The Formation of Single and Multiple Stars Bo Reipurth Institute for Astronomy Astrobiology Seminar...

The Formation of Single and Multiple Stars

Bo Reipurth

Institute for Astronomy

Astrobiology Seminar University of Hawaii Manoa 20 September 2004

Overview

• The Formation of Binary and Multiple Stars

• High Velocity Jets

• Disk Accretion Events

• Clustered Star Formation

Evolution of Young Low Mass Stars

Courtesy of Pat Hartigan

* Class 0 objects are deeply embedded protostars only about 10, 000 yr old surrounded by large infalling envelopes

* Class I objects are partly embedded protostars less than 100, 000 yr old surrounded by massive circumstellar disks and a remnant envelope

* Class II objects are T Tauri stars typically a few million years old with disks

* Class III objects are T Tauri stars with less circumstellar material

Historically, there has beena strong bias towardslooking at young stars assingle stars.

Young Binary Stars: Some History

• The study of young binaries began with Joy & van Biesbrock (1944) who found 5 visual binaries among the newly recognized T Tauri stars.

• In 1962, George Herbig added another 24 T Tauri binaries, demonstrating that binaries among young stars are not uncommon.

• Observations of young binaries remained decidedly out of main stream astronomy until 1993, when within one month three major surveys appeared.

• By now, studies of young binaries are leading a paradigm shift in star formation.

IT IS A KEY FACT THAT 2/3 OF ALL STARS ARE BINARIES

The RZ93 Survey - I

• Reipurth & Zinnecker (1993) [A&A 278, 81] imaged 238 young stars at 0.9 m and detected 37 binaries with separations between 1” and 12”.

• Taking into account distances to the binaries, this implies a PMS binary frequency of 16% in the projected separation range of 150 AU to 1800 AU.

The RZ Survey - II

• The distribution of separations for PMS binaries is complete to the vertical dotted line.

• The separation distribution function shows a steep rise towards smaller separations.

The RZ93 Study - III• The left figure shows a comparison between the distribution of PMS binary

separations and the distribution of G dwarf MS binaries from Duquennoy & Mayor (1991), who found a binary frequency of 53% (curves are shown for 60, 80 and 100%).

• The right figure shows the same for K dwarfs studied by Mayor et al. (1992), who find a binary frequency for these stars of 45%.

• An excess of PMS binaries relative to MS binaries is evident in the three bins where the survey is complete.

• At 130 AU the peak of the DM91 separation distribution function of 30 AU is at 0.2”-0.3”.

Binary Formation through Capture

• Capture was the earliest suggestion for forming a binary.

• All variations on this theme require some additional medium that can dissipate excess kinetic energy so that two unbound stars can become bound during a close passage.

• This medium can be a third star, a circumstellar disk, or the stars themselves if the encounter is close enough to raise tides.

• For field stars this concept is now discredited, but under certain circumstances the idea is still valid, e.g. in newly formed clusters.

• Most importantly, if one or both stars have disks, the disk matter exterior to the minimum separation of the encounter will become unbound and take away the excess kinetic energy.

Fragmentation and the Formation of Wide Binaries - I

• Fred Hoyle noted in 1953 that if collapse is isothermal then, as the density of the cloud is increased, the Jeans Mass [Mj propto T^(1/2) ^(-1/2)] will decrease, and smaller parts of the cloud will become gravitationally unstable.

• Hoyle envisioned a repetitive process known as “hierarchical fragmentation”.

• Although the detailed physics of fragmentation still poses problems, fragmentation in various forms is still the most widely accepted formation mechanism for wide binaries.

Fragmentation and the Formation of Wide Binaries - II

Accretion and Binary Evolution

• The existence of close binaries, e.g. spectroscopic binaries with periods of days or months, is not easy to explain.

• Such binaries do not appear to result from our current understanding of star formation.

• It is likely that such close binaries evolved from wider pairs through dynamical evolution.

• The main driver of this evolution is the circumstellar or circumbinary gas disks.

Disks in Young Binary Systems

• Young binary stars show a great diversity of disk properties, with a large part of the variation accounted for by binary separation.

• Many of the closest binaries (with separations less than a few AU) could harbor circumbinary disks.

• Binaries with separations larger than about 100 AU tend to have massive circumstellar disks around the individual components.

• But in the intermediate range, while disks do exist, they are strongly affected by the stellar companion, and consequently have limited mass.

Mass Transfer in Eccentric Binaries

• It is now recognized that the typical eccentricity of a binary star (whether young or old) is considerable (e ~ 0.2 – 0.6) and not small or zero as has often been assumed in early theoretical work.

• The non-zero eccentricity allows for substantial interactions between the stars and their circumstellar environments.

• Simulations of eccentric binaries show that a circumbinary disk can transfer mass onto the central binary through non-axisymmetric structures (streamers or gas arms).

• This resupply mechanism may explain the common presence of small circumstellar disks which should otherwise have short life-times.

Matthew Bate’s Simulation of An Accreting Binary

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are needed to see this picture.

The Case of DQ Tau - I

• DQ Tau is a normal classical T Tauri star, except that it is one of only two CTTS that are known to be a close double-lined spectroscopic binary (all other PMS double-lined spectroscopic binaries are WTTS).

• DQ Tau is a short period, eccentric binary with roughly equal-mass components.

• The orbital solution shows that the components are extremely close, ~8 Rsun, at periastron.

The Case of DQ TAu - II

• For such close separations it is difficult to understand how the components can continue to accrete, since circumstellar disks should be destroyed.

• However, light curves and spectral variability studies show that DQ Tau undergoes periodic accretion events.

• The star brightens and shows increased Hemission and veiling with the binary period, synchronized with periastron of the binary. This is interpreted as gas channeled along the streamers from a circumbinary disk to the individual components.

HERBIG-HARO JETS AND MULTIPLE STARS

• The very youngest stars sometimes drive powerful jets. Such jets represent a fossil record of the accretion events in the source.

• Examples are HH 111, HH 1, L1551-NE, and Haro 6-10.

The Case of HH 111

• Near-infrared images of the HH 111 source region shows two sources, A and B. A is driving the major jet and is heavily extincted, B shows little extinction.

• Both sources are detected by the VLA in the radio continuum, so both are young and lose mass.

• Analysis of the brighter source A shows that it has two radio jets coming out at large angles to each other. It follows that A is a close binary, so the whole system is a hierarchical triple.

The Case of HH 1

• The HH 1 jet and a neighboring flow HH 144 emanate from two deeply embedded source radio continuum sources.

• One of the two sources is extended along the axis of the HH 1 jet, and it shows very large proper motions.

• Optical/infrared HST images have revealed a third tiny jet flow. Proper motion studies show that it comes from the extended radio source, indicating that it is a binary.

• In other words, the source region contains a hierarchical triple system.

The Case of L1551-NE

L1551-NE is a deeply embedded source in a dense dust ridge. It drives a series of HH knots and a giant HH flow. In the IR it shows an illuminated outflow cavity. VLA observations reveal the source to be a binary. Mm data have found a third source nearby

The Case of Haro 6-10

• Haro 6-10 drives a giant HH flow. Near the source the flow breaks up into two components with different directions. It appears as if there is precession in play.

• VLA observations have revealed that there are three sources, forming a not quite hierarchical triple system.

Disintegration of Multiple Systems

• Detailed analysis of giant jet energy sources show that 80-85% are observed to be binaries or multiples, i.e. without corrections for incompleteness [Reipurth 2000, AJ 120,3177].

• Since much fewer stars are binaries and multiples on the main sequence, it follows that some of them must disintegrate.

• Non-hierarchical systems oscillate between two phases: interplay and close triple approach, and the latter can lead to ejection.

• Disks go through serious warping and truncation during close triple approaches, leading to accretion events, and thus outflow activity. Giant jets are presumably formed in this way.

Burrau’s Problem (simulation by Michael Sterzik)

• The motion of a non-hierarchical triple system is chaotic and can only be studied numerically.

• In a few simple cases, one can find an analytical description.

• Burrau’s problem considers three objects of 3, 4, and 5 mass units, with separations of 3, 4, and 5 length units, located in a right-sided triangle.

• The motion illustrates the three classes of interplay, close triple approaches, and ejection

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The Formation of Brown Dwarfs

• The decay of a small non-hierarchical system occurs statistically after 100 crossing times, a time unit that is determined by the masses and separations of the components. For newborn systems, this typically amounts to 10,000 or 20,000 years.

• At such early times, the stars are still busy building up their masses.

• Stars that are slightly more massive will occupy the center of infall more often, and thus grow even faster.

• Such competitive accretion leads to delays for the smallest objects. If these “runts of the litter” are ejected before reaching 0.08 Msun, they will forever remain as brown dwarfs.

[Reipurth & Clarke AJ, 122, 432, 2001].

The Bate, Bonnell, Bromm 2003 Simulation - 1

• Matthew Bate, Ian Bonnell, and Volker Bromm [MNRAS 339, 577, 2003] have calculated the collapse of a 50 Msun cloud core with a diameter of 0.375 pc.

• The cloud is turbulent.

• The simulation runs for 266,000 years.

• It was carried out on the UK Astrophysical Fluid Facility, and required 100,000 CPU hours, about 10% of the total time available during one year.


• As the simulation proceeds, the turbulent motions in the cloud form shock waves that slowly damp the supersonic motions.


• When enough energy has been lost in some regions of the simulation, gravity can pull the gas together to form a dense sub-core.


• The formation of stellar embryos begins in the densest sub-core.

• The embryos rapidly accrete gas and grow in mass.


• As the stars and brown dwarfs interact dynamically with each other, many are ejected from the cloud.


• At the same time that star formation continues in the dense sub-cores, members that have been ejected scatter throughout and beyond the cloud.


• The following slides show enlargements of specific regions of interest.

• They are 16 times smaller than the previous images, which showed the entire cloud.

• Each of the following squares are 5100 AU on the side.

• The first star formation event in the cloud starts with the birth of a binary.


• Both gas filaments and disks form stars and brown dwarfs. Some of the remaining gas falls in around these protostars to form circumstellar disks, helping to build up the masses of the protostars.


• Stars and brown dwarfs fall into a cluster.

• At this stage the objects range in mass from nearly the mass of the Sun down to as small as the mass of Jupiter.

• A star with an edge-on disk is ejected towards the left of the field.


• An unstable system of 5 stars (lower right) breaks up and ejects stars from the cloud in three different directions.


• After a pause, star formation begins again with the gathering together of more gas (center).

• The disks of gas that form around the protostars also contain a lot of mass, and gravity can also cause protostars to form within these disks.


• The orbits of these objects formed in circumstellar disks are unstable, and they are quickly ejected from the system.

• Because they are ejected out of the cloud before they have assembled their final masses, many of these objects will remain substellar, i.e. as brown dwarfs.


• The gas flows in the complex potential of the cloud are highly structured.

• Many of the stars and brown dwarfs are surrounded by disks which are truncated during encounters.


• A brown dwarf with a large disk is ejected to the lower left.


• The simulation was terminated after 266,000 years.

Cluster Formation

[Bate, Bonnell, Bromm 2003, MNRAS, 339, 577].

The simulation runs for 266,000 years, i.e. a second of the movie corresponds to 10,000 years.

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FUORS IN EARLY STELLAR EVOLUTION

• FUors are eruptive variables in star forming regions, brightening by 5-6 mag

• They have F- or G-type supergiant spectra without emission lines (Herbig 1977).

• In the near-IR they have strong broad CO bandheads in absorption (Reipurth & Aspin 1997).

• Only about a dozen FUors or FUor-like objects are known.

• FUor events are likely to be repetitive, but too few are known for good statistics

Decay Times and Activity

• The two best studied FUors are FU Orionis itself, which erupted in 1936, and V1057 Cyg, which exploded in 1970.

• The two objects have similar amplitudes, but whereas FU Ori is still near maximum light, V1057 Cyg has decayed dramatically, as seen in the lightcurve here.

• Even near minimum, V1057 Cyg has extreme mass loss, with dramatic P Cygni profiles that is highly variable. The figure shows spectra between 1996 and 2001, from Herbig et al. (2001).

Herbig et al. 2001

Triggering Mechanisms

• Several triggering mechanisms have been proposed:

[1] A throttle mechanism in which the accretion rate through a disk varies around the more stable infall rate from an envelope (Hartmann & Kenyon 1996)

[2] A large-scale thermal ionization instability may affect the disk and lead to occasional emptying-out of the inner disk regions (Bell & Lin 1994)

[3] A companion star on an elliptical orbit may perturb the inner disk regions during periastron passage (Bonnell & Bastien 1992)

Binary FUors• Several FUors have companions, including the

famous Z CMa and L1551 IRS5.

• In two cases, both components in a binary are known to be FUors: RNO 1B/C (Kenyon et al. 1993) and AR6A/B (Aspin & Reipurth 2003).

• Given the rarity of FUors, this would be extremely unlikely if the events were not related, i.e., whatever triggered the eruption in one should be connected to what triggered the eruption in the other.

• The separations of the above FUor pairs are so large that it would take several thousand years from a periastron passage to reach their present locations. FUor eruptions probably do not last that long.

Orbital Evolution of Newly Bound Binaries

• An ejection will in some cases lead to a bound system and sometimes to an escape.

• A quadruple non-hierarchical system will often break up into two binary systems, which sometimes detach from each other, and sometimes form a bound hierarchical quadruple system.

• The dynamic evolution is a stochastic process that cannot be predicted, only studied statistically.

• In the process of being bound, such newly formed binaries will have highly eccentric orbits (e~0.9).

• These newborn binaries will evolve with significant viscous interactions, and as a result start to spiral together.

Courtesy P. Artymowicz

Herbig-Haro Jets and FUors

• As the components spiral towards each other on highly eccentric orbits, they trigger disturbances at periastron leading to increased accretion and outflow, eventually forming the highly collimated Herbig-Haro jets.

• Eventually they get so close (<10AU or so) that they disturb the inner regions where magnetic fields are anchored, and the stars lose the ability to produce jets, even though they still have major mass loss events.

• The final periastron passages before the individual stellar disks are converted to a circumbinary disk will appear as major accretion events, i.e. as FUors.

HH 34L1551 IRS5

In this picture, FUors occur right after the period of jet formation. Some FUors like L1551 IRS5 still drive small jets.

Binary FUors and the Time Scale Problem

• The key to the disintegrating multiple star scenario is that it can solve the time scale problem: how does it happen that two stars next to each other can both turn into FUors but being too far apart to affect each other?

• The solution is that FUor pairs are really quadruple systems which transformed from an unstable non-hierarchical configuration to an orderly hierarchical configuration.

• The quadruple then evolves slowly through viscous interactions until the components are so close that they erupt in FUor outbursts at about the same time.

1

2

3

Simulations of Disk-Disk Encounters - I

• Pfalzner and collaborators have explored the encounters of two stars with disks using SPH simulations.

• In the present simulation, a 1 Msun star with a disk with 50 AU radius moves past a similar object with a velocity of 10 km/sec in a grazing collision. The encounter generates strong spiral structures and leads to severe disk disturbance and truncation of the outer disk regions.

Pfalzner, Henning, Kley 2000

Simulations of Disk - Disk Encounters - II

Courtesy Anthony Whitworth and Henri Boffin

The Companion to FU Orionis

• Wang et al. (2004) discovered a star 0.5” (230AU) from FU Ori and suggested it could be a companion.

• We used Subaru to do photometry and spectroscopy and indeed found the new companion to be a pre-main sequence star, most likely a K-type star with IR excess

(Reipurth & Aspin 2004)

Is the FU Ori Companion related to the Eruption?

• If the companion was ejected in 1936 when FU Ori erupted, then it must have a tangential velocity of 15 km/sec. This is highly unlikely, since we do not see T Tauri stars zooming around in star forming regions.

• But the companion could have been ejected a few thousand years ago. FU Ori must in this picture be a close binary with a separation of roughly 10 AU, i.e. the system should be a triple. In the intervening years the newborn binary has had time to spiral together, reaching now the period of FUor outbursts.

• We should be able to detect this close binary spectroscopically, since the mean velocity would be around 5 km/sec. However, in a highly eccentric orbit almost all the velocity change occurs near periastron, which took place in 1936. Currently little velocity variability is expected.

Should FUor Eruptions be Periodic?

• In principle, FUor outbursts should take place at regular intervals at each periastron passage.

• However, other factors play in, including how long it takes to replenish and reconfigure the disturbed disks following the previous closest encounter.

• If a disk requires much longer to reconstitute than the time interval to the next periastron passage, then there will not be another eruption for a while.

• In this picture, FUors should be a very heterogeneous group, a veritable zoo of objects with broad similarities, but distinct differences in specific characteristics.

Has the Sun always been Single?

• Statistically, the Sun is more likely to have been born in a small multiple system

• The obliquity of the Solar System: the mean plane of the planetary orbits is tilted 7 degrees from the Sun’s rotation axis

• This could be understood if the Sun was ejected from a small group of newborn stars

Dynamical Interactions in Pre-Main Sequence Evolution

Dynamical interactions between multiple components and their stochastic nature are essential for understanding a variety of phenomena in early stellar evolution:

1 - FUor outbursts

2 - The formation of brown dwarfs

3 - The jet phenomenon

4 - The diversity of disk properties in CTTS and WTTS

5 - The binary separation distribution function

6 - The obliquity of the solar system

Attempting to understand star formation through the birth of single stars is nolonger a sufficient approach

STAR FORMATION IN CLUSTERS

• Isolated star formation in dark clouds is not so common.

• Most stars are born in clusters of stars similar to the Orion Nebula cluster, containing a few thousand stars.

Newborn Stars in Clusters are Irradiated

CourtesyJohn Bally

HH 502 inOrionACS/HST

Astrobiological Implications

• Most stars in our Galaxy are born in rich clusters of a hundred to a thousand stars

• Most stars in our Galaxy are binaries, and perhaps all stars are born in small multiple systems

• Whenever we think about conditions for other life forms in other planetary systems, it is important to not employ a heliocentric bias.

• The Sun most probably was not born in isolation, but rather in a large cluster, and possibly/probably in a small multiple system that disintegrated shortly after birth.

Upcoming Meetings

• CHONDRITES AND THE PROTOPLANETARY DISK

November 8-11 2004 Kaua’i, Hawaii

http://www.lpi.usra.edu/meetings/chondrites2004

• PROTOSTARS AND PLANETS V

October 24-28 2005 Hilton Waikoloa Village, The Big Island, Hawaii

http://www2.ifa.hawaii.edu/CSPF/ppv/ppv.html

The Formation of Single and Multiple Stars Bo Reipurth Institute for Astronomy Astrobiology Seminar...

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