Young Low-mass Stars Research Articles

Modeling ϵ Eri and Asteroseismic Tests of Element Diffusion

Taking into account the helium and metal diffusion, we explore the possible evolutionary status with a seismic analysis, of the MOST (Microvariability and Oscillations of STars) target: the star epsilon Eri. We adopt different input parameters to construct models to fit the available observational constraints in, e.g., T-eff, L, R and [Fe/H]. From the computation we obtain the average large spacings of epsilon Eri to be about 194 +/- 1 mu Hz. The age of the diffused models was found to be about 1 Gyr, which is younger than the age determined previously by models without diffusion. We found that the effect of pure helium diffusion on the internal structure of the young low-mass star is slight, but that of metal diffusion is obvious. The metal diffusion leads the models to have much higher temperature in the radiative interior, and, correspondingly a higher sound speed there, hence a larger frequency and spacings.

Encounters in the ONC – observing imprints of star-disc interactions

The external destruction of protoplanetary discs in a clustered environment acts mainly due to two mechanisms: gravitational drag by stellar encounters and evaporation by strong stellar winds and radiation. If encounters play a role in disc destruction, one would expect that stars devoid of disc material would show unexpectedly high velocities as an outcome of close interactions. We want to quantify this effect by numerical simulations and compare it to observations. As a model cluster we chose the Orion Nebula Cluster (ONC). We found from the observational data that 8 to 18 stars leave the ONC with velocities several times the velocity dispersion. The majority of these high-velocity stars are young low-mass stars, among them several lacking infrared excess emission. Interestingly, the high-velocity stars are found only in two separate regions of the ONC. Our simulations give an explanation for the location of the high-velocity stars and provide evidence for a strong correlation between location and disc destruction. The high-velocity stars can be explained as the outcome of close three-body encounters; the partial lack of disc signatures is attributed to gravitational interaction. The spatial distribution of the high-velocity stars reflects the initial structure and dynamics of the ONC. Our approach can be generalized to study the evolution of other young dense star clusters, like the Arches cluster, back in time.

UV Excess Measures of Accretion onto Young Very Low Mass Stars and Brown Dwarfs

Low-resolution spectra from 3000 to 9000 Å of young low-mass stars and brown dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission arising in the Balmer and Paschen continua yields mass accretion rates ranging from 2 × 10−12 to 10−8 M☉ yr−1. These results are compared with HST STIS spectra of roughly solar-mass accretors with accretion rates that range from 2 × 10−10 to 5 × 10−8 M☉ yr−1. The weak photospheric emission from M dwarfs at <4000 Å leads to a higher contrast between the accretion and photospheric emission relative to higher mass counterparts. The mass accretion rates measured here are systematically ~4-7 times larger than those from Hα emission line profiles, with a difference that is consistent with but unlikely to be explained by the uncertainty in both methods. The accretion luminosity correlates well with many line luminosities, including high Balmer and many He I lines. Correlations of the accretion rate with Hα 10% width and line fluxes show a large amount of scatter. Our results and previous accretion rate measurements suggest that ∝ M1.87 ± 0.26 for accretors in the Taurus molecular cloud.

An assessment of Li abundances in weak-lined and classical T Tauri stars of the Taurus-Auriga association

Accurate measurements of lithium abundances in young low-mass stars provide an independent and reliable age diagnostics. Previous studies of nearby star forming regions have identified significant numbers of Li-depleted stars,often at levels inconsistent with the ages indicated by their luminosity. We aim at a new and accurate analysis of Li abundances in a sample of ~100 pre-main sequence stars in Taurus-Auriga using a homogeneous and updated set of stellar parameters and model atmospheres appropriate for the spectral types of the sample stars.We compute Li abundances using published values of the equivalent widths of the Li 6708 A doublet obtained from medium/high resolution spectra. We find that the number of significantly Li-depleted stars in Taurus-Auriga is greatly reduced with respect to earlier results. Only 13 stars have abundances lower than the interstellar value by a factor of 5 or greater. All of them are weak-lined T Tauri stars drawn from X-ray surveys; with the exception of four stars located near the L1551 and L1489 dark clouds, all the Li-depleted stars belong to the class of dispersed low-mass stars, distributed around the main sites of current star formation. If located at the distance of Taurus-Auriga, the stellar ages implied by the derived Li abundances are in the range 3-30 Myr, greater than the bulk of the Li-rich population with implication on the star formation history of the region. In order to derive firm conclusions about the fraction of Li-depleted stars of Taurus-Auriga, Li measurements of the remaining members of the association should be obtained, in particular of the group of stars that fall in the Li-burning region of the HR diagram.

A LARGE-SCALE OPTICAL-NEAR-INFRARED SURVEY FOR BROWN DWARFS AND VERY LOW MASS STARS IN THE ORION OB1 ASSOCIATION

We report the initial results of a large-scale optical-near infrared survey to extend the known young population of the entire Orion star-forming region down to the substellar domain. Using deep optical I-band photometry and data from the 2MASS survey, we selected candidates across ~14.8 deg^2 in the ~8 Myr old Ori OB1a subassociation and over ~6.7 deg^2 in the Ori OB1b subassociation (age ~3), with completeness down to 0.05Mo and 0.072Mo respectively. We obtained low resolution optical spectra for a subsample of 4 candidates in Ori OB1a and 26 in Ori OB1b; as a result we confirmed 3 new members in Ori OB1a, one of which is substellar, and 19 new members in Ori OB1b, out of which 7 are at the substellar limit and 5 are substellar. We looked into the presence of accretion signatures by measuring the strength of the Ha line in emission. Accordingly, we classified the new members as having Classical T-Tauri star (CTTS) or Weak Lined T Tauri star-like (WTTS) nature. We found that all the new members confirmed in Ori OB1a are WTTSs, while 39 +25/-22 % of the new members in Ori OB1b exhibit CTTS-like behavior, suggestive of ongoing accretion from a circum(sub)stellar disk. Additionally we found that none of the members confirmed in OB1a show near-IR color excess while 38 +26/-21 % of OB1b members show H-K color excess. These results are consistent with recent findings for low mass young stars in Orion OB1. The similarity in CTTS-like properties and near-IR excess across the substellar boundary gives support to the idea of a common formation mechanism for low mass stars and at least the most massive brown dwarfs. Finally, we remark the discovery of two new members classified as CTTSs, both exhibiting W(Ha) < -140 A, suggesting significant ongoing accretion.

The c2dSpitzerSpectroscopic Survey of Ices around Low‐Mass Young Stellar Objects. II. CO2

This paper presents Spitzer IRS λ/Δ λ ∼ 600 spectroscopy of the CO2 15.2 μm bending mode toward 50 embedded young low-mass stars, taken mostly from the "Cores to Disks" (c2d) Legacy program. The average abundance of solid CO2 relative to water in low-mass protostellar envelopes is 0.32 ± 0.02, significantly higher than that found in quiescent molecular clouds and in massive star-forming regions. A decomposition of the observed CO2 bending mode profiles requires a minimum of five unique components. Roughly of the CO2 ice is found in a water-rich environment, while most of the remaining is found in a CO environment with strongly varying relative concentrations of CO2 to CO along each line of sight. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO2:CO component and suggest a model in which low-density clouds form the CO2:H2O component and higher density clouds form the CO2:CO ice during and after the freezeout of gas-phase CO. The abundance of the CO2:CO component is consistent with cosmic-ray processing of the CO-rich part of the ice mantles, although a more quiescent formation mechanism is not ruled out. It is suggested that the subsequent evolution of the CO2 and CO profiles toward low-mass protostars, in particular the splitting of the CO2 bending mode due to pure, crystalline CO2, is first caused by distillation of the CO2:CO component through evaporation of CO due to thermal processing to ~20-30 K. The formation of pure CO2 via segregation from the H2O rich mantle may contribute to the band splitting at higher levels of thermal processing (≳50 K) but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.

Deep MMT Transit Survey of the Open Cluster M37. II. Variable Stars

We have conducted a deep (15 r 23), 20 night survey for transiting planets in the intermediate-age (~550 Myr) open cluster M37 (NGC 2099) using the Megacam wide-field mosaic CCD camera on the 6.5 m MMT. In this paper we present a catalog and light curves for 1445 variable stars; 1430 (99%) of these are new discoveries. We have discovered 20 new eclipsing binaries and 31 new short-period (P < 1 day ) pulsating stars. The bulk of the variables are most likely rapidly rotating young low-mass stars, including a substantial number (500) that are members of the cluster. We identify and analyze five particularly interesting individual variables, including a previously identified variable that we suggest is probably a hybrid γ Doradus/δ Scuti pulsator, two possible quiescent cataclysmic variables, a detached eclipsing binary (DEB) with at least one γ Doradus pulsating component (only the second such variable found in an eclipsing binary), and a low-mass (MP ~ MS ~ 0.6 M☉) DEB that is a possible cluster member. A preliminary determination of the physical parameters for the DEB+γ Doradus system yields MP = 1.58 ± 0.04 M☉, MS = 1.58 ± 0.04 M☉, RP = 1.39 ± 0.07 R☉, and RS = 1.38 ± 0.07 R☉.

Chemical and Thermal Structure of Protoplanetary Disks as Observed with ALMA

We predict how protoplanetary disks around low-mass young stars would appear in molecular lines observed with the ALMA interferometer. Our goal is to identify those molecules and transitions that can be used to probe and distinguish between chemical and physical disk structure and to define necessary requirements for ALMA observations. Disk models with and without vertical temperature gradient as well as with uniform abundances and those from a chemical network are considered. As an example, we show the channel maps of HCO$^+$(4-3) synthesized with a non-LTE line radiative transfer code and used as an input to the GILDAS ALMA simulator to produce noise-added realistic images. The channel maps reveal complex asymmetric patterns even for the model with uniform abundances and no vertical thermal gradient. We find that a spatial resolution of $0.2-0.5\arcsec$ and 0.5--10 hours of integration time will be needed to disentangle large-scale temperature gradients and the chemical stratification in disks in lines of abundant molecules.

A Comprehensive View of Circumstellar Disks in Chamaeleon I: Infrared Excess, Accretion Signatures, and Binarity

We present a comprehensive study of disks around 81 young low-mass stars and brown dwarfs in the nearby ~2-Myr-old Chamaeleon I star-forming region. We use mid-infrared photometry from the Spitzer Space Telescope, supplemented by findings from ground-based high-resolution optical spectroscopy and adaptive optics imaging. We derive disk fractions of 52 (+/-6) % and 58 (+6/-7) % based on 8-micron and 24-micron colour excesses, respectively, consistent with those reported for other clusters of similar age. Within the uncertainties, the disk frequency in our sample of K3-M8 objects in Cha I does not depend on stellar mass. Diskless and disk-bearing objects have similar spatial distributions. There are no obvious transition disks in our sample, implying a rapid timescale for the inner disk clearing process; however, we find two objects with weak excess at 3-8 microns and substantial excess at 24 microns, which may indicate grain growth and dust settling in the inner disk. For a sub-sample of 35 objects with high-resolution spectra, we investigate the connection between accretion signatures and dusty disks: in the vast majority of cases (29/35) the two are well correlated, suggesting that, on average, the timescale for gas dissipation is similar to that for clearing the inner dust disk. The exceptions are six objects for which dust disks appear to persist even though accretion has ceased or dropped below measurable levels. Adaptive optics images of 65 of our targets reveal that 17 have companions at (projected) separations of 10-80 AU. Of the five <20 AU binaries, four lack infrared excess, possibly indicating that a close companion leads to faster disk dispersal. The closest binary with excess is separated by ~20 AU, which sets an upper limit of ~8 AU for the outer disk radius. (abridged)

Circumstellar disks around young low-mass stars: Observed properties and lifetime

Unveiling the physical processes leading to planet formation is one of the major challenges of modern astrophysics. Until the last fifteen years, the origin of planetary systems was mostly investigated from observations and modelling of our own Solar System. However, the new generation of astronomical facilities is slowly but surely changing this. Observations of circumstellar disks around young stars similar to the Sun in its infancy are now routinely provided by large astronomical facilities, from the optical wavelengths to the millimeter range. These observations begin to have the sensitivity and angular resolution needed to provide invaluable information inside the regions of disks where planets should form. In this review, I show some new results which illustrate how fast this new domain is evolving and which kind of physical/chemical parameters can be explored. In particular, I present what recent observations tell us about disk lifetime and evolution. I conclude by some open questions which represent important astrophysical challenges for tomorrow in order to understand how planetary systems form.

Spatially extended polycyclic aromatic hydrocarbons in circumstellar disks around T Tauri and Herbig Ae stars

Aims. Our aim is to determine the presence and location of the emission from polycyclic aromatic hydrocarbons (PAHs) towards low and intermediate mass young stars with disks using large aperture telescopes. Methods. VLT-VISIR N-band spectra and VLT-ISAAC and VLT-NACO L-band spectra of 29 sources are presented, spectrally resolving the 3.3, 8.6, 11.2, and 12.6 μm PAH features. Spatial-extent profiles of the features and the continuum emission have been derived and used to associate the PAH emission with the disks. The results are discussed in the context of recent PAH-emission disk models. Results. The 3.3, 8.6, and 11.2 μm PAH features are detected toward a small fraction of the T Tauri stars, with typical upper limits between 1 × 10^(−15) and 5 × 10^(−17) Wm^(−2). All 11.2 μm detections from a previous Spitzer survey are confirmed with (tentative) 3.3 μm detections, and both the 8.6 and the 11.2 μm features are detected in all PAH sources. For 6 detections, the spatial extent of the PAH features is confined to scales typically smaller than 0.12−0.34, consistent with the radii of 12−60 AU disks at their distances (typically 150 pc). For 3 additional sources, WL 16, HD 100546, and TY CrA, one or more of the PAH features are more extended than the hot dust continuum of the disk, whereas for Oph IRS 48, the size of the resolved PAH emission is confirmed as smaller than for the large grains. For HD 100546, the 3.3 μm emission is confined to a small radial extent of 12±3 AU, most likely associated with the outer rim of the gap in this disk. Gaps with radii out to 10−30 AU may also affect the observed PAH extent for other sources. For both Herbig Ae and T Tauri stars, the small measured extents of the 8.6 and 11.2 μm features are consistent with larger (≥100 carbon atoms) PAHs.

An optical spectroscopic HR diagram for low-mass stars and brown dwarfs in Orion

The masses and temperatures of young low mass stars and brown dwarfs in star- forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type vs. temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main sequence objects. The objects have been placed on an H-R diagram overlaid with theoretical isochrones. The low mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from 0.018 to 0.44Msun. The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters.

Precise measurement of the dynamical masses in AB Doradus

AbstractThe radio stars ABDor A and ABDor B (=Rst137B) are the main components of the ABDoradus system. Both stars are double (ABDor A/ABDor C and ABDor Ba/ABDor Bb) and usual targets of astrometric instruments at optical (Hipparcos), infrared (VLT), and radio (VLBI) wavelengths. From a combination of all astrometric data available, we have obtained precise limits to the dynamical mass of both binaries in AB Doradus. The determination of the mass of ABDor C (0.090±0.003 M⊙) is important, since this object constitutes one of the few calibration points used to test theoretical evolutionary models of low-mass young stars. Follow-up observations both in radio (VLBI) and optical wavelengths (VLT) should determine with high precision the dynamical mass of the four components of this system.

A close look at the heart of RCW 108

Context. The IRAS 16362-4845 star-forming site in the RCW 108 complex contains an embedded compact cluster that includes some massive O-type stars. Star formation in the complex, and in particular in IRAS 16362-4845, has been proposed to be externally triggered by the action of NGC 6193. Aims. We present a photometric study of the IRAS 16362-4845 cluster sensitive enough to probe the massive brown dwarf regime. In particular, we try to verify an apparent scarcity of solar-type and low-mass stars reported in a previous paper (Comeron et al. 2005, A&A, 433, 955). Methods. Using NACO at the VLT we have carried out adaptive optics-assisted imaging in the JHKSLbands, as well as through narrow-band filters centered on the Brγ and the H2 S (1) v = 1 → 0 lines. We estimate individual line-of-sight extinctions and, for stars detected in the three JHKS filters, we estimate the contribution to the KS flux caused by light reprocessed in the circumstellar environment. We also resolve close binary and multiple systems. We use the K luminosity function as a diagnostic tool for the char- acteristics of the underlying mass function. Results. IRAS 16362-4845 does contain young low-mass stars. Nevertheless, they are far less than those expected from the extrapo- lation of the bright end of the K luminosity function towards fainter magnitudes. We estimate a total stellar mass of 370 M� . Nearly all the cluster members display Lexcesses, whereas KS excesses are in general either absent or moderate ( 9, likely to be a Class I source. Conclusions. The fact that solar-type and low-mass stars are present in numbers much smaller than those expected from the number of more massive members hints at an initial mass function deficient in low mass stars as compared to that of other young clusters such as the Trapezium. The origin of this difference is unclear, and we speculate that it might be due to external triggering having started star formation in the cluster, perhaps producing a top-heavy initial mass function. We also note that there are no detectable systematic differences between the spatial distributions of bright and faint cluster members. Such absence of mass segregation in the spatial distribution of stars may also support external triggering having played an important role in the history of the RCW 108 region.

X-ray flares in Orion low-mass stars

Context. X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC). Aims. Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3 M_sun) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission. Methods. We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares. Results. The high energy tail of the energy distribution of flares is well described by a power-law with index 2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data. Comparing low mass stars with stars in the mass interval (0.9-1.2M_sun), we establish that, at ~1 Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies. Conclusions. Our observational results are consistent with the following model/scenario: the light curves are entirely built by over- lapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.

High spatial resolution mid-infrared observations of the low-mass young star TW Hydrae

Aims.We want to improve knowledge of the structure of the inner few AU of the circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have suggested the existence of a large inner hole, possibly caused by interactions with a growing protoplanet.

GJ 900: A new hierarchical system with low-mass components

Speckle interferometric observations made with the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences in 2000 revealed the triple nature of the nearby ($\pi_{Hip}=51.80\pm1.74$ mas) low-mass young ($\approx200$ Myr) star GJ 900. The configuration of the triple system allowed it to be dynamically unstable. Differential photometry performed from 2000 through 2004 yielded $I$- and $K$-band absolute magnitudes and spectral types for the components to be $I_{A}$=6.66$\pm$0.08, $I_{B}$=9.15$\pm$0.11, $I_{C}$=10.08$\pm$0.26, $K_{A}$=4.84$\pm$0.08, $K_{B}$=6.76$\pm$0.20, $K_{C}$=7.39$\pm$0.31, $Sp_{A}$$\approx$K5--K7, $Sp_{B}$$\approx$M3--M4, $Sp_{C}$$\approx$M5--M6. The ``mass--luminosity'' relation is used to estimate the individual masses of the components: $\mathcal{M}_{A}$$\approx0.64\mathcal{M}_{\odot}$, $\mathcal{M}_{B}$$\approx0.21\mathcal{M}_{\odot}$, $\mathcal{M}_{C}$$\approx0.13\mathcal{M}_{\odot}$. From the observations of the components relative motion in the period 2000--2006, we conclude that GJ 900 is a hierarchical triple star with the possible orbital periods P$_{A-BC}$$\approx$80 yrs and P$_{BC}$$\approx$20 yrs. An analysis of the 2MASS images of the region around GJ 900 leads us to suggest that the system can include other very-low-mass components.

What can X-rays tell us about accretion, mass loss and magnetic fields in young stars?

AbstractUntil recently, X-rays from low-mass young stars (105–106 yr) were thought to be a universal proxy for magnetic activity, enhanced by 3-4 orders of magnitude with respect to the Sun, but otherwise similar in nature to all low-mass, late-type convective stars (including the Sun itself). However, there is now evidence that other X-ray emission mechanisms are at work in young stars. The most frequently invoked mechanism is accretion shocks along magnetic field lines (“magnetic accretion”). In the case of the more massive A- and B-type stars, and their progenitors the Herbig AeBe stars, other, possibly more exotic mechanisms can operate: star-disk magnetic reconnection, magnetically channeled shocked winds, etc. In any case, magnetic fields, both on small scale (surface activity) and on large scale (dipolar magnetospheres), play a distinctive role in the emission of X-rays by young stars, probably throughout the IMF.

Young Stellar Groups around Herbig Ae/Be Stars: A Low‐Mass YSO Census

We present NIR and MIR observations of eight embedded young stellar groups around Herbig Ae/Be stars (HAEBEs) using archived Spitzer IRAC data and 2MASS data. These young stellar groups are nearby ($\leq$ 1 kpc) and still embedded within their molecular clouds. In order to identify the young stellar objects in our sample, we use the color-color diagram of J - [3.6] vs. Ks - [4.5]. The Spitzer images of our sample show that the groups around HAEBEs, spectral types earlier than B8, are usually associated with bright infrared nebulosity. Within this, there are normally 10 - 50 young stars distributed close to the HAEBEs ($<$ 1 pc). Not only are there young stars around the HAEBEs, there are also young stellar populations throughout the whole cloud, some are distributed and some are clumped. The groups around the HAEBEs are sub-structures of the large young population within the molecular cloud. The sizes of groups are also comparable with those sub-structures seen in massive clusters. Young stars in groups around HAEBEs have generally larger SED slopes compared to those outside, which suggests that the young stars in groups are probably younger than the distributed systems. This might imply that there is usually a higher and more continuous star forming rate in groups, that the formation of groups initiates later, or that low mass stars in groups form slower than those outside. Finally, there is no obvious trend between the SED slopes and the distance to the HAEBEs for those young stars within the groups. This suggests that the clustering of young stars dominates over the effect of massive stars on the low-mass young stars at the scale of our study.

Proper motion studies of outflows from Classical T Tauri stars

In a previous paper (McGroarty & Ray 2004, 420, 975) we examined the environment of a number of evolved low-mass young stars, i.e. Classical T Tauri Stars, to see if they are capable of driving parsec-scale outflows. These stars - CW Tau, DG Tau, DO Tau, HV Tau C and RW Aur - were previously known to drive only micro-jets or small-scale outflows of < 1' or 0.04 pc at the distance of the Taurus-Auriga Cloud. We found that they drive outflows of 0.5pc - 1pc, based on the morphology and alignment of newly discovered and previously known HH objects with these sources and their micro-jets. Here, we use a cross-correlation method to determine the proper motions of the HH objects in these five outflows (HH 220, HH 229, HH 702, HH 705 and HH 826 - HH 835) which in turn allows us to confirm their driving sources. Moreover, the tangential velocities of HH objects at large distances from their origin are currently poorly known so these proper motions will allow us to determine how velocities evolve with distance from their source. We find tangential velocities of typically 200 kms^-1 for the more distant objects in these outflows. Surprisingly, we find similar tangential velocities for the micro-jets that are currently being ejected from these sources. This leads us to suggest that either the outflow velocity was much higher 10^3 years ago when the more distant objects were ejected and that these objects have decelerated to their current velocity or that the outflow velocity at the source has remained approximately constant and the more distant objects have not undergone significant deceleration due to interactions with the ambient medium. Numerical simulations are needed before we can decide between these scenarios.