Hâgard, from hâ- prefix denoting "reversal; to," sometimes creating nuance
[Dehxodâ], + gard present stem of gardidan, gaštan
"to change; to turn;" Mid.Pers. vartitan;
Av. varət- "to turn, revolve;" cf.
Skt. vrt- "to turn, roll," vartate "it turns round, rolls;"
L. vertere "to turn;" O.H.G. werden "to become;"
PIE base *wer- "to turn, bend."
کروند ِ هاگرد
Fr.: facteur de conversion
1) A numerical factor that, by multiplication or division, translates one
unit or value into another.
2) In → molecular cloud studies, a factor used to convert
the → carbon monoxide (CO) line intensity to
→ molecular hydrogen (H2)
→ column density; usually denoted
XCO = I(CO) / N(H2). This useful factor relates
the observed CO intensity to the cloud mass. A general method to derive XCO
is to compare the → virial mass
and the 12CO (J = 1-0) luminosity of a cloud.
The basic assumptions are that the CO and H2 clouds are co-extensive, and
molecular clouds obey the → virial theorem.
However, if the molecular cloud is subject to ultraviolet radiation,
selective → photodissociation may take place, which will change
the situation. Moreover, molecular clouds may not be in
→ virial equilibrium.
To be in virial equilibrium molecular clouds must have enough mass, greater than
about 105 solar masses.
The way → metallicity affects
XCO is a matter of debate, and there is no clear correlation
between XCO and metallicity. Although lower metallicity brings about
higher ultraviolet fields than in the solar vicinity, other factors appear to be
as important as metallicity for the determination of XCO.
In the case of the → Magellanic Clouds,
14 ± 3 × 1020 cm-2 (K km s-1)-1, which is
larger than XCO (LMC) = 7 ± 2 × 1020 cm-2
(K km s-1)-1.
An independent method to derive XCO is to make use of the
gamma ray emission from a cloud. The flow of → cosmic ray
protons interacts with interstellar low-energy hydrogen nuclei in clouds creating neutral
→ pions. These pions quickly decay into two gamma rays. It is therefore
possible to estimate the number of hydrogen nuclei and hence the cloud mass
from the gamma ray counts. Such a gamma-ray based conversion factor is estimated to be
2.0 × 1020 cm-2 (K km s-1)-1
for Galactic clouds, in good agreement with the result obtained from the
virial method. However, the gamma ray flux is not well known in general, so this
method is uncertain as well. See, e.g., Fukui & Kawamura, 2010 (ARAA 48, 547).
The conversion of a → number system
with a given → base to another system with a
different base; such as the conversion of a → decimal number
(base 10) to a → binary number system
In order to convert a number into its representation in a different
number base, we have to express the number in terms of powers of the other base.
For example, to convert the decimal number 100 to base 3, we must figure out how to
express 100 as the sum of powers of 3. We proceed as follows:
1: Divide the decimal number to be converted (100) by the value of the new base
2: Get the remainder from Step 1 (that is 1) as the rightmost digit (least
significant digit) of new base number.
3: Divide the quotient of the previous divide (33) by the new base.
4: Record the remainder from Step 3 (0) as the next digit (to the left) of the new base number.
Repeat Steps 3 and 4, getting remainders from right to left, until the
quotient becomes zero in Step 3 (2 and 0).
The last remainder thus obtained (1) will be the most significant digit of the new base number.
Therefore, 10010 = 102013.
Conversely, to convert from another base to decimal we must:
1: Determine the column (positional) value of each digit.
2: Multiply the obtained column values (in Step 1) by the digits in the corresponding columns.
3: Sum the products calculated in Step 2. The total is the equivalent value in decimal.
For example, the binary number 1100100 is determined by computing the place
value of each of the digits of the number:
(1 × 26) + (1 × 25) + (0 × 24) +
(0 × 23) + (1 × 22) + (0 × 21) +
(0 × 20) = 64 + 32 + 0 + 0 + 4 + 0 + 0 = 100.