System Of Knots Research Articles

The nature of the cometary knots in the Helix planetary nebula (NGC 7293)

The system of cometary knots in the Helix planetary nebula (NGC 7293) has been systematically observed using ground-based images and long-slit, high-resolution spectroscopy. CCD images in the [N ii] 6584-A line, taken with the ESO NTT, are used to determine the spatial distribution of the knots; images in the [O iii] 5007-A line distinguish their position within the nebula on the basis of the absorption of the central [O iii] 5007-A-emitting zone of the Helix nebula. The kinematics of 50 of the brighter knots and their tails were studied primarily with the Manchester Echelle Spectrograph (MES) as well as the ESO NTT using the EMMI spectrograph. Three regions were covered by multiple exposures of a 10-element multislit with MES to obtain [N ii] 6584-A line profiles along 300 individual slit positions. In addition, many long-slit MES spectra, placed diametrically across the nebula, were obtained to compare the kinematics of the knots with the large-scale kinematical structure of the nebula. The global expansion of the system of knots is around 14 km s−1, some 17 km s−1 less than for the overrunning gas, and they are distributed in a thick central disc. The velocity field of the system of knots is similar to that of molecular (CO) emission but with a lower expansion velocity. Most knots are external to the central [O iii] 5007-A-emitting region, but some still show localized [O iii] 5007-A emission from their heads. The kinematical structure of two of the latter knots as revealed by profiles of the [O i] 6300-A, [N ii] 6584-A, [O iii] 5007-A and Hα emission lines is considered in detail and compared with their CO emission-line profiles. The interstellar absorption by the core of a further knot is derived from an [O iii] 5007-AHST image combined with MES [O iii] 5007-A profiles. Accurate masses, densities and ages are determined as a consequence. Furthermore, a kinematical model shows that the [N ii] 6584-A-emitting flow around a dusty, molecular globule (the core of an ionized knot) arises mainly from the expanding ionized gas of the Helix nebula engulfing the knot. The flow, parallel to the globule's photoionized surface, is mildly supersonic. The relationship between the system of knots and the large-scale structure is discussed.

The nature of the cometary knots in the Helix planetary nebula (NGC 7293)

The system of cometary knots in the Helix planetary nebula (NGC 7293) has been systematically observed using ground-based images and long-slit, high-resolution spectroscopy. CCD images in the [N ii] 6584-A line, taken with the ESO NTT, are used to determine the spatial distribution of the knots; images in the [O iii] 5007-A line distinguish their position within the nebula on the basis of the absorption of the central [O iii] 5007-A-emitting zone of the Helix nebula. The kinematics of 50 of the brighter knots and their tails were studied primarily with the Manchester Echelle Spectrograph (MES) as well as the ESO NTT using the EMMI spectrograph. Three regions were covered by multiple exposures of a 10-element multislit with MES to obtain [N ii] 6584-A line profiles along 300 individual slit positions. In addition, many long-slit MES spectra, placed diametrically across the nebula, were obtained to compare the kinematics of the knots with the large-scale kinematical structure of the nebula. The global expansion of the system of knots is around 14 km s−1, some 17 km s−1 less than for the overrunning gas, and they are distributed in a thick central disc. The velocity field of the system of knots is similar to that of molecular (CO) emission but with a lower expansion velocity. Most knots are external to the central [O iii] 5007-A-emitting region, but some still show localized [O iii] 5007-A emission from their heads. The kinematical structure of two of the latter knots as revealed by profiles of the [O i] 6300-A, [N ii] 6584-A, [O iii] 5007-A and Hα emission lines is considered in detail and compared with their CO emission-line profiles. The interstellar absorption by the core of a further knot is derived from an [O iii] 5007-AHST image combined with MES [O iii] 5007-A profiles. Accurate masses, densities and ages are determined as a consequence. Furthermore, a kinematical model shows that the [N ii] 6584-A-emitting flow around a dusty, molecular globule (the core of an ionized knot) arises mainly from the expanding ionized gas of the Helix nebula engulfing the knot. The flow, parallel to the globule's photoionized surface, is mildly supersonic. The relationship between the system of knots and the large-scale structure is discussed.

The global motions of the cometary knots in the Helix planetary nebula (NGC 7293)

Systematic measurements of the radial velocities of a significant sample of the Helix cometary knots have been made in a sustained period of exceptionally good ‘eeing’. This kinematical information has been combined with [O iii] 5007-Å images of the whole of the Helix nebula to identify the location of each knot. A global expansion velocity of ≈ 10 km s−1 for the system of knots has been found, which is far less than the 24 km s−1 general expansion of the Helix nebula. A model is favoured in which the cometary knots are relics of condensations in the precursor, red giant/AGB, slow, dense wind now overrun by faster shells (and perhaps fast winds) generated after the formation of the planetary nebula.

Fabry-Perot images of NGC 1275 and its puzzling high-velocity system

The Fabry-Perot imager is used to obtain a velocity sequence of calibrated narrow-band CCD images to cover 3000 km/s velocity space between the redshifted H-alpha emission lines of NGC 1275, its extended associated system of low-velocity (LV) filaments, and the high-velocity (HV) system of knots, projected on the same line of sight in the sky. The lack of intermediate-velocity emission-line gas between the two systems leads to an upper limit of 1.5 x 10 exp -16 ergs/sq cm s sq arcsec (3 sigma) on stripped ionized gas due to the dynamical interaction between NGC 1275 and its HV companion galaxy. It also confirms previous reports that the level of continuum light arising from stellar and nonstellar sources must be very low in otherwise bright, strongly concentrated emission-line knots with unresolved diameters of 425/h pc. The H-alpha luminosities of the emission-line regions of the two systems were measured and star formation rates derived in order to investigate quantitatively the physical relation between the HV galaxy, NGC 1275, and the surrounding cooling flow filaments.

Counting and Accounting: A Speculation on Change in Recordkeeping Practices

an organized people is counting: how many people are there? how many sheep and goats? how many boats? how many members of this group or circle? Recording the count is also necessary, whether for tax and revenue purposes or for purposes of control. A people need not have a written language to be able to record a count; for example, the people of pre-Columbian Peru, masters of textile arts, used a system of knots on strings or cords to record census and tax information. But many of the oldest known written documents concern