ORBITAL MOTIONS IN BINARY AND MULTIPLE PROTOSTARS

L. F. Rodríguez (IAUNAM, Morelia)

L. Loinard, M. Rodríguez, & P. D’Alessio (IAUNAM, Morelia)

S. Curiel, J. Cantó, & A. C. Raga (IAUNAM, México City)

J. M. Torrelles (IEEC, Spain), J. M. Girart (U. Barcelona, Spain)

David J. Wilner & Paul T. P. Ho (CfA, USA)

High angular resolution (0.1”) Very Large Array observations of young stellar systems that allow measurement of orbital proper motions and estimate of stellar masses.

Union of two fields where Arcadio Poveda has made significant contributions

BACKGROUND

• Most information on stellar masses comes from studies of orbital motions

• Work at optical band toward visible stars has been going on for 200 years

• In the last decade, near-IR speckle and adaptive optics has been used to investigate T Tauri binaries

• What about heavily obscured protostars, not detectable even at near-IR wavelengths?

RADIO OBSERVATIONS

• Remarkably, protostars can be tracked at radio wavelengths due to three processes:

1. Gyrosynchrotron from active stellar magnetosphere

2. Free-free emission from ionized outflows

3. Thermal emission from circumstellar disks

No extinction. However, processes (2) and (3) produce extended sources. These emissions can or cannot be present.

Very Large Array

0.1” resolution at 2 cm

SOURCES:

• L1551 IRS5

• YLW 15

• L1527 (= IRAS 04368+2557)

• IRAS 16293-2422

• T Tauri

L1551 H+ [SII]

Devine et al. (1999)

H

[SII]

Cont.

Reipurth & Bally 2001

ESO NTT

L1551 IRS5

• Near-IR source (Strom et al. 1976) that excites bipolar flow (Snell et al. 1980)

• Located at Taurus at 140 pc

• Bolometric luminosity of 30 Lsun

• Embedded in dense core (1000 AU)

• Believed to be single star, now it is known to be a binary system

Rodríguez et al. 1998

Compact dust disks

Free-free from ionized outflow dominates cm range, while thermal emission from dust in disk dominates mm range

See Poster 14

L1551 IRS5 VLA-A 2 cm

Proper Motions

• Large proper motions due to large scale motion of region with respect to Sun and agree very well with Jones & Herbig (1979)

• However, proper motions not identical for N and S components, indicating relative (orbital) motions

Orbital Proper Motions

• Observed changes in separation and position angle imply relative velocity in the plane of the sky of 2.3+-0.5 km/s

• A (very) conservative lower limit to the total mass can be derived from (M/Msun)>0.5 (V/30 km/s)^2 (R/AU)

• We obtain (M/Msun)>0.1

An attempt to correct for projection effects...

• Assume plane of orbit parallel to plane of disks (Bate et al. 2000)

• Circular orbit

• => M = 1.2 Msun; P = 260 yr

• In the main sequence, luminosity will be of order 1 sola luminosity, while now Lbol is of order 30 Lsun => accretion main source of luminosity

YLW 15 VLA-A 3.5 cm

1990.41

2002.18

YLW 15

YLW 15

• Relative velocity in the plane of the sky of 6.4+-1.8 km/s, implying:

• M > 1.7 Msun

• Assuming observed separation about true separation, P < 360 yr

• Lbol = 13 Lsun

See poster 2

L1527 VLA-A 7 mm

Relative Velocity in Plane of the Sky = 4+-2 km/s

M > 0.1 Msun, most likely 0.5 Msun

Lbol about 2.5 Lsun

Up to now, binary systems, what about multiples (i. e. triples)?

• IRAS 16293-2422

• T Tauri

IRAS 16293-2422, VLA-A, 3.5 cm, average proper motion subtracted

IRAS 16293-2422

• Relative velocity of about 15 km/s and separation of about 30 AU between components A1 and A2, implies relatively large mass of about 4 Msun

• However, A1 has been proposed in the past to be shock with ambient medium

T Tauri: Prototype of its class

T Tauri is triple (Koresko 2000)Data from Dûchene et al. (2002):

Dûchene et al. (2002)

V = 20 km/s => M > 4 Msun

What are we seeing in the radio?

• Comparison between radio and near-IR, as well as circular polarization characteristics of southern source indicates that in the radio we are always seeing T Tau Sb

• Even when in the radio we do not see component Sa, it is possible, combining radio and near-IR to obtain orbit of Sb relative to Sa

• This relative orbit comes from detailed astrometric measurements and corrects for relative motion of Sa with respect to N

What makes us think the orbit changed?

• Last two points do not fit previous ellipse

• Area/time for last two points larger than for previous points

• Large mass (4 Msun) required for bound motions, while 2 Msun required before 1995

Arguments against:

• Something may be wrong with measurements

• Suggested ejection very unlikely, although evidence for ejections exists in literature (Allen et al. 1974; Hoogerwert et al. 2000)

• Johnston et al. (2003) model all data points with a single ellipse

Future observations will solve the issue

• We are undertaking new radio and near-IR observations to follow motion of T Tau Sb

• In this scheme of orbital change (or even ejection or escape), T Tau Sa must be a binary, making the T Tauri system a quadruple

CONCLUSIONS

• Orbital motions in protostars will provide important constraints on the early phases of stellar evolution

• We are getting reasonable results, but must follow cases of IRAS 16293-2422 and T Tauri

• Now we are limited by modest signal-to-noise ratio, but this situation will greatly improve with EVLA, ALMA, and SKA