Herbig-Haro Object

What HH objects are and what they mean

An HH object is a Rosetta Stone hiding in plain sight. To the naked eye on an HST image it looks like a tidy chain of knots, sometimes with a curved cap at the far end, sitting at the edge of a dark molecular cloud. To the physics, it is the visible signpost of an otherwise invisible event: a protostar buried in dust is launching a tightly collimated jet at 150 - 400 km/s, the jet has travelled tens of thousands of AU through the surrounding cloud, and at each knot the leading material has rammed into a denser parcel of ambient gas. The collision shock-heats the gas to ~10⁴-10⁵ K and excites a forest of optical forbidden lines. Hence the brightness.

This makes HH objects valuable in two ways. First, they reveal the geometry, mass-loss rate, and history of jets from young stellar objects that are otherwise too dust-obscured to study directly. Second, they trace the feedback channel that lets accretion proceed: a star cannot collapse without losing angular momentum, and the bipolar jet (typically carrying 1-10% of the accreted mass) does much of that bookkeeping.

Worked example — HH 1/2 in Orion

The prototype pair HH 1 and HH 2 lie ~3 arc-minutes apart in the L1641 cloud of Orion, at distance d ~ 460 pc. They are the two opposite ends of a bipolar jet driven by an embedded Class 0 source named VLA 1 (also called HH 1/2-VLA 1), midway between them. We can pin down the kinematics with HST proper-motion data alone:

Angular separation HH 1 ↔ VLA 1   θ ≈ 1.5'  = 9.0 × 10⁼⁴ rad
Linear separation at 460 pc           L = d · θ ≈ 460 pc × 9.0 × 10⁼⁴
                                          ≈ 0.41 pc = 1.27 × 10¹⁶ m
Proper motion (HST 1994 ↔ 2007)    μ_pm ≈ 0.15 "/yr
Tangential velocity                   v_t = d · μ_pm
                                          = 460 pc · (0.15 "/yr / 206265")
                                          ≈ 327 km/s
Adopting inclination ~25° from POS:  v_jet = v_t / cos(25°)  ≈ 360 km/s

Dynamical age                         t = L / v_jet
                                          ≈ (1.27 × 10¹⁶ m) / (3.6 × 10⁵ m/s)
                                          ≈ 1100 yr

So the HH 1 knot has been travelling outward for about a millennium — a typical HH dynamical age — and the kinematics agree (within factor of 2) with the jet velocities measured spectroscopically. The driving source itself has a total bolometric luminosity ~40 L_⊙, consistent with a deeply embedded ~0.5 M_⊙ protostar in late Class 0 / early Class I phase.

The physics of the shock

An HH bow shock is one of the cleanest astrophysical examples of a hydrodynamic shock in a partially-ionised medium. The jet, with pre-shock velocity v_jet, density n_j, slams into ambient cloud gas with density n_a. Conservation of momentum across the shock front gives the post-shock temperature:

T_s ≈ (3/16) (μ m_H / k_B) v_s²

For v_s = 150 km/s, μ ≈ 0.6 (ionised gas):
T_s ≈ (3/16) × (0.6 × 1.67 × 10⁻²⁷ kg / 1.38 × 10⁻²³ J/K) × (1.5 × 10⁵ m/s)²
    ≈ 5.1 × 10⁵ K

That is hot enough to ionise hydrogen and excite the optical forbidden lines we see. As the post-shock gas cools through ~10⁴ K, recombination radiation lights up Hα; collisional excitation of S⁺, O⁰, and N⁺ levels pumps the forbidden-line ratios that diagnose density (typically n_e = 10³-10⁴ cm^-3) and shock velocity. The [SII]/Hα ratio > 0.4 is the standard observational signature distinguishing HH shocks from photoionised HII regions where [SII]/Hα < 0.2.

For a jet that pulses — mass loss not constant but varying with period τ — faster ejecta catch up with slower earlier material, producing internal "working surfaces" along the jet that show up as regularly spaced knots. The spacing ΔL ≈ τ · v_jet and is typically 10-1000 AU for periods of 10-200 yr.

A short tour of the HH catalogue

HH #	Region	Distance (pc)	Notable	Driving source class	Discoverer / first imaged
HH 1 / HH 2	Orion (L1641)	460	Prototype bipolar pair; HST proper motions	Class 0 (VLA 1)	Herbig 1951, Haro 1952
HH 7-11	NGC 1333, Perseus	235	String of 5 knots from SVS 13	Class I	Strom et al. 1974
HH 24-26	Orion (L1630)	414	Complex multi-jet system	Class I-II	Herbig 1974
HH 30	Taurus	140	Edge-on disk + bipolar jet; HST textbook image	Class I-II T Tauri	Mundt 1983, HST 1995
HH 34	Orion	414	Most photographed HH jet; chain of 7 knots	Class I	Reipurth 1985
HH 47	Vela molecular ridge	450	Curved sinuous jet; HST proper motions 100-300 km/s	Class I	Schwartz 1977
HH 211	Perseus IC 348	320	One of the youngest known (Class 0); SiO + CO	Class 0	McCaughrean 1994
HH 212	Orion (L1630)	400	Spectacularly symmetric, jet visible in SiO with ALMA	Class 0	Zinnecker 1996

How the field came together

• 1951. George Herbig at Lick photographs a small nebula in Orion with peculiar emission spectrum; recognizes it as a new class but does not yet understand the origin.
• 1952. Guillermo Haro at Tonantzintla independently catalogues the same and similar objects in Orion; the joint discovery becomes the basis for the "Herbig-Haro" designation.
• 1976. Don Osterbrock and others recognize the spectra as shock-excited, not photoionised, based on [SII]/Hα ratios.
• 1977. Schwartz proposes that HH objects are produced by collimated stellar winds from young stars.
• 1980s. Reinhard Mundt and colleagues at Calar Alto first directly resolve jets feeding HH objects (HH 30, HH 47).
• 1982. Blandford-Payne magnetocentrifugal wind theory provides the launching mechanism.
• 1994-2000. HST imaging campaigns by Reipurth and others measure proper motions of dozens of HH objects, confirming jet velocities of 100-400 km/s.
• 2010s. ALMA resolves the molecular SiO and CO outflows that accompany the optical HH jets in Class 0 sources too embedded for HST — e.g. HH 212.
• 2024. JWST NIRCam and MIRI imaging detects HH bow shocks through tens of magnitudes of extinction, finding new HH objects in clouds previously opaque to optical telescopes.

Why HH objects matter for star formation

• Visible tracer of invisible accretion. Most Class 0/I protostars are buried in 10-100 mag of optical extinction. The HH jet escapes the cocoon and lights up far from the source, letting us infer accretion rates and ages from the jet's properties alone.
• Angular momentum disposal. Spherical collapse is impossible because of conservation of angular momentum; the protostellar jet carries off much of the cloud's J, allowing further infall.
• Feedback into the parent cloud. Each bow shock injects ~10⁴³-10⁴⁵ erg into the cloud, contributing to turbulence support and eventually disrupting the natal core.
• Chemistry laboratory. Shock chemistry produces SiO, SO, CH₃OH, and other molecules at concentrations far above quiescent cloud values — HH jets are some of the brightest extra-galactic SiO sources at sub-mm wavelengths.
• Disk wind launching diagnostic. The width, opening angle, and rotation of HH jets test models of how disks launch and collimate magnetocentrifugal winds.
• Episodic accretion timeline. Knot spacings record bursts of higher accretion (e.g., FU Orionis events) at 10-200 yr intervals throughout the Class I phase.

Common misconceptions

• "HH objects are emission nebulae like H II regions." No — the excitation is shock, not photoionisation. The diagnostic [SII] / Hα > 0.4 is much higher than in H II regions.
• "The driving star is at the centre of the knots." No — the central source often sits between widely separated HH objects (HH 1 and HH 2 are 3' apart but their driver lies midway). The chain traces the jet path, not the star's position.
• "HH knots are static." Each knot moves outward at hundreds of km/s. HST proper-motion campaigns directly measure this displacement over a decade.
• "Only one HH object per young star." No — multi-pulse jets produce chains of dozens of HH knots, and embedded sources can host multiple jets simultaneously (HH 24-26 complex).
• "HH objects are short-lived blips." Individual knots fade in 10²-10⁴ yr, but the underlying jet runs continuously over 10⁵-10⁶ yr of the Class 0-I phase.
• "They form planets." No — planets form in the disk, not the jet. HH objects are the loss channel for material that doesn't accrete.

Open questions

• Launching radius. Does the jet originate at the inner edge of the disk (X-wind), throughout the disk (disk wind), or in the stellar magnetosphere (stellar wind)? Different models predict different rotation signatures at the jet base, which ALMA is beginning to resolve in HH 212 and others.
• Asymmetry. Many HH systems are bipolar but the two lobes have different velocities — HH 47 in particular shows the redshifted lobe slower by 30%. Why isn't the launching mechanism symmetric?
• FU Orionis link. Are knot spacings actually triggered by FU Ori outbursts? Statistical samples are still small.
• Jet rotation. Several HH jets (DG Tau, RW Aur) show rotational signatures consistent with magnetocentrifugal launching. But the measurement is at the limit of current spectroscopy.