий

развития

вспышки

и

исследовано

возможное

влияние

изменений

в

структуре

течения

на

блеск системы. Моделируемые

изменения

светимости,

вызванные

формированием

системы

ударных

волн

DEPENDENCE OF THE FLOW DEPENDENCE OF THE FLOW STRUCTURE ON THE WIND VELOCITYSTRUCTURE ON THE WIND VELOCITY

The wind velocity regime is defined by the parameter

W=W=vvwindwind

/v/vorborb

(1).

Results of 3D modeling:

WW>1>1

–

wind accretion

WW<1<1

–

disk accretion.

Confirms earlier results .

In classical symbiotics

the value of giant’s wind velocity is close to the orbital velocity so both regimes are possible.

1Бисикало

и

др., 1994Бисноватый-Коган

и

др., 1979

In quiescent state accretion rate is within the steady burning range

At some moment accretion rate increases and oversteps the limits of the steady burning range.

The time scale of the accretion rate increase should correspond to thecharacteristic time of outburst development.

OUTBURST IN A CLASSICAL SYMBIOTIC SYSTEMOUTBURST IN A CLASSICAL SYMBIOTIC SYSTEM

What is the reason of the accretion rate What is the reason of the accretion rate growthgrowth??

Z AND LIGHTCURVE DURING THE OUTBURST IN Z AND LIGHTCURVE DURING THE OUTBURST IN 200200

Sokoloski

et al, 2004

In symbiotics:

WD has very small diameter as compared with the giant’s one so it observations at orbital phases where the white dwarf is located behind a portion of the cool wind would allow us to study different layers of the circumstellar

material.

Components have very different spectral characteristics and can be easily disentangled.

WD provides a bright UV continuum source upon which the absorption from the cool circumstellar

gas is superimposed. FUV

and UV ranges are very rich in resonance and diagnostically informative atomic and molecular transitions which would not be visible without presence of strong UV continuum.

В

настоящее

время, все

большую

роль начинает

играть

численноное

исследование

Симб

зв. Почему: 1) компьютеры;

2) Необходимость

интерпретации всевозрастающего

ОНД