Rotational Velocities of the Red Giants in Symbiotic Binary Stars

Радослав К. Заманов(Институт по Астрономия, Българска Академия на Науките)

в сътрудничество с:

M.F. Bode (Liverpool, UK), C.H.F. Melo (ESO, Chile)

A. Gomboc (Ljubliana, Slovenia),R. Bachev, I. K. Stateva, R.Konstantinova-Antova (Sofia, Bulgaria)

October 2007

Symbiotic binary star = Red Giant + White Dwarf

Symbiotic stars are interacting binaries consisting of red giant transferring mass onto a white dwarf.

We are investigating the projected rotational velocities of the mass donors. Our aims are:

• To check theoretical prediction that the red giants in these binaries are co-rotating (for objects with known periods).

• To perform comparative analysis and to check if they are faster rotators (comparing with isolated giants and those in wide binary systems).

• To give clues for binary periods, individual mass loss rates, select candidates for X-ray observations.

Observations: 40 symbiotic stars have been observed with the 2.2m telescope

(ESO, La Silla) + FEROS spectrograph at resolution 50000.

Our sample: All objects from the Symbiotic star catalogue with

0h<R.A.<24h, Declination < 00, and brighter than V< 12.5 mag.

From literature -12 northern symbiotics. Our sample is flux limited, there should be no biases in

rotation.

ESO – La Silla

ESO La Silla - 2.2 m telescope

Observations: the 2.2m telescope + FEROS spectrograph

- resolution 48000; dispersion=0.03 A/pixel

- wavelength coverage: 3600 – 8900 AA in a single exposure

- signal-to-noise ratio = 50; V=12 mag; exposure=30 min

Fig. Theoretical spectrum and spectra of a few symbiotic in the near IR: wavelength 8760-8850 AA

Fig. Numerical mask and spectra of a few symbiotic in Wavelength interval 8760-8850 AA.

Two examples of the Cross-Correlation Function and the fitting gaussian. Left – AS 316 – v sin i = 9.8 ± 1.5 km/sRight – rapid rotator V417 Cen – v sin i = 75 ± 7.5 km/s

To measure the projected rotational velocity (v sin i) we used the CCF method and numerical template. The width of the CCF is connected (calibrated for FEROS) with the v sin i.

Fig. Check of our methods. The measurements of v-sin-iwith FWHM and CCF methods are in good agreement.

The rotational period of the red giant versus the orbit period for 16 symbiotic stars in our sample with known orbital periods (all they are S-type).The solid line represents synchronization (Porb=Prot).

Among these 16 objects there are two, which deviate considerably from co-rotation: CD-43 14304 and RS Oph. There are doubts that v sin i of CD-43 14304 could be wrong. RS Oph seems to be the only symbiotic system which is not synchronized.

D’-type symbiotics are characterized by an earlier spectral type

giant (F-K) and lower dust temperatures. Rotational velocities have been measured for five such stars (Zamanov et al. 2006).

Four of these five objects appeared to be very fast rotators, compared with the catalogues of v sini for the corresponding spectral types. At least three of them rotate at a substantial fraction (≥ 0.5) of the critical velocity. Hence, in D’-type symbiotics, the cool components rotate faster than the isolated giants of the same spectral class (as predicted by Soker 2002).

As a result of rapid rotation, they must have larger mass loss rate, probable enhanced in the equatorial regions. In addition, as a result of the fast rotation, magnetic activity is expected to exist in these giants.

Fig. v sin i versus the spectral type.Symbiotic stars – red crosses, black –

single giants.

Results:

238 K2III-K5III stars: v sin i = 1.0 - 6.7 km/s mean v sin i =1.70 km/s)The K giants in symbiotic stars: v sin i = 4.5 - 8.9 km/s mean v sin i =7.42 km/s)

The Koslmogorov-Smirnov test gives a probability of 10-4 (K-S statistics =0.60)

K-giant mass donors of symbiotic stars rotate faster than isolated K-giants !!!

Isolated giants spectral classes K2-K5 III (238 objects from catalogues of v sin i)

K2-K5 III giants in symbiotic stars (7 objects, our measurements)

isolated M giants: 1.8 < v sin i < 18 km/s (mean vsini=5.54 km/s)M-giants in symbio 3.0 < v sin i < 52 km/s (mean vsini=9.07 km/s)

The tests gives a probability of 0.09-0.01 that both distributions are coming from the same parent population.

Isolated giants spectral classes M0-M7 III (12 objects from catalogues of v sin i)

M0-M7 III giants in symbiotic stars(28 objects, our measurements mostly)

Discussion:

The reasons for faster rotation in giants in symbiotic systems could be:

- synchronization, if the time spent by the mass-losing star on the giant branch is longer than the synchronization time. In all symbiotic systems with orbital period Porb ≤ 100 years tidal interaction overcomes the angular momentum loss by the wind (Soker 2002).

- accretion during the MS phase of the present red giant: the more massive star in the system, the present WD, had transferred material at the stage when it had been red giant.

- backflowing material: hot component prevents part of the mass blown by the giant from acquiring the escape velocity for the binary system. This fraction of mass may acquire angular momentum, and if it is accreted back by the giant, it spins-up its envelope.

- angular momentum dredge-up when convective envelope approaches the core region of the giant.

- planet engulfment during the giant phase.

• We have measured the projected rotational velocities of 40 symbiotic stars (v sin i) by the means of CCF.

• Among 16 symbiotics with known orbit and rotation, there is only one (RS Oph) which is probably not synchronized.

• Our results show that the mass donors in the symbiotic stars rotate faster than isolated giants. The faster rotation is undoubtfull for D’-type (yellow) symbiotics and for those harbouring K-giant as mass donor. For those with M giant it is not so obvious.

CONCLUSIONS:

FUTURE WORK:To strengthen our results, more data on M-type isolated giants and more v sin i measurements of K-type mass donors in symbiotics are desirable. We intend to expand our sample with northern and fainter symbiotic stars.

Open questions:

1. Is there a bimodal distribution of v sin i of the isolated giants?

2. Hen 3-1674 rotates very fast v sin i = 52 km/s

M5III - R=139 Rsun, and mass 1-3 Msun, Vcrit=40-60 km/s. What is this object? a monster? or just an error somewhere?

3. Is there a connection between the rotation of the red giant and the density of the circum binary nebula and mass accretion rate?

THE END

Results:

238 K2III-K5III stars have vsini in the interval from less than 1.0 km/s up to 6.7 km/s with mean vsini =1.70 km/s (median= 1.50 km/s) and standartd deviation of the sample = 0.90 km/s. The K giants of 8 S-type symbiotic stars with mass donors K2III-K5III have vsini in the interval from 4.5 up

to 8.9 km/s with mean vsini=7.42 km/s (median=7.15 km/s) and stddev= 1.54 km/s.

The Koslmogorov-Smirnov test gives a probability of 10-6 (K-S statistics =0.60) that both distributions are coming from the same parent population. This means that the K-giant mass donors of symbiotic stars rotate faster than isolated K-giants.

Isolated giants spectral classes K2-K5 III K2-K5 III giants in symbiotic stars (238 objects from catalogues of v sin i) (7 objects, our measurements)

The isolated M giants: 1.8 km/s < v sin i < 18.1 km/s, mean vsini=5.54 km/s (median=3.10 km/s), stddev=5.22 km/s.

The M-type mass donors of symbiotics 3.0 < v sin i < 52 km/s, mean vsini=9.07 km/s (median=7.72 km/s), stddev=8.81 km/s.

The Kolmogorov-Smirnov test gives a probability of 0.0074 (K-S statistic =0.54) that both distributions are coming from the same parent population. This means that from statistical point of view the M giants mass donors of symbiotic stars rotate faster than isolated M-giants (at confidence level 99%).

Isolated giants spectral classes M0-M7 III M0-M7 III giants in symbiotic stars (12 objects from catalogues of v sin i) (28 objects, our measurements mostly)

Observations: 40 symbiotic stars have been observed with the 2.2m telescope

(ESO, La Silla) + FEROS spectrograph at resolution 50000.

Our sample: All objects from the Symbiotic star catalogue with

0h<R.A.<24h, Declination < 00, and brighter than V< 12.5 mag.

From literature -12 northern symbiotics. Our sample is flux limited, there should be no biases in

rotation.