AST 6336, Interstellar Medium, Spring 2015 Young stellar clusters (lectures by Nicola Da Rio ndario@ufl.edu) January 2, 4, 2015
Star formation A molecular cloud may become unsupported gas pressure + magnetic fields + internal turbulence insufficient to counterbalance gravity The cloud collapses and fragments due to internal motions. The denser knots prestellar cores may end up collapsing into one or multiple stars
Stellar clusters and associations Stars form in groupings (clusters and associations): 1. Stellar aggregates contain young stars These stars are young because of their photospheric characteristics: Brighter than main sequence larger radii - still contracting Show photospheric signatures of infalling gas H recombination lines Show evidence of warm dust surrounding them IR excess from heated dusty disks Show prominent X-ray emission typical of young convective stars These stars must have formed recently because they are surrounded by gas 2. Massive stars are found in stellar aggregates Massive stars are short lived (few Myr for M>20Msun) they must have formed recently Therefore the aggregates themselves must be young Not all these groupings survive as bound clusters Early dissolution of the stellar population into the field ``infant mortality, or slow expansion and stellar loss. This can be concluded from the comparison of the current star formation rate (stars per unit of time per unit of volume) with the number of stars in open clusters per interval of age. 90% of the current star local star formation output feeds the galactic field population Trapezium cluster (Orion Nebula Cluster) Pleiades (100Myr)
Embedded clusters At early stages tt~ < 1 MMMMMM young clusters are deeply embedded within their parental molecular cloud. The high dust extinction produced by the high column density of dust limits the detection of their stellar population at optical wavelengths. Absolute magnitude: Distance modulus: Dust extinction Sun: MM VV ~5 mmmmmm μμ = 5 log dd 5 + 10pc μμ = 0 + 100pc μμ = 5 1kpc μμ = 10 10kpc μμ = 10 Low: AA VV < 5 Intermediate AA VV ~5 15 High AA VV ~20 100 Trapezium cluster (Orion Nebula Cluster) Approximate limit of optical telescopes: V=20-25 mag (ground based); 28-30 (HST)
Embedded clusters Dust extinction is significantly lower at longer wavelengths. ONC optical Trapezium cluster (Orion Nebula Cluster) Note: at large distances within the disk of the MW, the buildup of foreground extinction from the disk of the galaxy itself becomes important too. ONC JH
Detection of embedded clusters, NIR surveys Near infrared photometric surveys enable the discovery of embedded young clusters. an excess of density of sources in a given location may be cluster candidate color-magnitude and color-color diagram, compared to adjacent fields, can identify the young population and help estimating its properties (number of stars, average age, ) AV direction ONC optical Trapezium cluster (Orion Nebula Cluster) IR excess sources ONC JH
IR excess young stars Some young stars exhibit an excess of flux at long NIR wavelengths (λλ > 2μμμμ), as their NIR colors cannot be reproduced invoking only dust extinction. Once the observed multi-wavelength fluxes are corrected for extinction (e.g., based on multi-band photometry at shorter wavelengths, or e.g., knowing the spectral type of the source from spectroscopic measurements, the spectral energy distribution (SED) of the excess is isolated. The excess has a circumstellar origin, and represents the emission of the dust in the inner part of the circumstellar disks, heated by the central star (TT dddddddd < TT ssssssss ) Since circumstellar disks have short lifetimes (<10Myr) as their material is either photoevaporated, accreted, or condenses in planets, circumstellar disk presence from IR excess is an indicator of youth. ρρ Ophiuchi dark cloud complex ONC optical Trapezium cluster (Orion Nebula Cluster) Upper Sco OB association IR spectral index αα IIII = dd log λλff λλ dd log λλ ONC JH
IR excess young stars Class I αα 2 10μμμμ > 0 Protostar: emission originates from accreting disk + envelope Class II 1.5 < αα 2 10μμμμ < 0 Pre-main sequence star: IR emission from dusty circumstellar disk ONC optical Trapezium cluster (Orion Nebula Cluster) Class III αα 2 10μμμμ < 1.5 Older pre-main sequence star. Disk partially cleared inside-out ONC JH
T and OB-associations Over a time of ~1Myr embedded clusters start to be optically revealed. We can distinguish different categories. T-associations: Systems that were born in low-density dark clouds that did not contain massive stars. Loose distributions of young pre-main sequence stars Typical example: the Taurus star forming region. OB-associations: The unbound, expanding grouping of newly formed stars, containing massive, bright OB stars. The high effective temperature of OB stars (TT eeeeee > 10,000KK) ionizes the remaining gas creating HII regions more massive than T associations. Actual clusters: When the stellar population remains bound (EE = KKKK + PPPP < 0) after gas removal. Note that a fraction (large or small) of stars may escape the stellar system in its early phases of evolution even when a bound clusters survives afterwards.
Taurus association Distance: 140 pc Low density of stars: 1-10 stars/pc 3 spans over 15 degrees on the plane of the sky Absence of massive stars. Many stars are distributed in relatively compact clumps which follow the density of gas ~1 Myr old (with age spread largely unknown) Luhman (2009)
Memberships of young associations Especially (but not only) for the case of loose T associations, the isolation of the young members from field (background and foreground) galactic field stars might be challenging. There are several memberships indicators that can be used; they are in many ways complementary, as each of them alone does not guarantee to accurately identify (if at all) the young nature of PMS stars. Limiting to low-mass PMS stars, these are: 1. Flux excesses and other spectral characteristics Disk excess: as it traces the presence of a circumstellar disk. Note that for more evolved class III sources, the signal might be weak, requiring observations at long wavelengths (MIR, FIR, sub mm) Accretion signatures: as matter falls from the inner disk on to the star, the loss of gravitational potential energy is converted into luminosity, with a typical spectrum including UV excess, optical continuum, and Hydrogen recombination lines. The Hα line at λ=6563 Angstrom excess, in particular, is most commonly used since practical to measure, and bright. A standard classification is as follows: Classical T-Tauri stars: E. W. Hαα > 20 AA Weak-line T-Tauri stars: 0 < E. W. Hαα < 20 AA Other accretion indicators are H and K-lines of Ca II at 3968 and 3934 Angstrom X-rays: young low mass stars are generally X-ray emitters, originating in plasma explosively heated and confined in magnetic loops following magnetic reconnection events. This is an analogous of solar flares, and is facilitated by the turbulent and highly convective interiors of low mass PMS stars 2. Kinematic common motions The velocity dispersion of the members of a young star forming region is generally of a few km/s, whereas the dispersion of field stars may be more than one order of magnitude higher. Proper motions and radial velocity surveys help to isolate the candidate members that share the same bulk motion. proper motions of the members in Taurus
OB associations An aggregate of young stars containing early (OB) type stars. B-type stars: TT eeeeee > 10,000 KK; MM 4MM O-type stars: TT eeeeee > 30,000 KK; MM 20MM The large mass of these stars requires the stellar population to be more numerous than T-associations; thus OB associations originate from more massive clouds. The UV radiation of early type stars is ionizes the surrounding gas, which then emits Hydrogen lines as it recombines forming an HII region. LH 95 (Large Magellanic Cloud) Note that massive star early evolution is fast enough that these objects are already on the main sequence as soon as the stellar system is non embedded. Cygnus OB2 NGC6914 Spectroscopic analysis can identify if an early type star is in main-sequence or already leaving the main sequence. In the first case, it follows a given temperature-luminosity relation, which can be exploited to determine their distance (but reddening must be corrected for).
Nearby young associations and Gould s belt T associations Many close star forming regions, as well as many of the nearby bright stars, lie on a ring-like structure called Gould s belt. It has a radius of ~300 pc, and is tilted by 20 with respect to the Galactic plane. Kinematic measurements show that the belt is expanding and slightly rotating. Age ~ 25Myr OB associations
Expansion and subgrouping of complexes of star forming regions Young star forming regions are often part of larger complexes including subgroups. An example is shown below: the Scorpius-Centaurus association. These subgroups generally formed as part of a joint structure of clouds, or molecular cloud complex. However, different subgroups may have formed at different times Δtt 10 Myr. As young association tend to expand over time, as a result of excess kinetic vs. potential energy, the concentration of an association is a (very rough) indicator of its age. Proper motions in this case can be used to track back the moment in the past when the stellar population was at its maximum density. Upper Scorpius
Hyades The proper motion are aligned towards a convergence point, where the open cluster appear to shrink towards. This indicates that the cluster is receding. The determination of the convergence point, together with measured proper motions and radial velocities can be used to determine the distance of the cluster: θ VV ττ = tan θθ ; VV VV ττ = μμ dd rr dd = VV rr tan θθ μμ
Blue: optical Red: HII Green: H 2 The Orion complex Trapezium cluster Orion Nebula
Blue: optical Red: HII Green: H 2 Extinction map (Lombardi+2007)
Blue: optical Red: HII Green: H 2 Orion OB association groups Orion OB Ia: ~12 Myr Orion OB Ib: ~8 Myr Orion OB Ic: ~3 6 Myr Orion OB Id: ~2 Myr - Orion Nebula
Blue: optical Red: HII Green: H 2 Megeath+ (2012) Young low mass stars in the Orion complex, part of the same population of the youngest Orion OB Id population, are identified through their NIR and MIR excess emission from circumstellar material. They are confined in filamentary structures which follow the Orion A and B clouds.
Subgrouping on smaller scales in the Orion complex Alves+ (2012) Analysis of the properties of low-mass pre-main sequence stars: lower AV and higher fraction of Class III sources vs Class II to the south of the Orion Nebula may indicate an older population.
Orion: young members follow the large kinematic structure of the remaining gas known members unknown membership 13 CO (Bally+1987) NGC 1981 NGC 1977 OMC 2/3 ONC NGC 1980 L1641-N L1641-S Radial velocity from spectroscopy
Orion: higher velocity dispersion of stars compared to gas STARS GAS ( 13 CO) L1641S L1641N ONC
Large Magellanic Cloud d=50kpc [Fe/H]=-0.4
Large Magellanic Cloud d=50kpc [Fe/H]=-0.4 30Doradus R136
Large Magellanic Cloud d=50kpc [Fe/H]=-0.4 SN1987A
Large Magellanic Cloud d=50kpc [Fe/H]=-0.4 LMC-4 superbubble
Large Magellanic Cloud d=50kpc [Fe/H]=-0.4 LMC-4 superbubble LH95 OB association DSS ~ 3 3 Credit: Davide de Martin (ESA/Hubble )
Ages of young stellar associations 1. Embeddedness (high AV = young). 2. IR excess (temporal evolution Class I Class II Class III). 3. Proper motions to track back the expansion of a stellar system 4. Presence of unevolved massive stars sets upper limits to cluster age tt MMMM MM 2.5 5. Turn off age: the luminosity (and mass) of the turn off point from the main sequence decreases with time 6. Pre-main sequence H-R diagram vs evolutionary models
Pre-main sequence ages The long contraction time needed for PMS stars to reach the MS can be used as a clock to measure stellar and cluster ages. BUT: 1) theoretical models for PMS star are more uncertain than those for MS stars 2) determination of TT eeeeee and log LL for PMS stars is challenging
Open clusters The remain of OB associations that have not undergone complete disruption. Size: 2-10pc. Despite their survival (ages 30-1000Myr) not all of them are bound. No or little molecular gas left. No active star formation. The members are coeval. ~1200 known systems (but sample is limited by galactic disk extinction), distributed. Precious laboratories of stellar evolution: simple stellar populations over a range of ages. Accurate photometric measurements allow to test theoretical models of stellar evolution and spectra of stars. Draper (1930): discovery of dust extinction: smaller (= farther away) young clusters appear dimmer. Pleiades (130 Myr) NGC4755 (16Myr) Hyades (650Myr)
Embedded massive stars Massive stars at very young ages can be still deeply embedded Their UV radiation can create Ultra Compact HII regions Smaller than HII regions (rr < 0.3pppp) Radiation absorbed and re-emitted at longer wavelengths (MIR, FIR, Radio)
Runaway massive stars 25% of O-type stars are not spatially associated with young clusters or clouds. Many are located farther from the Galactic plane, and show velocities vv = 50 100 km/s Their velocity originates from: Multi-body stellar encounters As ex-companions of supernovae